LiPo Discharge Current

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I have never worked with LiPo batteries before, so this may be a typical beginnner battery question...

 

I am working on another "weekend project" to build a 4 x LED flashing strobe light to mount on a small quad-copter drone.

The light is to make it easier to see the drone at a distance, and to make its location visible at night.

(Beyound visual sight and night flying are both done with an FAA waiver.)

 

The LiPo battery I was thinking of using is a TinyCircuits product, available on DigiKey.

The "Data Sheet" says it is a Lithium Polymer cell, 3.7 V (Nominal), 500 mAh capacity.

There is no "C" rating provided.

It is about 1 inch x 1 inch x 0.2 inches in size.

 

The Question:

How much current will a little LiPo cell source?

Will a "small", single cell LiPo provide enough current to power the LED Driver Chip and hence the 4 LEDs simultaneously? 

 

My initial plan is to use a Tiny13 to drive an LED Driver chip to flash the 4 LEDs, on 100 mSec / Second, 1 Hz flash rate.

The LED Driver will drive the 4 LEDs in series, so they will all flash together.

The LEDs are White, 3W, 700 mA LEDs, that I am planning to drive at 150 mA, (derated).

(Thank you East Coast Jim for the LEDs!)

 

So although there is only one, 150 mA load, the LED Driver chip will have to boost the Vin, (LiPo ~ 3.7 V), up to ~ 12 V

to drive the string of LEDs.

 

I have the Tiny13's and the LEDs.

I haven't yet purchased the LED driver chip, the LiPo charger chip, or the LiPo battery itself.

This  project is still in its early planning phase!

PCB quantity:  Several, but for my use, not a commercial product.

 

I was hoping that those with some experience with small LiPos would say:

Not a problem, it ought to work fine

or

Are you crazy?  Think of it more like a coin cell than a car battery!

 

Suggestions before I build a peperweight are always appreciated!

 

Thanks,

 

JC

 

The Concept:

 

 

Edit: Image size

 

Last Edited: Tue. Nov 24, 2020 - 04:56 PM
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DocJC wrote:
How much current will a little LiPo cell source?
500mA max

Rechargeable lithium polymer battery catalog from PowerStream, lithium polymer batteries and cells in stock with specifications

[after mid-page]

Part 2: Cells used in Mobile Phone Polymer Li-ion Battery Packs

 

P.S.

DocJC wrote:
(Beyound visual sight and night flying are both done with an FAA waiver.)
Thank you.

Ten years ago, had a conversation with an airline captain about his thoughts on drones.

 

P.P.S.

re LTC4071, Microchip has Li-ion chargers for small cells though not shunt (iow pass)

 

"Dare to be naïve." - Buckminster Fuller

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re tiny13, tiny13A doesn't have a UART whereas tiny102 and tiny104 do (bootloader)

tiny202 adds one-wire to UART.

A goal could be in-field updates via a USB UART dongle on the notebook PC.

 

ATtiny13A - 8-bit AVR Microcontrollers

AN_42736 AT12498: ATtiny104 Low Footprint Bootloader

ATTINY202 - 8-bit AVR Microcontrollers

 

"Dare to be naïve." - Buckminster Fuller

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You may want to go with a 2C or better battery. Assuming the 500ma/hr one you have is a 1C, it is designed to put out 500ma max current. A 2C will give you 1000ma without harming the battery. Just search "LiPo C ratings" for more info.  

"If you find yourself in an even battle, you didn't plan very well."
https://www.gameactive.org
https://github.com/CmdrZin

Last Edited: Tue. Nov 24, 2020 - 06:42 PM
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The LED Driver will drive the 4 LEDs in series, so they will all flash together.

Can you turn on the LEDS one at a time?  That would limit the peak current by 4x, which can be extremely helpful (though a big cap can help).

Limiting the peak current, also means the power losses could be about 16x lower, in some sections that would see 1/4 the peak current (wiring, transistors, etc).

 

It may be that  the highest efficiency is not needed (nor getting close to the lowest battery droop voltage), so this thought might be overly complicated.

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

Last Edited: Tue. Nov 24, 2020 - 07:03 PM
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Thank you, everyone!

 

That 500 mA current limit was the number I was looking for, thank you!

With the LED Driver putting out 12 V x 150 mA = 1.8 W.

If one assumes 80% efficiency, (hopefully a little bit better), then the power input to the LED driver = 2.25 W.

 

So a 3.7 V LiPo (1xcell), would have to run at about 608 mA during the time the LEDs are on.

 

So that, unfortunately, is a bit over the 500 mA number, albeit used for a 10% duty cycle per Second.

The LED Driver I was looking at is single channel, so the LEDs have to be stacked.

I could easily use two of the chips, but I was hoping to keep the PCB space low, (it’s a small drone).

I thought about rolling my own constant current drivers with an NFet, low side driver, and a sense resistor, and either a small signal transistor or an op-amp for the Gate driver, but that starts to be a lot of little parts to put on the board, and the NFet is run in the linear mode, so I‘d have to do some thermal calcs on it.

Heat sinks obviously add space and weight, weight is the enemy on this project.

Then there is the issue that I’ve never been good at thermal calcs…, and I’d rather my learning curve be limited to LiPos and driving high current LEDs.  

 

I selected the battery above in large part due to its size, and from my initial reading of the data sheets, it looked like charging a single cell from a USB source was pretty easy.  I’ll have to look at it again, as perhaps a 2 cell charger is also easy, once I find the right chip.

I’ll look at the MC parts, the reason I selected the shunt charger was that I don’t care about rapid charging, and it was a trivial solution.  Some of the chargers required extensive support parts, which will take up space, especially since I like 1206 parts for my hand & frying pan soldering.

 

I’ll have a look at the other suggested Tinies. 

I envision the code as being pretty trivial, also, (famous last words), as I am not anticipating adding mission critical, failure detection, failure correction / mitigation, etc. to the project.  It is truly envisioned as a simple, small, LED flasher, (although flashing LEDs at 150 mA, not 20 mA, and incorporating a Lithium battery recharger, (What could possibly go wrong? <smirk>).

 

In any event, I hope to write the code, burn it, and be done with it!  As the device is portable, and I usually put an ISP header on the PCB, it wouldn’t be difficult to update the software at my work station, and I don’t envision changing any code in the middle of the device being used.  I do appreciate the reminder to think about a Bootloader, however.

 

LiPo C ratings is now added to my search vocabulary, thank you.  I’ll have a look for some small footprint 2 C battery packs, and see what their capacity and weight are.  That does, of course, mean that I’d now how to squeeze a small voltage regulator on the PCB, also…  The world is full of trade-offs!

 

I’ll just tell my wife I need a bigger drone, I’ve already outgrown this dinky one!

 

Thanks, everyone!

 

JC

 

 

 

 

 

 

 

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Seems like a lot of trouble when a pair of AA alkaline batteries would give you 2.5ahr capacity, simple in field replacement, no charging or charging circuits, no chance of sudden flame outs!

K.I.S.S

Jim

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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There's some websites on making led torches that get anal about the finer points of everything. I somehow think it swings to more emotional than technical at times. Seems everyone has their favourite battery, led, driver etc. Anyways, some Googling should unearth these sites and hopefully there might be some gems of information - especially with constant current drivers. I expect there are a number of chips out there - in a mains powered smart led I opened recently, there were some Chinese chips that did just about everything - just add resistor and inductor. These probably cost cents.

 

As for max current out of the battery - that can depend on a few factors. Mainly heat rise - pull more current and there's more loss in the wiring and the battery impedance, so it gets hotter the more current you pull. Some batteries may have a circuit that limits the max current - the LiFePo4 battery I have limits it to 5A continuous. I'd think exceeding the constant rating by 20% at a 10% duty cycle is most likely ok.

 

If your battery doesn't have a current limit circuit, then be aware that it can source many amps into a short circuit. Fusing is required.

Last Edited: Tue. Nov 24, 2020 - 09:25 PM
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If one assumes 80% efficiency, (hopefully a little bit better), then the power input to the LED driver = 2.25 W.

Not sure if you mentioned a short flash/blink...if the led just does a "photoflash", then you could add a moderate cap on the input side that spits out the high current (into the booster) during the light pulse.

 

roughly:  Cin  =  Iinput * PULSEduration/DeltaV  (delta is the allowed Vdrop going into the booster).   If the pulse is very long, it won't be so good--you'd need a huge cap, or allow a huge Vdroop going into the Vboost

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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All good points, and I appreciate the input.

 

The drone system is full of rechargeable batteries, (The drone itself, the controller, and the tablet/display), so one needs a process for recharging everything post usage.  Adding one more device to plug in seemed easier that buy batteries forever...  Plus, its about time I learned a little bit more about LiPo's.  The kicker is that if one wants a secondary mode with a forward looking "headlight" that is always on, then the power consumption really goes up, (no longer a 10% duty cycle), so going rechargeable seems to make sense.

 

There are certainly many Lithium based battery charger chips, and LED driver chips, available.  I've never delved into switching power supplies, and it seems that both of these are descendants of those, and they come in all of the various topologies.  It is taking me a while to gain even a basic understanding of the field, but that's what learning curves are all about.

 

I have done a bit of reading, mostly data sheets, but I'll have a look at some of the projects that are available to see what hurdles those designers encountered and overcame.

 

Your mention of a Fuse is a good one.  Truly I wanted this to be small, really small, and light weight.  My original plan was to use a polarized connector on the LiPo battery and call it a day.  No other reverse polarity protection, or overcurrent protection, (poor decision, especially since we're talking about Lipos!).  You are guilting me into doing the right thing.  Sigh.  Reminds me of the other current Thread discussing grams of potatoes, only I'm counting grams of components!

 

I still remember banks of 2200 uF or 4700 uF caps on 7805 based power supplies...  I'm not sure what duration of "flash" is adequate for spotting the aircraft against a light blue sky.  The contrast differential is what helps one's eye to spot the drone (led) while scanning the sky.  During the day I suspect, (totally unproven), that a decent duration flash will be helpful.  Once the project is up and running, flying, it will be easy to test 10, 20, 50, & 100 mSec pulses. 

 

An alternative that I have seen is a single, bright, LED on the top of the drone.  But then one ideally has some sort of diffuser to take some of the vertically directed light and aim it outwards, sideways, so that a ground based observer can see it.  

 

Plan B, using a zenon flash tube, is beginning to look a whole lot better!

 

JC  

 

Edit: Typo

 

   

 

 

Last Edited: Wed. Nov 25, 2020 - 02:49 AM
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Plan B, using a zenon flash tube, is beginning to look a whole lot better!

Sure seems like it could be a contender, if you set it up for a "weak" flash.  A high power flash can spit out a big blast of juice--for some project I was tasked with making flashlamps to theoretically optically polish glass...the result was glass sputtered with tungstun (hardly a good outcome).   xenon  (we used a tank of Argon, if I recall).

 

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

Last Edited: Wed. Nov 25, 2020 - 03:21 AM
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gchapman wrote:
Ten years ago, had a conversation with an airline captain about his thoughts on drones.

 

Yes. If you really want an earful, ask a paraglider pilot: we hate them. Too small to see, and a bad habit of their users (a) not being aware of local flying rules, particularly as to where they're forbidden to fly and (b) liking to follow paragliders landing or taking off for those really good shot. Carbon fibre blades spinning at speed and aramid lines in the 0.4-1.2mm thickness range are not a good combination... /rant

 

DocJC wrote:
I'm not sure what duration of "flash" is adequate for spotting the aircraft against a light blue sky.  The contrast differential is what helps one's eye to spot the drone (led) while scanning the sky.  During the day I suspect, (totally unproven), that a decent duration flash will be helpful.  Once the project is up and running, flying, it will be easy to test 10, 20, 50, & 100 mSec pulses. 

 

A flash which is too short provides information that something is there, but provides insufficient time for the eye to locate/track it. I don't know what the practical limits are, but particularly with something small, fast, and moving quickly I would want something with a longer flash than a shorter. The duration of a strobe light on a commercial aircraft might be a good guide?

 

edited to explain: Too short a flash tells you it's there. A longer flash tells you where it's going and how fast it's going there.

Neil

Last Edited: Wed. Nov 25, 2020 - 06:23 AM
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Doc,

 

to truly know if your system will be good enough you will need to know more about the battery.

1 maximum charge rate allowed for both quick charge and normal charge

2 maximum discharge rate

3 capacity under load

 

1 is to see how fast you may charge them. I guess the drone pack is a quick charge so Over 1C so charging is done within an hour or so.

If you charge at only 50mA( which I got from the earlier posts) you will be looking at minimum 11h charging time for the lights when the battery is done. You would prefer a C/2 charging current as that would mean a 2,5hour charging cycle.

All depending on the need you then can decide if you do quick charge or that you go with a C/5 charging current as that most of the times si best for the cell.

 

2 there might be a protective circuit that cuts off power to the system when the battery is drained to quick. And a to big load will degenerate the cell fast so the power will reduce quickly.

 

3 this is more or less related to 2 the 500mAh spec might be at C/10. I did not see that at first glance in the data. but it might be that at discharge rates getting close to C the max available power might drop.

 

Also keep in mind that 150mA@12V   will be 450mA@4V and 600mA@3V.

You also never want to get the cell voltage to the cutoff voltage as that is bad for the cell. I did read that there is internal protection to handle that, but you normally want to avoid getting that low.

Make a protection measure with a hard switch to ensure the cell is not drained 100%

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DocJC wrote:

The "Data Sheet" says it is a Lithium Polymer cell, 3.7 V (Nominal), 500 mAh capacity.

There is no "C" rating provided.

It used to be the case that the maximum you could source from LiPo was about 10C so a 500mAh battery would be able to source 5000mA (5A). But these days most LiPo are good for at least 20C (10A in this case) and the best 40C (20A).

 

The extraordinarily conservative 500mA (1C) that gchapman describes is not something I recognise. No one would be flying drones, planes, helis if the available energy density were only 1C.

 

(of course the downside of 20C/40C/etc is duration. At 1C a 500mAh battery will source 500mA for an hour (that's what it says on the tin) but if you pull 10C you get 5A but only for 6 minutes, 20C lasts 3 minutes and 40C just 1.5 minutes. But with that much energy it can be a pretty wild ride at 20C/40C! What actually happens is periods of sedate motion when you could be conserving energy at 1C then bursts of real excitement when you pull much more)

 

For lighting I'm guessing you'll operate at some small fraction of 1C and the capacity will last for hours but you can effectively consider the sourcing ability almost limitless. But how those lights would burn bright if you dumped the entire battery load in 90 seconds!!

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meslomp wrote:
2 there might be a protective circuit that cuts off power to the system when the battery is drained to quick. And a to big load will degenerate the cell fast so the power will reduce quickly.
LTC4071 has a low cell voltage cut-off.

LTC4070 with two external PFET to increase the current by an order of magnitude; a protected FET has current limiting (load switch)

 

LTC4071 Li-Ion/Polymer Shunt Battery Charger System with Low Battery Disconnect | Analog Devices

LTC4070 Li-Ion/Polymer Shunt Battery Charger System | Analog Devices (Figure 9)

 

"Dare to be naïve." - Buckminster Fuller

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For this use case, concur for pulsed current limited by the cell's power dissipation; large cells, low internal resistance, large surface area.

Aids :

  • air flow
  • sun shade
  • convection

 

 

"Dare to be naïve." - Buckminster Fuller

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DocJC wrote:
I’ll just tell my wife I need a bigger drone, I’ve already outgrown this dinky one!

 

That was me, they kept getting a little bigger and better.   Finally ended up with a DJI Mavic Pro.  You could just about make a theater quality movie with the video camera on there.  (I liked it so much I bought the Osmo Pocket too, snazzy).  There's a video of one flying up to about 8500 feet.  I get nervous at about 400.

 

Has a bunch of clearance lights, but at a little distance, it's still hard to pick out.  I was making a time lapse video of some traffic over a bridge in FL. at night, and I lost it (just sight of it) among the other lights out there.

 

If you like the idea of a vertical/horizontal diffuser, look at some of those cheap solar powered lights for yards.  I know some of them have a ..... I guess you'd call it something like a chrome plated cone in there, that does just that.  They use one LED, and the point of the cone is centered over the LED, and that directs the light out in a 360 circle.

 

Might be able to simplify things by using one really bright LED instead of 4.    And some of those lights are like a buck each.

 

 

Just gettin' started, again....

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Xenon vs Argon

I think classic “flash tubes”, such as those used in classic cameras, and emergency vehicles, use / used xenon, and have a wide spectrum of emission, and therefore appear white.

Argon, IIRC, glows blueish when electrified.

The old Neon bulbs, I think, had neon gas with a little bit of argon mixed in…

I guess the possibilities are endless, which is why bar / pub / tavern lights come in so many colors!

 

I promise not to harass any paraglider pilots! 

I bet you don’t like lasers pointed at you either!  (Sorry, just being a smart a…)

 

The goal is really to be able to spot the drone in the distance, when it tends to fade into the sky and is hard to be seen.  I can tell from the panel which direction it is pointing, and in which direction it is moving, (which aren’t the same, because like a copter is can go in any direction…), so I don’t need to gain that info visually from the lights.  I’m hoping just a bright, brief, 1 Hz flash, will suffice.

 

I found it a little bit odd that the TinyCircuits LiPo didn’t tell the battery’s “C” rating, as after doing some reading that seems to be a pretty important spec.  I could go with a different battery, with a “real” data sheet, but I don’t think I will need that level of documentation and detailed info for this project. (Hopefully).

 

I presume the 500 mA “max” current sourcing would be for “in spec” designs, where one is concerned about maximizing the battery’s life, recharging cycles, and safety from physical expansion and fire, etc.  I am not surprised that the drone / RC model industry pushes the batteries to the limit, as flight performance is probably a higher priority that battery integrity for that user group.

It is still, from a design perspective, nice to know how many mAs, (or Amps!), one can pull from the battery without its internal resistance limiting the current and dropping the supplied voltage “too much”, (another poorly defined term!).

 

The tentatively selected battery does have its own internal safety monitoring circuit, (primarily to prevent over discharge), and the tentatively selected LiPo charger chip also has circuitry to prevent overcharging.  I looked though 20-30 Analog Devices LiPo charger chips and the nice thing about the one selected is that it needs a by-pass cap and a resistor, that’s it!  K.I.S. to the max!  One can add an NTC to throttle back the charge current if the battery gets too warm, (2 more parts). 

I don’t think a long charging time will be an issue, as my flight time is ½ hr / drone battery, and I only have two batteries at the moment.  The others that I fly with also have pretty short flight times.  As the entire project is only a couple of dollars / board, I can have several of them, if needed, and Velcro them to the airframe.

 

Some of the charger chips have many options and features, but can require 20+ external components, and that isn’t what this project needs.

 

I can’t imagine flying my drone at 8500 feet above ground level!  The USA - FAA rules are a maximum altitude of 400 feet AGL.  If one is inspecting a building or a tower then one can fly up to 400 feet above that object, within a 400 foot (IIRC) radius of the building / tower.  I don’t think my little drone has the battery capacity to fly up to 8500 feet, and back down, before the battery would be drained! 

 

It is already a little bit nerve racking deciding how soon one has to start flying back to the starting point without running out of power before making it. 

 

I appreciate everyone’s input and guidance!

 

JC

 

 

 

 

 

 

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Can you remote control the LEDS/flasher, so they don't have to run all the time?  Run them only when you need them, or want to verify position?

Maybe drop some magnesium?

 

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:
Maybe drop some magnesium?

Yea that would work, but I'm not sure the forestry service would like that! angel

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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A magnesium flare would certainly increase its visibility!

 

Likely so would a Lithium Cell fire!

 

In this case the larger drones can toggle a device on and off, this small one of mine doesn't have that capability built-in.

 

Of course, if we simple add an ESP8266 for WiFi to the LED flasher, and add an Adroid App to talk to it, as a simple new feature request, we can easily add remote On/Off capability!

 

JC

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Polymer Li-Ion Cell: 3.7V 400 mAh (PL-402248-2C) --- UN38.3 Passed (NDGR) (Batteryspace)

...

Discharging Rate
0.8 A Max.  (2.0 C rate)

...

The 500mAh cell is currently out-of-stock :

Polymer Li-Ion Cell: 3.7V 530mAh (1.96Wh, 503035-2C)

 

"Dare to be naïve." - Buckminster Fuller

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I'd not seen that battery site, thank you for the link!

 

That 530 mHa battery looks great.

Heck, I could use the 400 mHa cell, also.

It is nice to see a more complete data sheet than the battery I was lloking at has.

 

Thank you!

 

JC

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DocJC wrote:
The goal is really to be able to spot the drone in the distance, when it tends to fade into the sky and is hard to be seen.  I can tell from the panel which direction it is pointing, and in which direction it is moving, (which aren’t the same, because like a copter is can go in any direction…), so I don’t need to gain that info visually from the lights.  I’m hoping just a bright, brief, 1 Hz flash, will suffice.

 

Just remember, JC, that you're not the only user of the air. You know where it's going, but other pilots don't... so a short unlocalisable flash may be a distraction for another pilot, adding to his workload.

 

Have you asked your helicopter drivers what they think of this? They've probably got better input on this than I have.

 

Neil

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Is there really that much air traffic in the dark? (Which is presumably why the lights are needed in the first place).

 

In the past I've bought kits that do the commercial airliner "flashing strobes" thing to make a model flyable at light. Solid red/green sure helps the orientation thing!

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you're not the only user of the air.

 

Well understood! 

Normal recreational drone pilots, and "Part 107" commercial drone pilots, are not permitted to fly at night.

When I fly for fun it is during the day.

 

When flying at night the drone Pilot In Command is required to have an FAA waiver, and the drone is required to have the strobe/beacon.

Even when flying at night the aircraft's altitude is maintained < 400 AGL.

Normal aircraft should be well above that.

 

When we are flying in the copter at night there are so many lights around it truly won't be noticed.

The copter is at several thousand feet AGL unless we are on take off or final approach.

My copter pilots obviously don't like drones around the airport or their landing zone, but otherwise its "not a factor", and they move on with their scan or other tasks.

 

BTW, (off topic), when I fly the drone at night I'm working, not doing it for fun.

 

Many drones have small LEDs on them to help make them visible, and to help with discerning the orientation of the drone.

Many of them have a small "beacon" LED to help make them visible, even  during the day time.

Especially the small ones are easy to lose track of when one looks down at the controller and then looks back up at the sky.

 

The controller for one of the larger drones actually includes an ADSB (transponder) receiver, so the controller software pops up a box telling you there is a C-172 X miles SW @ Y altitude.

Very cool what one can accomplish with current technology.

That would not have been fesible even 10 years ago!

 

All that said, Yes, I hear your concerns!

My goal is to fly safely, whether I'm in the aircraft or on the ground!

 

JC 

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Just an update:

 

I'd post a YouTube video, but its not worth the effort for a couple of flashing LEDs!

 

Its labeled a Prototype because the holes for the Mode PB Switch are too small... (Darn).

 

I ended up using a Tiny402 instead of the Tiny13.

It has more memory and the simpler UPDI interface.

 

I added a PPTC "Fuse" in series with the LiPo battery.

 

I went with two LED driver chips, (for 4 LEDs), so that I could decrease the burst energy draw when the LEDs are on.

The micro controls the flashing, and the four LEDs are never on simultaneously.

 

A special shout out to East Coast Jim who sent me the 3 W LEDs.

 

I'll keep one board for my Drone, and give several to one of the Special Response Teams I work for.

 

In small quantities, the board costs about $50 in parts, which was higher than I expected!

The energy usage from the LiPo, however, and the overall performance, are both much better than that obtained with 4 resistors for current limiting!

 

The LEDs are mounted on headers, and for short flashes do not require an additional heat sink.

The LiPo included its own protection circuitry, and it also has an NTC thermistor taped to it to also let the LiPo charger chip keep tabs on the battery's temperature. 

In use, the LiPo is taped to the back of the PCB, and the board is Velcro'd to the Drone.

 

Thanks, everyone, for your guidance!

 

JC

 

 

 

 

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Very nicely done Jay. But of course we would expect nothing less of you.

Ross McKenzie ValuSoft Melbourne Australia

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Well done!

DocJC wrote:
... and give several to one of the Special Response Teams I work for.
Sometimes a prototype can lead to some volume from a customer (follow-on work)

DocJC wrote:
In small quantities, the board costs about $50 in parts, which was higher than I expected!
In the BOM, is the PCB relatively expensive?

Reason : US PCB fabs can compete

DocJC wrote:
The energy usage from the LiPo, ...
What's the LiPo's capacity?

For all, meet the mass requirement?

 


PCBShopper – A Price Comparison Site for Printed Circuit Boards

 

"Dare to be naïve." - Buckminster Fuller

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Thank you, the kind words are certainly appreciated!

 

can lead to some volume 

Truely I am better at design and prototyoing than at production of more than a handful of anything!

Needless to say I learned a lot on this project, and have a few improvements in mind if I elect to built another, Ver II, model.

Production would best be farmed out to another Freak!

 

I haven't built any PCBs this past calendar year, what with Covid keeping me pretty busy.

Perhaps I was just rusty, or perhaps I'm just getting older, but it took 2+ hrs to paste (no stencil), place, and frying pan solder the board!

Clearly I need a Pick-and-Place machine for Christmas if there should ever be any interest in a production run!

 

The PCB's were very inexpensive, and the key non-micro parts were small SMDs, hence I went directly from a paper design to the PCB.

I usually like to breadboard prototypes, but that didn't seem feasible for this project.

 

I used PCBWay.  The boards are 40 x 50 mm. It was 10 PCBs for $4 USD, plus DHL Shipping & Handling ($18 USD), and PayPal ($2 service charge), for a total of $24 USD / 10 boards.

That is 2-sided, through vias, top silk screen, color of your choice. etc.

If I could convince my wife to move to CHina I wouldn't have to pay DHL shipping and handling charges!

 

Weight is a key parameter for this project!

I ended up using a 500 mAh battery that is about 2 cm x 3 cm x 0.75 cm in size, including its own little LiPo safety circuit board, (that prevents over charging and over discharging the cell).

There were several sites that had 500 mAh cells listed, but they were out of stock.

Mouser had the TinyCircuits battery in stock.  Decision made.

I could easily go with a smaller battery, but I still have limited experience in how much current the smaller cells can source for the LED flashes.

 

The 3W LEDs can handle a continuous current of 700 mA, but that is with a big heat sink.

Needless to say, heat sinks are heavy, hence I derated the current to 150 mA, which is still incredibly bright.

Coupled with a low duty cycle on time and no heat sink is required, and the battery life per charge is markedly extended.

 

My test software is flashing the micro's small LED for 50 mSec, then the Driver #1 pair of LEDs for 50 mSec, pause 50 mSec, and then the Driver #2 pair for 50 mSec, wait (cool down) for 300 mSec, and then repeat.

That gives two flashes / Sec.

Ignoring the little LED on the PCB thats a 200 mSec / Sec on time, 20% duty cycle.

 

On its first charge it has been flashing for 6 hours so far, and the LiPo voltage has dropped from about 4.2 V to 3.8V.

 

My drone is a Mavic Mini, which weighs 248 grams on my scale, (0.546 lbs).

A Small UAV that weights < 0.55 lbs doesn't have to be registered, (in the USA).

The Team's drones are larger, and registered.  

The added weight for them is essentially immaterial regarding flight time.

 

If I want to fly it at night, or beyound visual sight, (both under an FAA waiver), it does have to be registered.

 

The PCB and battery weight ~ 28 grams.  

So putting it on the drone automatically puts the drone over the minimum weight limit registration exemption and also makes registration required just to add it to my drone.

(248 + 28 = 276 grams, = 0.61 lbs, which is over the 0.55 lbs limit)

 

It was quite a fun little "weekend" (1 month!) project!

 

JC

 

Edit: Typo

Last Edited: Thu. Dec 17, 2020 - 03:24 AM
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DocJC wrote:
... or perhaps I'm just getting older, ...
Not age, health.

DocJC wrote:
Clearly I need a Pick-and-Place machine for Christmas if there should ever be any interest in a production run!
Lots of PCB fabs have attached PCBA manufacturing even for small quantity (early prototype, PCB placed, parts literally poured from a bag onto a tray, automated PnP) (IIRC, distributors can kit into a bag)

DocJC wrote:
I usually like to breadboard prototypes, but that didn't seem feasible for this project.
Is feasible other than the LiPo charger IC (smallest pitch of all?)

Magnifying glass ... first prototype will be larger than second (PCB) though might still fit (maybe stack substrates)

DocJC wrote:
Needless to say, heat sinks are heavy, ...
by sheet metal may be reasonable especially if trial between heat sink, cell's capacity, dawn/dusk capability (ambient light)

DocJC wrote:
On its first charge it has been flashing for 6 hours so far, ...
a locator feature (landed onto thick brush)

DocJC wrote:
My drone is a Mavic Mini, which weighs 248 grams on my scale, (0.546 lbs).
The structure for some is fiberglass resin (FR PCB)

 


MacroFab | Redefining Electronics Manufacturing & AssemblyUsing AREF as another Digital Input | AVR Freaks

Rapid PCB - Home

Fine Circuits | Printed Circuit Board (PCB) fabrication Bartlett, IL

Quick Prototyping Solutions | Circuit Cellar

PCB, Printed Circuit Board Manufacturer & Assembly | RushPCB USA via How to Handle Small Components | Electronic Design

Introducing MOUSER as supported distributor for Precious Parts | AISLER

 

Surface Mount Prototyping PCBs | BusBoard Prototype Systems

ELM - Wiring Techniques

 

Designing Raspberry Pi 400 - Raspberry Pi

by Simon Martin [Senior Principal Engineer]

3rd Nov 2020

...

Raspberry Pi 400 contains a heat spreader that dissipates the heat across the whole unit, front and back, so that no part of it will feel too hot to touch. 

...

[picture]

An early unit going through thermal analysis

 

🐉 | Drones • Tech • Robots | 🤖 (@dragon_drones) • Instagram photos and videos

 

"Dare to be naïve." - Buckminster Fuller

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Nice build!!

 

Not sure what booster chip you use.  Could you save some $$$ using one booster chip/inductor, etc  & just fet grounding the leds you want to light?  Sharing is good.

 

I've  got free shipping from JCLPCB a couple of times, not sure exactly how....that made my handful of boards from them really cheap.  And they arrived within 10 days or so to boot.

 

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

Last Edited: Fri. Dec 18, 2020 - 12:44 AM
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What a great idea.

 

The LED driver, an LT1937, drives the LEDs and then has a current sense resistor, 0.63 ohms for 150 mA, to Ground.

I could use a low side NFet and adjust the resistor for the small Vdrop across the NFet.

That would infact get rid of one of the physically large inductors, a small chip, and a few small parts.

I will keep that thought in mind for Ver II !

 

JC

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I could use a low side NFet and adjust the resistor for the small Vdrop across the NFet.

No need...much better to put your two fets above the common sense resistor...the resistor only develops 95mV typ, so it only lowers your Vgs gate turn on a miniscule amount...the fet won't effect the current sensing.  You need some care if both fets are turned off...since of course it will try to create super-high voltage to achieve the sense current (so you can use the SHDN to stop it).  You can also turn on both (A AB B) during a transition.   They also have you covered in this manner:

 

Open-Circuit Protection In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback voltage will be zero. The LT1937 will then switch at a high duty cycle resulting in a high output voltage, which may cause the SW pin voltage to exceed its maximum 36V rating. A zener diode can be used at the output to limit the voltage on the SW pin (Figure 5).  

 

 

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