16 Rocket Semi-Simultaneous Launch Controller

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So about to buy parts for a Cub Scout model rocket launch control system! Yay!!!

It is made of multiple boxes, each with a big 12V battery and an Arduino, networked together. At the launch pads, each Pad Box needs to power both the static 5V control logic AND 12V through relays to between one and eight model rocket igniters. Although the RSO has the Master Arm, _each_ Scout/LCO has their own Launch Button.

 

The big difference between systems I have seen elsewhere versus our needs: it seems almost all of the other multiple-pad controllers I've seen--one-person launches for all. But we want to continue to allow each Cub Scout (after the RSO arms) to control their own individual launch buttons. Most Scouts comply with the shared countdown & "drag race." However there are often Scouts who want to "confirm their own control" or "call attention to themselves" by launching early or late! A little early or late, no problem....these "control freaks" may become real rocketeers someday! (But we do want the RSO's master-arm to keep it within bounds).

 

So all of the individual launch buttons are merely gathered as inputs to the RSO's Arduino control. Only when the Pad Boxes have their local power turned on do the Arduinos (or anything else) come on. Only when the RSO pushes the Master Arm do the Pad Boxes' Arduinos get commanded to power on the relay boards. Similarly, only when the RSO's countdown clock enables the two second window, then do the collection of launch button signals get forwarded to the Pad Boxes. So remove any of FIVE keys or momentary buttons, or the control logic, then NOTHING happens. Add in extra distance, buzzers, strobe lights, "Apollo-style" countdown/message display, Scouts launching from under a tent, "peanut gallery" watching/waiting under another tent... Well you see we take safety _very_seriously_ indeed!

 

 

"Two heads are better than one..."  So here it is, please give me your recommendations...  Three Arduinos of some model(s), RS485 interfaces, driver/relay boards, all of it is open to suggestions.  "Not too proud to beg."  Thanks in advance.

 

 

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Looks like fairly elementary logic, really.  Given the cable lengths, I think RS-485 would work fine.  Don't try to run it too fast - 9600 baud is far faster than human reaction time anyhow.  Keep most of the code on the RSO's Arduino.  And you're not going to get 5VDC out of a transformer hooked up to 12VDC...  wink  S.

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Cool project!

 

You probably don't need a full-blown Arduino at each location, thought the cost of those is relatively low, especially a nano.   You might consider laying out PCB board with an AVR and some relay's and relay drivers as a custom launchpad controller (of which you'd have several)....they could be wired up to master control using rs485 serial comm, or maybe even a radio link (careful with interference).

 

Each scout's launch button box (per table)  might be able to connect to a common 4 wire 5V/Gnd/I2C shared comm.   Normally I2C isn't so good for a table length of connected items, but your data is essentially zero (you are not moving megabytes) so it would prob be fine.  Each launch button box could have some different LEDS or maybe a 7seg display that indicates launcher number or other info.  

Switches from arcade games are robust & will take a beating & are cheap

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Thanks guys! Yes, fairly simple stuff, but never hurts to ask....others might see something I missed (Safety).  Or they may know of ways to save costs (A Scout is....Thrifty...)

 

Planning to use a "real" electro-mechanical key switch for the RSO's Master Arm, one that cannot be removed in the On position.  For the Scouts/LCOs, planning to use a 1/4" TRS plug as a removable "key."  The plug will properly cross two of the wires.  But if someone tried to insert a pocketknife and "pick" the lock, the third wire will lockout that channel.

 

Do need a momentary button for the Scouts/LCOs, but not settled on one yet.  Like the look of "missleman" red flip-covered toggles.  But also like the look of a big push-button, especially if "ready" lit.  The flip-covers don't seem big enough for the push-buttons.  Or maybe fit the Continuity LED into the end of the 1/4" TRS plug (since no wires to come out anyway)?

 

Yes, almost all logic is in the RSO's Arduino.  In Modbus, this is the Master.  RSO Box will send Enable commands to the Pad Boxes, poll from the Pad Boxes for Continuity status on the igniters, and send to the Pad Boxes the Launch relay commands.  So the Pad Boxes are simple Modbus Slaves. 

 

I do need to define how the igniters' Continuity is measured...  I show the igniter as on the Common pin of the relay.  The NO pin dumps the full 12V 2A ignition current.  But the NC pin needs to measure Continuity and feed it to the Arduino.  Definitely need Open/High Resistance (no rocket igniter/fallen lead) and Closed/Negligible Resistance (rocket igniter ready).  Would be nice to have Short/Little Resistance (igniter bent/leads touching).  Probably means moving from digital inputs to analog inputs, and maybe some extra measuring circuit.  Will have to account for resistance variance due to 5'-10' igniter cables from closest launch pad to the farthest launch pad.

 

And I have to find or build the Countdown/Message Display.  Haven't found a 6"H, daylight-readable, six character, alpha-numeric display (SETUP, HOLD, T-SS.d, LAUNCH, T+SS.d).  Or maybe jump to a 6"H LED matrix panel?  That would be more to drive....but then again the Pad Box Arduinos aren't under much processing load.  Seems I2C in order to keep the pin count down.

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If you have money, you can use these flip digits/dots...I spent one summer wiring them up for FORD factory signs.  I still have a bunch of the nice boxes they came in  (Signalex  from the Staver co)..this was in the mid 80's

https://flipdots.com/en/products...

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Will have to account for resistance variance due to 5'-10' igniter cables from closest launch pad to the farthest launch pad.

I doubt you need to do this, or that you can measure this without some pretty specialized analog front end circuitry.

The good thing is you really don't need to do this.

 

Each launch pad could/should measure it's own igniter status:

Open, i.e. not connect, the alligator clip (or whatever) fell off the igniter wire, or the igniter wire is already used

Short, i.e. the igniter wires and/or the clips are touching each other

Good-to-go, i.e. there is a valid test loop, with an igniter wire hooked up properly.

 

The test loop typically (?) is a resistor divier with the igniter wire being one of the two resistors.

Your test loop passes a very small current through the test loop to measure the three statuses.

The current is enough to make the measurements, but no where near enough, by an order of magnitude or two, to heat the igniter wire up.

 

One would measure the test loop with an analog input.

One would typically use another I/O pin to supply the test signal.

 

If you use a DPDT relay, then the unarmed / test mode connects the igniter leads to the test circuit.

When the Master Armed is valid, and the local Armed is valid, and the Launch Button / Signal is valid, then use the DPDT relay to connect the igniter to the 12V source.

The relay shouldn't connect the 12V firing supply to the line unless the test circuit says the igniter circuit is good.

 

One might include a switch in series with the relay's coil as a "Final Arm" switch for the local controllers.

 

The individual launch controllers can obviously have a status LED (s).

One LED, perhaps a WS2812B digital multi-colored LED, can show the igniter circuit status:

Green: Good

Flashing Red: Error, Fault, Open Circuit

Steady Red: Error, Fault, Short Circuit

 

Or whatever code you elect to ues, with a label on the bottom of the box with the codes written out.

 

One LED shows the Master Armed signal status:

Red:  Error, this controller is not communicating with the Master, it will never get a Master Go signal

Yellow: Standby, this controller is awaiting a Master Go signal

Green: Good-to-Go, the Master sent a Master launch authorized signal

 

The individual launch controllers likely need a Piezo beeper, also, for instant feedback to the user that yes, there push button press was acknowledged.

 

If one turns the local lauch controller on and off, one has to ensure that the circuit's startup process can not accidentally provide power to the igniter.

This is not a trivial design criteria, nor is it's implimentation.

 

Good Luch with your project!

 

Post some photos of the launch!

 

JC  

Last Edited: Wed. May 5, 2021 - 06:44 PM
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Detecting an open is very easy, detecting a short is tougher, since the ignitor is "almost" a short (may be as low as a fraction of an ohm, but prob more like 1 ohm)

So at low currents, the diff between a short & good setup will only be maybe 10-50mv (and a "short" might not be zero & additionally includes wiring & connector resistances).

Therefore, some precision, accuracy & care is needed to reliably detect a shorted condition.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. May 5, 2021 - 07:07 PM
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avrcandies wrote:

Detecting an open is very easy, detecting a short is tougher, since the ignitor is "almost" a short (may be as low as a fraction of an ohm, but prob more like 1 ohm)

So at low currents, the diff between a short & good setup will only be maybe 10-50mv (and a "short" might not be zero & additionally includes wiring & connector resistances).

Therefore, some precision, accuracy & care is needed to reliably detect a shorted condition.

 

Yes, that's my fear/complication.  The Estes igniters are to be kept under 50mA for testing.  If I do it off of an Arduino's 5V output, then indeed maybe not much difference to measure from a short...

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I would suggest using the same length wire from pad to launch controller, coil any excess, then impedance will be similar for each lead, may make detecting short a bit easier. 

Sounds like a cool project, please send launch pic's when completed! 

Have fun.

Jim

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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I believe you can still measure it, you may just not get too many counts ( @5V ref, the adc is about 5mv/count)...you can add a low offset 10x/20x/50x or amp ahead of the adc to give more numeric leeway.  Another chan can be non-amplified, for the high voltage end.

 

Many AVR's have opamp gain stages for the ADC , but I'm always suspicious about their offset voltage ratings, so really don't use them (you could cal out any such offsets in software).  

 

for 30 cents , I can get a very accurate , known gain, and  fraction of mv offset (such as INA180  ...many others)

https://www.ti.com/lit/ds/symlin...

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. May 5, 2021 - 08:02 PM
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LydaRA wrote:
But we want to continue to allow each Cub Scout (after the RSO arms) to control their own individual launch buttons.
Excellent; more range safety as each scout is responsible for where their rockets land (a scout may see what the RSO doesn't or can't see; scouts can be be educated, trained, mentored, and practiced in ballistic trajectories and safety)

LydaRA wrote:
these "control freaks" may become real rocketeers someday!
Excellent; such operators have ARM, FIRE, and can simply safe by disabling FIRE (reasons - hangfire, misfire, rockets' trajectories are incorrect, etc)

LydaRA wrote:
Scouts launching from under a tent,
Use case - a loose tent stake can whip on the cord (laceration); recommend replacing the tent with a tarp that the RSO can visually confirm as stowed (a scout has their poncho)

LydaRA wrote:
Well you see we take safety _very_seriously_ indeed!
A BMX helmet with visor may be enough PPE; uncertain if ear plugs are necessary.

Recommend a bucket of sand at all; scout might be able to deal with a situation well before the RSO completes the run to the scout.

Recommend an alternate RSO (can recall when I forgot a step in a SOP due to excitement)

LydaRA wrote:
RS485 interfaces, driver/relay boards,
All that is subject to EMI most likely by cellular telephones (a 2 watt transmitter); add EMI suppressors and interlocks (electrical, electronic, software)

For this use case, RS-485 cables can be damaged; consider some form of sheathing.

Real rocketeers have a relatively short shielded and armored and sheathed cable between a small control box and the rockets (for when they don't want to be close to the rockets); similar for operators of trucks that haul mid-size containers (remote control of the electro-mechanical/hydraulic system)

LydaRA wrote:
"Not too proud to beg."
Add a location to your profile; parts may simply "arrive"wink

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:
Normally I2C isn't so good for a table length of connected items, but your data is essentially zero (you are not moving megabytes) so it would prob be fine.
If still a concern, there are I2C bus buffers.

avrcandies wrote:
Switches from arcade games are robust & will take a beating & are cheap
Likewise for vandal-resistant IP-rated switches though at some expense.

 

"Dare to be naïve." - Buckminster Fuller

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LydaRA wrote:
Like the look of "missleman" red flip-covered toggles. 
Very easy to safe by slapping the FIRE switch's cover.

LydaRA wrote:
Or maybe jump to a 6"H LED matrix panel?
Smart phone? (meets all requirements but height ... tablet would ... feature phone might be acceptable)

A web browser can load a single-page application (SPA) with data over WebSockets.

Efficient web server technology for resource-constrained microcontrollers - Embedded.com

How to install a secure embedded Web server on a $3 WiFi device - Embedded.com

 


Developers • Why KaiOS - KaiOS (feature phone)

 

PIC32MZ W1 is an alternative to ESP32 though PIC32MZ W1 doesn't compete on price :

wireless/apps/tcp_server at master · Microchip-MPLAB-Harmony/wireless · GitHub

 

"Dare to be naïve." - Buckminster Fuller

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Inexpensive VNA have a TDR capability (distance to short)

NanoVNA | Very tiny handheld Vector Network Analyzer

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:
Many AVR's have opamp gain stages for the ADC ,
PGA versus AVR DB's op amp.

avrcandies wrote:
(such as INA180  ...many others)
next on price are CSA from ON Semiconductor then further on for Microchip Technology CSA.

Current Sense Amplifiers | ON Semiconductor

 

"Dare to be naïve." - Buckminster Fuller

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avrcandies wrote:

 

I believe you can still measure it, you may just not get too many counts ( @5V ref, the adc is about 5mv/count)...you can add a low offset 10x/20x/50x or amp ahead of the adc to give more numeric leeway.  Another chan can be non-amplified, for the high voltage end.

 

Many AVR's have opamp gain stages for the ADC , but I'm always suspicious about their offset voltage ratings, so really don't use them (you could cal out any such offsets in software).  

 

for 30 cents , I can get a very accurate , known gain, and  fraction of mv offset (such as INA180  ...many others)

https://www.ti.com/lit/ds/symlin...

 

 

 

So some additional circuit between the NC relay pin and the Arduino's analog input pin?  Like the extra driver circuit between the Arduino's digital outputs and the relays...  The relay drivers come on a 8x board.  Is there a comparable 8x resistance measurement opamp?

 

 

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gchapman wrote:

LydaRA wrote:
Scouts launching from under a tent,
Use case - a loose tent stake can whip on the cord (laceration); recommend replacing the tent with a tarp that the RSO can visually confirm as stowed (a scout has their poncho)

LydaRA wrote:
Well you see we take safety _very_seriously_ indeed!
A BMX helmet with visor may be enough PPE; uncertain if ear plugs are necessary.

Recommend a bucket of sand at all; scout might be able to deal with a situation well before the RSO completes the run to the scout.

Recommend an alternate RSO (can recall when I forgot a step in a SOP due to excitement)

 

Technically, we use a jumbo "carport" frame and tarp as a "tent."  And the guy lines are kept inline with the general ropes defining the range, firing line, and gallery areas. 

 

BSA delegates regulation to National Association of Rocketry.  Helmets (or any other head or hearing protection) are not required.  Locally we are just applying our "safe, safer, saferest" [sic] mentality.  At the manufacturer's recommended 10 feet, NAR's recommended 15 feet, and we seek even more distance between the launch pads and the controlling Scouts....these little 1/2A, A, or B engines are not big hazard.  But we still seek to prevent even "safe" parachutes from coming down on someone unexpectedly. 

 

The reason for trying to up the number of rockets on the range is to increase our throughput of happy Scouts.  But just as I'm looking to offset that increased risk with more "systems" safeties, I am also unwilling to give up the safety discussions with the Scouts, their Leaders, and their Parents.  Some might say I beat a dead horse, but between requisite range safety and this being a "learning" moment about engineering...  Like with COPE & Climbing, staff always cross-check each other's work.  Yes, we explicitly tell _everyone_ present that we _all_ are responsible for each other's safety--and _all_ are empowered and expected to shout out if they see something unsafe!

 

 

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gchapman wrote:

LydaRA wrote:
RS485 interfaces, driver/relay boards,
All that is subject to EMI most likely by cellular telephones (a 2 watt transmitter); add EMI suppressors and interlocks (electrical, electronic, software)

For this use case, RS-485 cables can be damaged; consider some form of sheathing.

Real rocketeers have a relatively short shielded and armored and sheathed cable between a small control box and the rockets (for when they don't want to be close to the rockets); similar for operators of trucks that haul mid-size containers (remote

 

Yes!  Notice how my design have the Pad Boxes right up close by the rockets.  So many wires....best kept short.  Then just need a single cable to send in commands from the RSO & LCOs.  Thinking that'll be STP CatX cabling.  It is cheap, durable, and available in longer lengths (so we could even go to larger engines if desired by the big Boy Scouts).  Or could use XLR/DMX cables.  Again, durable enough to drag around for stage show after stage show....pyrotechnics not subject to _so_much_EMI....

 

I see many custom launch control systems that are moving to Wi-Fi.  But although I have four FCC radio licenses, no interest in adding that complexity here.

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Should also note the Pad Boxes will have a multi-step power switch:

 

Off:  Obvious, ZERO power allowed from the battery

Local Test:  Powers Arduino & Continuity Test circuitry only.  Keeps boot transients from accidental launch triggering.

Remote Control:  Allows receiving of sequential commands from the RSO Box:

     "Master Arm" adds power to Relay Driver board

     "Countdown Complete" (2seconds window) adds power to NO bus

    

And finally, the RSO Box can command the Pad Box's Arduino to trigger whichever rocket's relay....again only during the "Countdown Complete" window.

 

 

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LydaRA wrote:
Thinking that'll be STP CatX cabling.  It is cheap, durable, and available in longer lengths (so we could even go to larger engines if desired by the big Boy Scouts).
fyi, single-par UTP  is likewise though 100BASE-T1, 10BASE-T1S, and 10BASE-T1L require specific PHY to an Ethernet MAC (probably no Arduino for single-pair Ethernet [SPE])

LydaRA wrote:
Or could use XLR/DMX cables.  Again, durable enough to drag around for stage show after stage show.
Concur

LydaRA wrote:
I see many custom launch control systems that are moving to Wi-Fi.
... and Wi-Fi's security holes result in corrections; Wi-Fi can have greater than significant range by mesh networking and/or antenna gain.

 


Ethernet Cables Speed Data in Harsh Environments | Electronic Design (SPE)

Single-pair Ethernet PHY Offers the Industry’s Leading Ultra-low TC10-compliant Sleep Current and is Functional Safety Ready | Microchip Technology (100BASE-T1)

New Ethernet Standard Solves Range Limitation for Industrial Applications | Electronic Design (10BASE-T1L)

 

WFI32E Curiosity Board - Robust Wi Fi® Authentication with WPA3 - YouTube (1m46s)

Enable Robust Wi-Fi Authentication with WPA3 | PIC32MZW1_Workshop/07_projects at master · MicrochipTech/PIC32MZW1_Workshop · GitHub

WNDW - Wireless Networking in the Developing World

 

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:

fyi, single-par UTP  is likewise though 100BASE-T1, 10BASE-T1S, and 10BASE-T1L require specific PHY to an Ethernet MAC (probably no Arduino for single-pair Ethernet [SPE])

 

Wow, I missed that one!  But not sure that we could still see an elementary-level, < 2oz., 15" H model rocket from 1 full KM away!  It may only go to about 200' on those 1/2A engines.  If visible though, that should give pretty good certainty for the AltiTrack.

 

 

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I believe you can still measure it, you may just not get too many counts

Obviously one has to get a stack of igniters, from different manufacturing batches, and measure their cold resistance.

I suspect it will be a lot higher than one expects!

That is good for the project!

 

When I tinkered with an igniter circuit a few years ago the tungston igniters had > 100K ohms cold resistance, yet burned orange/white hot when powered!

 

The test circuit placed the igniter as the middle of a 3 resistor chain.

That offset the low end reading.

 

My notes, for testing, showed the following ADC thresholds:

0 - 4 = 0V, Open circuit

5 - 75 = Igniter Good

76 - 1023 = Short Circuit, wires shorted.

 

The resistor stack could likely have been improved to better set the three ranges, but the above worked fine in practice for this project.

 

Obviously one needs a small cap across the lower leg of the 3-resistor divider, basically a LPF for the ADC input, to help eliminate noise.

 

And, as one isn't doing high speed signal processing, it is wise to take multiple measurements and massage the data before making one's status decision.

IIRC, I through out the high and low values, and averaged the rest.

How you do that is up to you!

 

Be safe!

 

Have fun!

 

JC

 

Edit: Typos

 

Last Edited: Thu. May 6, 2021 - 02:27 AM
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DocJC wrote:

The test circuit placed the igniter as the middle of a 3 resistor chain.

That offset the low end reading.

 

My notes, for testing, showed the following ADC thresholds:

0 - 4 = 0V, Open circuit

5 - 75 = Igniter Good

76 - 1023 = Short Circuit, wires shorted.

 

The resistor stack could likely have been improved to better set the three ranges, but the above worked fine in practice for this project.

 

Love that info.  Thanks!

 

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LydaRA wrote:

Or could use XLR/DMX cables.

 

Probably expensive.  However - AC extension cord available from Home Depot (or whatever your local home improvement store is) by the reel will (with a bit of stuffing - test first, and use dish detergent as a lubricant) sort of fit into off-the-shelf XLR connector housings.  Have your Scouts earn their soldering badges by making the cables  wink .   It's pretty good cable, actually - three conductors at 14ga. and well-protected (ish) for not much money.  Coils nice, too.

 

Have fun!  S.

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Scroungre wrote:

Coils nice, too.

 

Coiling cables.....nooooo.  I'm a climbing instructor as well.  So we'll butterfly bundle those cables!  Fewer twists, kinks, and knots.  :-D

 

 

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Data cabling is not rope.  It should not be treated the same ways.  indecision  S.

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Scroungre wrote:
Data cabling is not rope.  It should not be treated the same ways.  indecision  S.

 

Yes, and no.  For more fragile, lightweight cables yes treat more gingerly.  However for heavyweight cables, like the electrical extension cords recommended, the gauge and overall diameter _are_ more like rope than most data cabling.  And if one is going to cutoff the manufacturer-provided ends and solder on XLR connectors, as recommended....well that screams to me to prevent tangles and the "hood stress" that can result!  Try butterflying an extension cord for your hedge-trimmers or lawn-edger....you'll never go back to that tangled coil mess again.

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NO! Back away from the keyboard!  Don't say it!

 

OK, I'll say it anyway.

 

An ESP8266 chip in each box is likely less expensive than the power cords!

The added benefit is ZERO cords to coil/butterfly, lay out, untangle, clean, trip over, etc.

 

Either each launch pad, or each cluster of 8 launch pads, has a wireless link back to the Master Controller.

 

This would be a lot more work on the front end, to code the wireless network, but it would be soooo much easier in the field!

 

JC

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DocJC wrote:

NO! Back away from the keyboard!  Don't say it!

OK, I'll say it anyway.

 

Hahahahaha!  Rotflmao

 

Yes, I _love_ wireless.  I have four FCC licenses (ham, clocks, site radios).  However for something "dangerous," that may have to be maintained by someone other than myself a few years from now...  I'll stick to the simplicity of a couple of cables.  Got to deal with those other analog igniter and LCO cables anyway.

 

Ironically, driving home today, a new concept ran through my mind:  Instead of "reinventing the wheel" building the Scouts' individual Keys, Continuity LEDs, and Launch buttons....why not just build an interface to clip the _existing, OEM controllers' alligator leads onto?  Reduces what I have to build, yet still gains most of the other benefits from my RSO and Pad boxes.  Still only a single cord from RSO/LCOs to the Pads.  Still a single RSO battery instead of scads of AA batteries in individual controllers (well still have the big batteries in the two Pad Boxes).  Still have the added safety of the RSO's "Master Arm" key.  Still have the Arduinos enforcing the "Countdown Complete" 2 second window to allow Launch commands.  Still have the strobe, siren, & other warnings.  However....then all that mass of loose OEM controllers spread across the tables, instead of a single "stick" panel.  Not sure...

Attachment(s): 

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DocJC wrote:
An ESP8266 chip in each box is likely less expensive than the power cords!
An ESP32 module is only a bit more in price with additional capability; ESP32-S2 adds a RISC-V co-processor (greatly reduces sleep current)

PIC32MZ W1 has a nearly completely functional ADC.

DocJC wrote:
...  to code the wireless network, ...
Network stacks exist and are relatively simple to configure; the complexity can be moved from Wi-Fi MCU to web apps by data over WebSockets.

Web apps are usually in JavaScript though TypeScript is available (memory-safe computer languages)

 


esp32-s2 | olimex

Understanding ADC Performance in Wi-Fi® MCUs - YouTube (4m26s)

PIC32MZ W1 Errata (page 4, top left)

 

A fresh look at embedding a web server - Embedded.com (WebSocket)

Memory safe computer languages | AVR Freaks

 

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:

Network stacks exist and are relatively simple to configure; the complexity can be moved from Wi-Fi MCU to web apps by data over WebSockets.

 

Web apps are usually in JavaScript though TypeScript is available (memory-safe computer languages)

 

True enough.  But not sure I could get anything "web app" through acceptance analysis by the local BSA council.  A couple of we techies have struggled for a while to get the council website converted to a more modern, delegatable platform; and for the implementation of better camp registration platform...

 

Back when I worked for a private school, I ran a middle school tech club.  Among the things we taught them: Android app development.  I had their rapt attention....until someone linked the accelerometer to their playlist.  Shake, Shake, "YEOW," "Hot Pants!"  Although smartphones and tablets were encouraged for note-taking in classes, we all cringed as for the rest of the year the hallways suffered random Shake, Shake, Scream...  But oh what _CAN_ be done!

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Trying to learn KiCAD....still rough, but getting better. Finally enough drafted to share (and drown in the red ink as you all mark it up)... Thanks in advance for constructive criticism!

1620685862204.png

Showing just two of eight independent parallel igniter sub-circuits. And considering variations....DPDT vs. SPDT....Arduino source current for continuity measurements vs. dedicated constant current source....building driver/relay sub-circuits vs. buying an 8x driver/relay board...

 

Obviously just focused on the pre-launch continuity tests and one the launch signal handling. There _ARE_ _MULTIPLE_ power discos, removable keys, momentary "deadman" switches, and program logic safety measures as well!

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You didn't include any photo or anything to look at

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

You didn't include any photo or anything to look at

Hmmm, it shows inline with the post for me.  But including it again here as an attachment.

Attachment(s): 

Last Edited: Tue. May 11, 2021 - 02:21 AM
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Hmmm, it shows inline with the post for me.  But including it again here as an attachment.

So you are saying when you clicked on it in the post you see it opening up??  Or just seeing the post it already opened? (not sure what you mean by inline)

 

 

Needs a lot of help...the ohm tests should connect direct to the ignitor not through the relay...clamp them at the avr, if needed.

 

The ignitor gnd should just connect to gnd...no relay.   The buzzer can connect to the AVR so you can control it as desired. There is some slight benefit to having it direct to the relay, in terms of safety/reliability, if is it a safety warning, failsafe from software troubles.  But that may be a small consideration here.

 

Why is the ignitor driven in series with the buzzer??---that seems quite wrong (unless a high current, low drop  buzzer)  How do you expect to power the ignitor--through the buzzer?

 

if you use a fet to drive the relay, the diode is not needed (the fet contains a zener diode)--unless it is a monster sized relay, or  fleapower fet.

Actually the fet can drive the ignitor quite well--no relay even needed.  Relays can get stuck and are mechanical (good or bad).

 

Isn't the ignitor going to use connectors?  where are all your connectors?  Why need an adrduindo?  Put an AVR on your PCB.  Or even patch in a nano (which isn't much more than an avr on a very small board).

 

Forget about this cad right now---you waste lots of time.  Draw on some paper, quick & easy until you have something & see your options.  What you labor for 3 hours can be sketched up in 10 minutes. 

CAD forces you to hang on to a lot of bad choices---yeah this circuit is junko, but I just spent 6 hours drawing it so I'm not going to change that opamp for a window comparator!!  All those logic gates could be replaced with one counter chip, but they are already drawn.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. May 11, 2021 - 04:15 AM
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avrcandies wrote:

...the ohm tests should connect direct to the ignitor not through the relay...clamp them at the avr, if needed.

Trying to keep the 5V ##mA Arduino separated from the 12V ###A car battery.

 

avrcandies wrote:

The ignitor gnd should just connect to gnd...no relay.   The buzzer can connect to the AVR so you can control it as desired. There is some slight benefit to having it direct to the relay, in terms of safety/reliability, if is it a safety warning, failsafe from software troubles.  But that may be a small consideration here.

 

Why is the ignitor driven in series with the buzzer??---that seems quite wrong (unless a high current, low drop  buzzer)  How do you expect to power the ignitor--through the buzzer?

Exactly, the buzzer is the "oh shit, backoff" warning should the relay contacts weld (or any other condition) that sends power to the igniter while someone is at the launch pads!  Doesn't need to be super loud (those in danger are standing over the Pad Box & buzzer), but does need to pass as much of the 2A current as possible to the igniter.

 

avrcandies wrote:

if you use a fet to drive the relay, the diode is not needed (the fet contains a zener diode)--unless it is a monster sized relay, or  fleapower fet.

Actually the fet can drive the ignitor quite well--no relay even needed.  Relays can get stuck and are mechanical (good or bad).

The NAR chapter here swears by automotive relays.  Again, seems to make sense to separate low-V&I logic from the high-I launch currents.

 

avrcandies wrote:

Isn't the ignitor going to use connectors?  where are all your connectors? 

Yes, the Pad Box will probably run all eight igniter leads out a single 12pin 16Ga aviation connector.  Eight pins need to carry 12V 2A each directly to individual alligator clips.  Then a shared bus for the other side of the circuits, spliced for parallel pigtails and alligator clips at each rocket.  I showed a note that gave the 15' cable resistance "+ connectors."  Will have to account for connections inside the Pad Box, the aviation connector, the pigtail splice down to the alligator clip...

 

avrcandies wrote:

Why need an adrduindo?  Put an AVR on your PCB.  Or even patch in a nano (which isn't much more than an avr on a very small board).

Indeed.  Was just trying to speed the concept design & prototype....like with a solder-less breadboard...  Obviously the Arduino Uno doesn't have enough analog pins, but it was what KiCAD had in their library.  More likely to do a Mega (my initial board of choice when prototyping) or a Micro (12 analog pins).  Also need to work in a RS485/Modbus net of one RSO Box "master" and two of these Pad Box "slaves"--spread 25'-150' apart....easy shields/feathers to bolt on.  If I use an Arduino/compatible board + 8 channel driver/relay board + RS485 board....then find an 8 channel resistance measurement/opamp/comparator board....well then the "custom PCB might not be much more than a set of header sockets to join it all together!  Then again, more custom (while more work, and more to safety check) PCB work would allow more cheap automotive components and be easier to service any component failures.

 

avrcandies wrote:

Forget about this cad right now---you waste lots of time.  Draw on some paper, quick & easy until you have something & see your options.  What you labor for 3 hours can be sketched up in 10 minutes. 

CAD forces you to hang on to a lot of bad choices---yeah this circuit is junko, but I just spent 6 hours drawing it so I'm not going to change that opamp for a window comparator!!  All those logic gates could be replaced with one counter chip, but they are already drawn.

Definitely could do this with the classical two buttons, incandescent bulb, and battery--but can do so much better, fun & safety--and learn something new to boot!  Not too proud to seek help here....not set on these drawings, just trying to clarify our discussions.  Lay the alternative ideas on me!  :-D

 

I've just got to pull this together before disappointing the Cub Scouts in a few weeks.  Thanks for the help!

Last Edited: Tue. May 11, 2021 - 04:57 AM
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Express PCB has a schematic drawing program that might be easier to use.

You don’t have to use their PCB layout program, or get your PCB’s made there.

It is just another (free) option for drawing schematics.

 

Agree, the Igniter Ground doesn’t need to go through the relay.  Open of closed it still connects that lead to ground.

 

The Arduino Uno Pin 9 says +5V @ 40 mA Output.

That is the MAX that that pin can source in spec.

With the 220 ohm resistor in the output, and ignoring the igniter’s resistance, the maximum current from that pin will be 23 mA, (I=V/R,  5/220 = 22.7 mA).

 

Did you actually take a handful of igniters and measure their cold resistance?  Is it really 0.8 ohms?  I’d have thought it was higher.

 

Agreed, again, don’t power the igniter through the buzzer.  You want the full 12V going to the igniter.

 

I’m fine with the “extra” diode across the relay.  Perhaps an 1N4004?

 

If you end up stacking the igniter in the middle of two resistors, for a 3 resistors in series stack, to help get the ADC reading up above Ground a little bit higher, then you will likely want to route the igniter’s ground lead through the relay.  In standby / measure the continuity mode the lower resistor is in the series circuit.  In fire mode the igniter lead connects directly to ground.

 

Which relay you select will determine its current draw, and hence the NFet to use.

 

Perhaps it is just me, but I love the old saying that a project can never have too many LEDs!

 

You will likely want a power on LED, and an Igniter Good LED, Open circuit LED, and Short Circuit LED.  Keep it simple so the kids know what the fault is when there is one.

You will likely want a good-to-go LED, for when the Range Master and Safety Officer, etc., all give the good-to-go signal, even if it only lights up for a few seconds.  It will be helpful, additionally, for trouble-shooting your system.

 

Arduinos don’t generally have a big heat sink on their regulators.  You will want to see how much current the Arduino and its LEDs and buzzer, etc. are drawing.  The higher Vin, 12V in your case, the more energy that the little regulator has to dissipate while supplying the output current for the 3 & 5 Volt buses.  The circuit might well be fine, but if you are drawing too much current then a separate power supply, with a decent heat sink, might be a better approach.

 

The ARef pin needs a 0.1 uF cap to ground to help decrease the noise on the analog, ADC, measurements.

 

One of the many nice features of the Arduino controller lineup is the fact that they have a decent hardware design, are built using a very small physical size, and can be purchased for dirt cheap, much cheaper than you could likely build your own PCB for.

One of the “down sides” is that if the igniters really are 0.8 ohms, then you will likely needs a good circuit to distinguish between a good igniter and a short circuit, (the clips touching under the rocket, for example).  The “difficulty” in making this test is compounded by the low sense current, and the wide range in igniter resistance from batch to batch, (although they will all glow orange/white hot when you apply 12V).

The Arduino’s don’t, IIRC, separate their digital and analog power supplies and grounds, so there will likely be more noise in your ADC measurements that if you designed a custom PCB, from the ground up.

That is likely a fine point, and with some testing you might well demonstrate that it doesn’t make a difference for this application, but you at least ought to be aware of the trade offs you are making as you undertake your design.

For this application you have loads of time to take multiple readings and average them, to help deal with the noise.

 

I think an important step in this project will be simply tinkering on the bench with the Mega328, (Arduino Nano/Uno/Whatever), and a bunch of igniters, and see what your ADC readings are.  Cross checking them with a DMM/Voltmeter, as you make your measurements, would also be a good idea.

 

Good luck with your project!

 

JC

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BTW, are you really using a car battery to power the igniters?

 

If so, then you also need to put a Fuse in each circuit, in case you get a short circuit at the igniter.  Car batteries can source an incredibly high amount of current!  If the igniter clips touch just before the ignition circuit is energized, it will quite literally burn up the wires.

 

I seem to recall launching with a few D cells in series, much more portable, and much more current limited.

On another project I did, with a different type of igniter, the circuitry used one 9V battery, and the igniter circuit via a relay, etc., had a second 9V battery.

It worked fine.

 

JC

 

 

You will notice three piezo beepers to make a little extra sound, (and one can modulate the beep with three of them...).

 

There are two, 9V battery holders.

 

The little tan box is the relay to connect the 9V battery to the igniter.

 

The user worked the bottom switches from Left to Right.

Configuration Dip Switch to set the mode, then power the board up, then the next push button, (function depended upon the mode, IIRC), then the "start" button, and finally the "final arm" slide switch, if the LED above it is green.

 

Lots of ways to skina cat!

 

JC

 

Edit:

Typos

And Hurrah!, I managed to insert an image with Copy/Paste.

 

 

Last Edited: Tue. May 11, 2021 - 05:19 AM
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yes wink

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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DocJC wrote:

Express PCB has a schematic drawing program that might be easier to use.

You don’t have to use their PCB layout program, or get your PCB’s made there.

It is just another (free) option for drawing schematics.

Will check.  Thanks.

 

DocJC wrote:

Agree, the Igniter Ground doesn’t need to go through the relay.  Open of closed it still connects that lead to ground.

Yes, See the second igniter example, with the SPDT relay.

 

DocJC wrote:

The Arduino Uno Pin 9 says +5V @ 40 mA Output.

That is the MAX that that pin can source in spec.

With the 220 ohm resistor in the output, and ignoring the igniter’s resistance, the maximum current from that pin will be 23 mA, (I=V/R,  5/220 = 22.7 mA).

Yes, since the Estes igniter is safe at <50mA, thought I'd use just half that....to be even "safer."

 

DocJC wrote:

Did you actually take a handful of igniters and measure their cold resistance?  Is it really 0.8 ohms?  I’d have thought it was higher.

Per Estes, yes their nichrome igniter is less that one Ohm.  I do have an entire box of them for testing, when I get to that point.  Will probably use a potentiometer, so the final value can be dialed in for different igniters.  Will also have the AVR code running on that analog input, so can adjust the three states' (Open, Ready, Short) definitions.

 

DocJC wrote:

Agreed, again, don’t power the igniter through the buzzer.  You want the full 12V going to the igniter.

Want this "last ditch" safety warning to be absolute.  Safety around children...

 

DocJC wrote:

I’m fine with the “extra” diode across the relay.  Perhaps an 1N4004?

Yeah, I'm not used to diodes included in transistor or relay packages.  So if the final components chosen don't have them, definitely want to ensure we prevent any kickback.  Safety around children...

 

DocJC wrote:

If you end up stacking the igniter in the middle of two resistors, for a 3 resistors in series stack, to help get the ADC reading up above Ground a little bit higher, then you will likely want to route the igniter’s ground lead through the relay.  In standby / measure the continuity mode the lower resistor is in the series circuit.  In fire mode the igniter lead connects directly to ground.

Ah, so back to a DPDT relay?  Yes, the buzzer is about all of the added resistance we should tolerate when actually trying to burn the igniter!

 

DocJC wrote:

Which relay you select will determine its current draw, and hence the NFet to use.

Suggestions? 

 

DocJC wrote:

Perhaps it is just me, but I love the old saying that a project can never have too many LEDs!

 

You will likely want a power on LED, and an Igniter Good LED, Open circuit LED, and Short Circuit LED.  Keep it simple so the kids know what the fault is when there is one.

You will likely want a good-to-go LED, for when the Range Master and Safety Officer, etc., all give the good-to-go signal, even if it only lights up for a few seconds.  It will be helpful, additionally, for trouble-shooting your system.

Yes.  There obviously are to be power disconnects, switches, LEDS, & buzzer in the Pad Boxes--where the most risk is.  But the primary focus for monitoring is in the RSO Box "master"....which (other than the local discos in the Pad Boxes "slaves") reads and displays--and controls--all power buses and commands.  To keep them involved (to encourage everyone "owning" safety, and to up the fun), the RSO Box copies the individual continuity statii to a LED in front of each Scout/LCO.  Similarly, the RSO Box collects key insertion and launch button presses from in front of each Scout/LCO....and subject to the RSO's removable key, Arming switch and "deadman" switch on the countdown clock; during the two second launch window, packs the launch button inputs into a Launch command packet forwarded to the Pad Boxes.  Also have a big strobe & siren on a pole to warn the range when countdown initiated.  Also planning a big 6"-12" high countdown clock/message display to stand in front of the launch pads (like the old NASA Apollo clock).

 

DocJC wrote:

Arduinos don’t generally have a big heat sink on their regulators.  You will want to see how much current the Arduino and its LEDs and buzzer, etc. are drawing.  The higher Vin, 12V in your case, the more energy that the little regulator has to dissipate while supplying the output current for the 3 & 5 Volt buses.  The circuit might well be fine, but if you are drawing too much current then a separate power supply, with a decent heat sink, might be a better approach.

Yes, I had read the "ideal" Arduino input would be 9V.  But got some feedback to simplify to a unified power supply, since it "can" run on 12V.  Just seems to me to demand that the Arduino needs some protection (maybe an auto-resettable fuse) on the line to the 12V supply.

 

DocJC wrote:

The ARef pin needs a 0.1 uF cap to ground to help decrease the noise on the analog, ADC, measurements.

Sounds good, thanks!  Definitely expecting the surge of 8 x relays + 2A to igniters will be a momentary whirlwind of EMI!  (One more reason I decline people suggesting I go Wi-Fi....even though I have four FCC licenses.)

 

DocJC wrote:

One of the many nice features of the Arduino controller lineup is the fact that they have a decent hardware design, are built using a very small physical size, and can be purchased for dirt cheap, much cheaper than you could likely build your own PCB for.

One of the “down sides” is that if the igniters really are 0.8 ohms, then you will likely needs a good circuit to distinguish between a good igniter and a short circuit, (the clips touching under the rocket, for example).  The “difficulty” in making this test is compounded by the low sense current, and the wide range in igniter resistance from batch to batch, (although they will all glow orange/white hot when you apply 12V).

The Arduino’s don’t, IIRC, separate their digital and analog power supplies and grounds, so there will likely be more noise in your ADC measurements that if you designed a custom PCB, from the ground up.

That is likely a fine point, and with some testing you might well demonstrate that it doesn’t make a difference for this application, but you at least ought to be aware of the trade offs you are making as you undertake your design.

For this application you have loads of time to take multiple readings and average them, to help deal with the noise.

Exactly.  The processing load & bandwidth here are soooo low....heck I even considered I2C for the 25' command bus between boxes.  But to answer any safety questions, decided not to even argue that far from recommended specs of the I2C bus...

 

DocJC wrote:

I think an important step in this project will be simply tinkering on the bench with the Mega328, (Arduino Nano/Uno/Whatever), and a bunch of igniters, and see what your ADC readings are.  Cross checking them with a DMM/Voltmeter, as you make your measurements, would also be a good idea.

Yep!  A full box of igniters & potentiometers....but even more grateful for any part numbers or values you all can recommend.  (I've not built many serious circuits since helping build motorized wheelchairs and ADA accommodations as a Red Cross volunteer in a residential hospital for the mentally retarded.  Oh how I get pulled into neat stuff when helping others!!!)

 

DocJC wrote:

Good luck with your project!

 

JC

Many thanks.  Keep the ideas coming everyone.

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DocJC wrote:

BTW, are you really using a car battery to power the igniters?

Hoping to use a smaller 12V motorcycle/lawn mower, or maybe even an emergency light/alarm panel battery.  Definitely need to supply 8 x 2A igniters + all of the relays and logic circuitry--without browning out.  However even when launching eight rockets at one time ("drag-racing"), it only lasts for a couple of seconds.  Hoping to consider this a higher "cold cranking amps" value, that lets me use a smaller battery without damaging it.  Max load also has low frequency: couple of seconds in five plus minutes, up to four times an hour, eight hours per day, few days over two weeks in the summer....then back to storage until the next Webelos Adventure Camp a year later.

 

DocJC wrote:

If so, then you also need to put a Fuse in each circuit, in case you get a short circuit at the igniter.  Car batteries can source an incredibly high amount of current!  If the igniter clips touch just before the ignition circuit is energized, it will quite literally burn up the wires.

Yes, I figure current limiting supplies, resistors (hate wasting heat), and fuses for each leg coming off of the 12V bus!  Suggestions?

Also considering making a "siamese alligator clip."  Something like an insulated office binder clip, with a termination strip barrier, and separate wire pads on each side.  Would make it quicker and easier to clip onto both sides of the igniters.  Would reduce the fallen clips (either side may hold on the other).  And eliminates the uncontrolled individual clips touching.  (Although we might still accidentally bend the igniter itself into a short...)

 

DocJC wrote:

I seem to recall launching with a few D cells in series, much more portable, and much more current limited.

On another project I did, with a different type of igniter, the circuitry used one 9V battery, and the igniter circuit via a relay, etc., had a second 9V battery.

It worked fine.

 

You will notice three piezo beepers to make a little extra sound, (and one can modulate the beep with three of them...).

There are two, 9V battery holders.

The little tan box is the relay to connect the 9V battery to the igniter.

The user worked the bottom switches from Left to Right.

Configuration Dip Switch to set the mode, then power the board up, then the next push button, (function depended upon the mode, IIRC), then the "start" button, and finally the "final arm" slide switch, if the LED above it is green.

Really nice!!!  Schematics available to learn from?

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js wrote:

yes wink

While in Scouting uniform or activities, I cannot join you in that Corona. 

 

But share something that might keep us from frying a koala or kangaroo....or that might help excite a child or parent to take up rockets (or electronics in general)!

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The NAR chapter here swears by automotive relays.  Again, seems to make sense to separate low-V&I logic from the high-I launch currents.

That by itself is somewhat old thinking ...FETs are used everywhere to control large voltages & high powers from small processor & logic...relays may be considered more "hardened" against abuse, since they can take  a severe overload/pounding/hammering/blowtorch and keep going.  In terms of general wear & tear FETs easily win the game, since they never wear out, don't stick, etc.

 

As far as the buzzer,,piezo ones typically use fairly low currents..(like 10ma, 50ma, etc)..How are you proposing to use them to drive amps through the ignitor?

 

Where you might like a relay is to short out the igniter, unless you are actively trying to do something (take measurement, launch, etc).  Open the relay and turn on a fet to run launch current through it.  Make it so if power is removed it is inherently shorted (it will need unshorted only for a few moments).  Then it is "impossible" to have any electrical mishap (ignoring things like stuck relays, or dropping steel wool onto the PCB).

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. May 11, 2021 - 08:27 AM
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avrcandies wrote:

The NAR chapter here swears by automotive relays.  Again, seems to make sense to separate low-V&I logic from the high-I launch currents.

That by itself is somewhat old thinking ...FETs are used everywhere to control large voltages & high powers from small processor & logic...relays may be considered more "hardened" against abuse, since they can take  a severe overload/pounding/hammering/blowtorch and keep going.  In terms of general wear & tear FETs easily win the game, since they never wear out, don't stick, etc.

One reason that the local NAR chapter recommends "overkill" 10-30A automotive relays. They should take the inrush spike, and the summer heat in a box in the middle of a sunny field. Even if they fail, cheap and easy to replace if connected in automotive sockets. "Safer..." but to be "saferest" [sic], I'm putting a buzzer in series with the igniter--a foolproof "oh shit, stand back" to be heard before anyone gets near the launchpad while a shorted circuit is still powering the igniter leads!

 

avrcandies wrote:

As far as the buzzer,,piezo ones typically use fairly low currents..(like 10ma, 50ma, etc)..How are you proposing to use them to drive amps through the ignitor?

Yeah, probably not a piezo one.  Not sure of an "automotive" or "industrial" part number--was hoping for suggestions...

 

avrcandies wrote:

Where you might like a relay is to short out the igniter, unless you are actively trying to do something (take measurement, launch, etc).  Open the relay and turn on a fet to run launch current through it.  Make it so if power is removed it is inherently shorted (it will need unshorted only for a few moments).  Then it is "impossible" to have any electrical mishap (ignoring things like stuck relays, or dropping steel wool onto the PCB).

Yes, my design will require numerous power discos, removable keys, momentary buttons, a clock, and code to enable the power bus, MOSFET, _and_ the relay to send the high-current to the igniter.  Yet another reason I'm thinking a dedicated "constant current" source on the continuity side of the relay is simpler and safer.  If the OEM says the igniter is safe to 50mA, and we have no source in the circuit beyond 25mA...

 

I think I prefer an isolated source of a constant "flashbulb safe" current for the continuity tests. Seems safer and frees pins on the Arduino. However there are a variable number of rockets to be launched, from eight (initial "drag race") down to possibly just one or two (after replacing igniters, re-clipping leads, correcting whatever misfires...). So with variable number of parallel paths, seems each continuity test sub-circuit will need its own low-voltage/low-current power source. Right? Any suggestions for a part number (I stuck in something representative, but KiCAD had just VAC chip....wrong item for this)?

 

Still think we'll need some sort of amplification to distinguish the three resistance levels on the igniter...

 

Thanks again--keep the ideas coming!

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Any 3 terminal regulator will work as a current source....adjustable type is much preferred, for greater Vcompliance usable range.   This is due to a low Vref voltage the adjustable reg maintains (often 1.25V)

 

MIC5377 is 10 cents in quantity,  has an enable on/off pin  (though you don't absolutely have to turn the 50ma off, for example yuse a shorting relay to just  send it to gnd)...but you can save batt power  using on/off

 

Anyhow, the vref here is 1V, so a 200 ohm resistor will give a constant 50ma out to gnd  (Vin must be more than about 1.3 V, and 5.5V abs max)

 

https://ww1.microchip.com/downlo...

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. May 11, 2021 - 04:37 PM
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LydaRA wrote:

Per Estes, yes their nichrome igniter is less that one Ohm.  I do have an entire box of them for testing, when I get to that point.  Will probably use a potentiometer, so the final value can be dialed in for different igniters.  Will also have the AVR code running on that analog input, so can adjust the three states' (Open, Ready, Short) definitions.

 

 

I've kept out of this because I have limited experience in the rocket launching states (except for Project LOHAN on The Register - look it up - which sadly was cancelled due to the unfortunate death of the project leader) but do I understand you to be adjusting the resistance value for a comparison value (to check the presence of a correctly valued igniter)? Rather than a rotary pot, easily nudged accidentally, I'd think about using something like a rotary switch. Twelve position switches are available, I think, and will provide  a much more robust solution. It's easy to wire a switch like that as a potentiometer, but also as individual values which might more easily be matched to your expected values.

 

Neil

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barnacle wrote:

LydaRA wrote:

Per Estes, yes their nichrome igniter is less that one Ohm.  I do have an entire box of them for testing, when I get to that point.  Will probably use a potentiometer, so the final value can be dialed in for different igniters.  Will also have the AVR code running on that analog input, so can adjust the three states' (Open, Ready, Short) definitions.

Rather than a rotary pot, easily nudged accidentally, I'd think about using something like a rotary switch. Twelve position switches are available, I think, and will provide  a much more robust solution. It's easy to wire a switch like that as a potentiometer, but also as individual values which might more easily be matched to your expected values.

Interesting idea.  Haven't heard of pots "drifting" inside of a box.  But "positive stops" are an intriguing idea....use & love them on my woodworking tools.  With big automotive relays, fuses, and sockets....certainly already requiring a fairly large PCB anyway.  Thanks for the suggestion.

Last Edited: Wed. May 12, 2021 - 12:37 PM
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Updated schematic...  Thoughts? 

 

Attachment(s): 

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Did you look at the MIC3077...it is only 25 cent in quantity !! It can run direct from your 5V supply.

 

https://www.digikey.com/en/produ...

 

Why have a pot on pin 10??...it will do nothing other than degrade your accuracy.   At least the pot pin 2 should be connected to pot pin 3 (don't leave a wiper connection by itself).  If you want different trip levels , you can adjust the current, or better yet , set the  trip level in the AVR (wire a switch or pot to the AVR).

 

Actually, you want to gnd the ignitor solidly (not through relay), so you have a known resistance (zero), rather than various contact resistances adding more uncertainty (since you are measuring fractional ohms).

 

The FET pulldown should be more like 1K, not 100K.  The continuity test lacks any filter caps, at min need an rc at the adc pin.

Use an INA180 amp 10x (or even 20x) to give you more used range on the ADC converter.  Did you see the INA4180 (quad amp) is only 70 cents @ 100?  There are many other similar amps, but these seem pretty price competitive & good offset (150 uV around gnd)

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

Did you look at the MIC3077...it is only 25 cent in quantity !! It can run direct from your 5V supply.

 

https://www.digikey.com/en/produ...

I was talked into eliminating the 5V source...  If doing a second power bus, the Arduino needs at least 6V, better 9V. (Still have misgivings, but I came for advice...)

 

avrcandies wrote:

Why have a pot on pin 10??...it will do nothing other than degrade your accuracy.   At least the pot pin 2 should be connected to pot pin 3 (don't leave a wiper connection by itself).  If you want different trip levels , you can adjust the current, or better yet , set the  trip level in the AVR (wire a switch or pot to the AVR).

 

Actually, you want to gnd the ignitor solidly (not through relay), so you have a known resistance (zero), rather than various contact resistances adding more uncertainty (since you are measuring fractional ohms).

Was advised to raise the base voltage range on the continuity test by adding a third resistor to the igniter path.  Made it a potentiometer so it could be adjusted for different types of igniters.  Might even make the first resistor a pot as well, so it is all adjustable.  Someone suggested a rotary encoder, with positive incremental stops.

 

avrcandies wrote:

The FET pulldown should be more like 1K, not 100K.  The continuity test lacks any filter caps, at min need an rc at the adc pin.

Use an INA180 amp 10x (or even 20x) to give you more used range on the ADC converter.  Did you see the INA4180 (quad amp) is only 70 cents @ 100?  There are many other similar amps, but these seem pretty price competitive & good offset (150 uV around gnd)

Sample sketch?

 

Thanks again.

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I was talked into eliminating the 5V source.

You certainly will have power to your avr chip??   If not take out a patent :)  Either the 5V or 3.3V is fine. 5V is generally better for signal to noise ratios in analog, meaning the smaller the signals, the bigger noisy neighbors look.

 

Was advised to raise the base voltage range on the continuity test by adding a third resistor to the igniter path.   ​​​​​​​

 That's not needed and adds more error than anything else.  You are measuring about 50mv before any amp & all currents are low then ...the adc will be able to see that very well compared to gnd (make sure gnds are tight). You just need a steady stable measurement.  The AVR will decide what value (reading) is too much/too little, etc.  If you need vastly different ranges (prob not), then you can adjust the current, probably under software to select either a "hi" or "low" setting.

You could use 2 of the regulators I mentioned...they could be individually be enabled (for hi or lo)  combine their current output through a schottky diode (might not be needed if regulator prevents backflow)

 

Note the regulator I mentioned is MIC5377  ...you can also get dual regulators (the symbol below is some random one I had)

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Thu. May 13, 2021 - 01:57 AM