Purpose of a cap in a circuit...??

[peter.rabbit]

I've built most of the circuit attached. It is, as titled a timed rocket igniter, and works well. I omitted the lower section of the circuit involving the third 555, as I didn't need the beeping buzzer.

I've been troubleshooting bouncing in the circuit, where the 2nd 555 would trigger on power-on, I solved this by actually following the schematic..... When I first breadboarded it I was short of some caps so made a bunch of substitutions, one of which was putting a 10uF cap at C10 instead of a 1uF - the circuit worked fine, but U2 would trigger on initial start-up.
I wonder now if this cap was damaged in an alligator clip incident during testing after stripboarding the circuit, that let the smoke out of one 555 and seriously injured the second....

My question though, is what cap C13 is there for? I'd left it out initially, and everything was working, and including it seems to make no difference....

Thanks for any advice anyone can offer....

 

[Arouse1973]

C13 is a smoothing capacitor which helps stabilise the supply voltage when the ICs outputs switch. They also supply large short bursts of energy needed for transients in the circuit like switching capacitive loads etc. and to reduce the supply drooping and causing circuit stability issues. I would leave it there if I was you.
Adam

 

[peter.rabbit]

It really connects across neg/pos rails doesn't it.... I wasn't sure if it was something to do with the lower part I've omitted.
I'll solder one in on your advice - thanks Adam.

 

[davenn]

yes it does, thats the way smoothing capacitors are connected

it smoothes the supply rail for the whole unit

Dave

 

[peter.rabbit]

Now... on this circuit, a problem....

Before I placed the 47u cap (C13) - I couldn't make U2 false trigger during power on. I could cycle the power switch however, and it would not trigger.
Now with C13 in place, if I power on shortly after powering off - few seconds, it will usually - not always - trigger. I still have not seen it trigger from a cold power up.

I've never seen U1 false-trigger, which is a nuisance, because that could be aborted - U2 directly fires the ignition! There are two separate disarming switches that prevent an actual accidental firing, if they are open that is.... I would love to stop this trigger on start-up altogether ideally...

Any thoughts as to why this may be?
Trouble-shooting advice?

 

[(*steve*)]

Clearly something unusual happens when the circuit is powered up from an intermediate voltage as opposed to it being powered up from cold.

Whilst you could try to track that down, a simple approach would be to change the power switch to a SPDT switch that shorts the power rail to ground (perhaps through a low value resistor) when the unit is switched off. This will quickly discharge the capacitor so all startups are effectively from zero.

 

[peter.rabbit]

I'm going to drag this back up.... progress....
I built the circuit as you see it - but I run it from 12V rather than 6V.
U2 is still false triggering on start-up if I rapidly cycle power on/off at 12.2V, but if I set voltage to 12.8 or higher, it will false trigger at every power on. This is a problem, because I run this from a fully charged SLA....

I suspect the resistors/diode on pin 2 of U2 have something to do with this? I did note before when I had accidentally installed a 10uF at C10 instead of 1uF it would trigger on start up also.....

 

[Arouse1973]

Unfortunately this might be some of the reliability issue you can get with 555 timers and why I never use them.....but we won't go in to that, I'll only get in trouble again .

If your design is a proven design and is built correctly and all care has been taken to clean the PCB to remove contaminant s then as long as the circuit is given time to discharge properly between power downs then the only other things it could be is either interference from other parts of the circuit or the 555 timer itself. If you can use a scope and post some screen shots of each pin of the 555 timers that are different signals compared to when it's working.
Adam