Idiot Freak now has to fix a 555 circuit

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Ok, Larry talked me into designing him a 555 "coupler trigger." He doesn't want anything surface mount or programmable. I should have told him to find an engineer 40 years older.

 

Now, of course he has a problem. The thing gives him an output pulse when he removes the power. Since this is on a track powered train, he's worried it will trigger whenever he hits a dirty spot on the track.

 

So how do you keep a 555 from triggering when you power it down?

 

Don't you hate being held responsible for the outcome of a decision you opposed?

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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And now he won't get off the phone till I tell him how to fix it.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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~William

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A P-channel Jfet will work just as good or even better if it can take 18vdc on the base. 

You place the P-channel Jfet just the same.

No current will flow with a high (digital 1) voltage on the base, when the voltage is low on the base the transistor closes to ground.  

~William

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Actually that wont work, as the 555 still has power to trigger befor the powers shut off. What about a 2200uf cap as a battery? So when it loses connection do to a 'dirty track its got some reserve current for a spit second. 

~William

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He's trying a 3300uF cap.

 

The trigger goes to a "revolution" receiver which is powered by a few thousand microfarads and keeps working through seconds of power outage.

 

(I don't even run my trains on track power.)

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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I was thinking a pulldown on the n chan gate. I seem to recall the output pin will hump out 200ma or so, so select the R for 150ma?

Imagecraft compiler user

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Have to further define the problem, first.

 

It looks like the 555 is driving a Fet which is driving a relay, and the relay is powered off the 18V rail.

 

So, is the problem the 555 retriggering on power down, or is the problem that when one intermittently loses power on the 18V rail the relay drops out?

 

With a simple diode to cap on the V+ rail a low power AVR can run for a long time.

And it can filter the input trigger signal.

And, depending upon the relay being used, and the duration of the black-outs it might drive the relay directly.

And...

And...

And...

 

JC

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The coil is a solenoid in a coupler. The symptom is it uncouples when he turns the power supply off. His worry is the coupler will uncouple when it runs across a dirty spot on the track.

 

MY thought of how to design this is a Tiny 10, a pulldown resistor on the output fet, and use the BOD to reset the processor when the voltage falls, but he wants something he can build himself and doesn't want to mess with programming it. The 555 is his idea.

Goodness, I used to play with 555s when I was a sophomore in HS. They were new then.

 

Of course, my design that he's been using for years is no timer but a 1000uF capacitor connected to + through a 1k resistor. When the input goes low, a PFet with a pullup resistor on the gate turns on and dumps the capacitor through the coil, but he says it's not strong enough to uncouple with tension on the coupler. Seems you don't want it to uncouple with tension on the coupler.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

Last Edited: Wed. Nov 5, 2014 - 10:44 PM
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You want the output to be disconnected as soon as the power interruption is detected, before the 555 triggers due to dropping power.

 

Add a diode before the 7805's input cap:

 

                              +------------+
18V    |\|                    |            |            5V
-------| |--------+-----------|   7805     |-------+-------
       |/|        |           |            |       |
                -----         +------------+     -----
                -----                |           -----
GND               |                  |             |    GND
------------------+------------------+-------------+-------

 

Use a small relay (or a FET as described above) to cut out the 555's output, but drive the coil (or gate) from the 18V supply taken before the diode.  As soon as it disappears, the relay (or FET) will cut out.  The 555 will continue to run for a while from the caps.

 

Small risk that if power returns just as the 555 falsely triggers.

 

Diode needs to handle solenoid's current.  Solenoid needs to activate with one diode voltage drop below 18 volts.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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I think a 555 is over-engineeering for this. I'd use a SAM4S16C Cortex M4.

 

Four legs good, two legs bad, three legs stable.

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John_A_Brown wrote:

I think a 555 is over-engineeering for this. I'd use a SAM4S16C Cortex M4.

 

 

My sentiments exactly!

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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I think a 555 is over-engineeering for this. I'd use a SAM4S16C Cortex M4.

smiley

 

JC 

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Tom,

 

I do not think there is a problem with the 55 at all.

It looks like the problem is loosing power.

 

If I recap correctly:

normally the 18V is applied. This keeps the solenoid activated. When a trigger signal is given the solenoid is briefly de-activated and the corresponding carriages should be released.

The problem is that effectively you have 2 trigger pulses, 1 is the 'trigger' line and the second is loosing power.

 

I do not know the hardware, but it seems that the circuit effectively works the wrong way around.

If no power is applied the solenoid should be in such state that it holds the stuff together, Also when power is applied and no trigger is given this should be the case.

In that case a power loss will not result in loosing the grip from the solenoid.

 

The trigger pulse should then cause the solenoid to switch in the 'release' mode.

 

Is it possible to change the hardware to do that?

or did I miss anything in my reading ( I just had my 6th work interruption in reading all replies, so that might be just the case)

 

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Yes, that is the complaint: The coil energizes on power loss.

 

Larry wants me to "perfect and market" this circuit. HA! If I were going to market it, it would be about a centimeter square and contain the regulator, mosfet and a Tiny 10.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

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meslomp wrote:
:normally the 18V is applied. This keeps the solenoid activated. When a trigger signal is given the solenoid is briefly de-activated and the corresponding carriages should be released.
Um, no.  The FET is an N-channel.  When the 555 is untriggered, it's output is low, the FET is off, and the solenoid is not energised.  When the trigger signal is given, the 555 output goes high briefly, the FET turns on, and the solenoid energises.

 

Hang on a second...

 

I'm no expert on the 555, but I don't quite recognise your schematic.  At first glance it looked like it was configured for monostable, but why do you have trigger tied to GND?

 

Shouldn't the circuit look more like this?:

 

EDIT: Another look at your schematic makes me wonder if you don't have them connected.  Is it just that the schematic was hastily drawn i.e. there's no node here?:

 

Quote:
If no power is applied the solenoid should be in such state that it holds the stuff together, Also when power is applied and no trigger is given this should be the case.

In that case a power loss will not result in loosing the grip from the solenoid.

I'm no expert in model trains, either.  But I expect that an energised solenoid decouples the coupler.

 

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Fri. Nov 7, 2014 - 03:03 PM
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What if you put a diode between out and vcc, with anode on out.  That way the output could never go higher than vcc.  Try it at your own risk!

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No, the trigger is not grounded. If it was, it wouldn't work with the power on.

 

I just laid out a board 11mm by 11mm (couldn't quite get it down to 10mm) with a fet, regulator, capacitor and tiny10. 

 

joeymorin wrote:

 

meslomp wrote:

:normally the 18V is applied. This keeps the solenoid activated. When a trigger signal is given the solenoid is briefly de-activated and the corresponding carriages should be released.

Um, no.  The FET is an N-channel.  When the 555 is untriggered, it's output is low, the FET is off, and the solenoid is not energised.  When the trigger signal is given, the 555 output goes high briefly, the FET turns on, and the solenoid energises.

 

 

Hang on a second...

 

I'm no expert on the 555, but I don't quite recognise your schematic.  At first glance it looked like it was configured for monostable, but why do you have trigger tied to GND?

 

Shouldn't the circuit look more like this?:

 

EDIT: Another look at your schematic makes me wonder if you don't have them connected.  Is it just that the schematic was hastily drawn i.e. there's no node here?:

 

 

Quote:

If no power is applied the solenoid should be in such state that it holds the stuff together, Also when power is applied and no trigger is given this should be the case.

 

In that case a power loss will not result in loosing the grip from the solenoid.

I'm no expert in model trains, either.  But I expect that an energised solenoid decouples the coupler.

 

 

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut. 

Last Edited: Fri. Nov 7, 2014 - 08:13 PM
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Torby wrote:
So how do you keep a 555 from triggering when you power it down?
Use a 2242.

Intersil

Timing & Digital ICs > Counters/Time Base ICs

http://www.intersil.com/en/parametricsearch.html?g=timing-and-digital&sg=counters-time-base-ics#g=timing-and-digital&sg=counters-time-base-ics

ICM7242

Long Range Fixed Timer

http://www.intersil.com/content/dam/Intersil/documents/icm7/icm7242.pdf (265kB)

...

When system power is turned off, any charge remaining on
the capacitor will be discharged to ground through a large
internal diode between the RC node and VSS.

...
The ICM7242 is a non-programmable timer whose principal
applications will be very low frequency oscillators and long
range timers; it makes a much better low frequency
oscillator/timer than a 555 or ICM7555, because of the on-
chip 8-bit counter.

"Dare to be naïve." - Buckminster Fuller

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I use a tiny 10wink

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut.