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556 delay circuit integration problem, works but only for limited time!

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More problems! I have managed to build two circuits. One of them is an op-amp triggering circuit that takes audio or IR inputs to trigger an output, this works beautifully on it's own. The other is a 556 timer circuit I found on the web, very lightly modified. Again, this works perfectly as intended.

Join the two together, by taking the output (OP) from the trigger circuit to the input of the delay circuit, and sharing a common 9v supply, all works fine for a few minutes, then it stops. Turn everything off, leave for 5 minutes, and again it'll all work for a few minutes, then stop.

Time seems to be the key, rather than use of the circuit - turn it on and leave it for a few minutes and it'll not work when you return to it, or you can use it 100 times in a few minutes before it stops working.

Any ideas?
 

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hi,
Are the two circuits designed to operate from the same voltage supply levels.??

A failure as you describe is often caused by a component overheating, then cooling, any sign of a part getting hot.??
Is the supply a battery and does the voltage remain steady over the failure period.?
 
Hi Eric,

Thanks for the response. To answer your questions:

Yes, both are designed to operate off 9v
No, I can't find any components getting hot.
Power supply has been both battery and transformer, both with the same results, with steady voltage.
 
Hi Eric,

Thanks for the response. To answer your questions:

Yes, both are designed to operate off 9v
No, I can't find any components getting hot.
Power supply has been both battery and transformer, both with the same results, with steady voltage.

hi,
OK.
Is there a simple way you can determine if the first stage is still operational after the fault occurs.
Trying to determine which of the stages is giving the trouble.?

EDIT:
Is the circuit connected like this.??
 

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hi,
Looking at the SCR trigger method from the 1st to 2nd stages I would suspect that stage 2 is not being triggered.

When the project fails disconnect the OP+ wire from the SCR and touch it to 0V, the 2nd stage [delay] should trigger OK.
 
yep, that is exactly how they are connected :)

Just tested again, you are correct, stage two would appear to not be being triggered, whilst the trigger LED for the first stage is lighting, double checking it, the second stage trigger led does not. Disconnecting the OP+ wire from the SCR and grounding it does indeed trigger the 2nd stage making it operational, reconnecting and again we have a few minutes of use before it fails... getting a headache LOL
 
yep, that is exactly how they are connected :)

Just tested again, you are correct, stage two would appear to not be being triggered, whilst the trigger LED for the first stage is lighting, double checking it, the second stage trigger led does not. Disconnecting the OP+ wire from the SCR and grounding it does indeed trigger the 2nd stage making it operational, reconnecting and again we have a few minutes of use before it fails... getting a headache LOL

hi,
Dont let a headache bother you too much..

What did the SCR control on the 1st stage when used on its own.??

We may need to modify the SCR area of the circuit...
 
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Let me explain a bit more fully how it all works ( or doesn't LOL)

X1 and X2 are sensor inputs - either Photodiode (X1) or microphone (X2). X1 acts as a beam break (sensor is infra red coupled emitter and receiver which items pass through) microphone is a simple sound detector.

Only one is used at a time, but either way the output OP+ goes to a a flash gun positive pin, the -ve pin goes to ground, so as soon as an event occurs the triac T2 shorts OP+ to ground and the on flash is fired. The second unit is simply a way of delaying the flash being fired, so the short to ground is instead fed into pin 6 of the 556 to trigger the timer.

Connecting the flash to OP+, it'll work all day, triggering faultlessly. Connect the flash to the INSOP or DELOP of the delay unit, then trigger it by manually shorting input to ground... once more it'll happily trigger all day long! Connect OP+ to input to essentially take over from my manual input, bang it all goes pear shaped.

Very confused (not that that is too hard LOL)

At least that's my understanding of it :p
 
Just to add... for simple curiosity, I added a momentary switch in the line between the OP+ > Input link, and ground. Once the unit has stopped working, pressing this switch and the delay circuit triggers fine, so obviously for some reason, the short to ground coming out of OP+ is just not happening when connected to the delay unit, even though it works flawlessly when not!
 
Just to add... for simple curiosity, I added a momentary switch in the line between the OP+ > Input link, and ground. Once the unit has stopped working, pressing this switch and the delay circuit triggers fine, so obviously for some reason, the short to ground coming out of OP+ is just not happening when connected to the delay unit, even though it works flawlessly when not!

hi,
Are you able to modify the circuit as shown in BLUE, also you may have to remove R3 the 1megR
 

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I'll have to go back to breadboard with it as the PCB is a bit cramped, but sure I can do that! Can I ask you to explain your rational for suggesting it? I'm trying to learn so all thought processes are quite useful to me.
 
I'll have to go back to breadboard with it as the PCB is a bit cramped, but sure I can do that! Can I ask you to explain your rational for suggesting it? I'm trying to learn so all thought processes are quite useful to me.

hi,
In the first circuit D1 and D2 act as an OR gate which drives T1 into conduction when either of two external inputs go high.
As T1 conducts it triggers the gate of the SCR, which on the original circuit connected a load to 0V.
Being an SCR it would remain conducting until the power was disconnected from the SCR load.
Scr's require a few tens of milliamps to to keep them in the conduction state, which will not happen when the load is a 1meg, 47nF into a 22K.
By changing the SCR to a transistor switch [ which has gain] it will drive a low going into the trig 1 input of the 556, via C1, so setting the first mono active.

If for some unexpected reason the SCR did remain conducting after being triggered, with R3 and R4 supplying sufficient holdon current, the SCR would not be able to supply another trigger pulse after the first one.

Basically I would consider that SCR output of the second stage is not suitable as a 556 trigger, thats why I am suggesting a transitor replacement.
 

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Beautifully explained, thank you :)

Just noticed your location, oh the irony of searching the net for help and finding it in the same county LOL. I shall go and make the mods you note and see what happens!
 
well, good news is it would appear you are correct! Since modifying the circuit as per your suggestion, I have not had it fail once. I will have to make a further slight modification as I'd also like a take off point between the two stages, which if using an older flash has the potential to have 300v + flashed across it so would make the transistor fairly short lived, but that shouldn't be too hard :)

The bad news is it would appear that neither the CA3130's nor the 556 like having a glass of my sons juice poured over them, so I shall be the one ordering new parts... joy.
 
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