Timer circuit problems

[geofflove]

Hi

I've just built a short delay circuit. The idea being that it will be sound triggered and then provide a short variable delay before triggering a flash for a camera. However.... I can't get the delay bit working as it should.

I've build this circuit..:

Delay circuits

However, I have not yet connected the input trigger. I've attached a couple of leds temporarily, one to each SCR. As soon as the battery is connect SCR1 triggers followed (corrrectly) after a brief delay by SCR2. Problem is why is SCR1 triggering before I ground the trigger and how do I fix it?

I can't find any mistakes in what I've built so is the design wrong? Any idea how I can trouble shoot it?

Thanks in hope!

 

[colin55]

Both halves of the 556 are wired up poorly and incorrectly.
The 556 (555) connected to pins: 1 to 7 has pins 1 and 2 connected together and the chip does not know what to do when the supply is connected. That's why it comes on with the output HIGH.
The other half of the 556 has pin 13 connected directly to an electrolytic and this does not form a time-delay circuit.
The author of the circuit has absolutely no idea how to connect a 556 correctly.

Colin Mitchell
TALKING ELECTRONICS.com
talking@tpg.com.au

 

[geofflove]

Well the timer part is working as stated.

Could you point me towards a schematic which would work?

Thanks

 

[colin55]

When you connect the power to the 556, do you accept the fact that the circuit will cycle and produce a flash.

 

[geofflove]

When you connect the power to the 556, do you accept the fact that the circuit will cycle and produce a flash.

Yes - that need not be a problem. The question is how to reset it. At present the 2 scr's trigger one after the other and stay on. How do I reset it so both are off ready for triggering? I've tried removing the link between pin 4 and V+ and then grounding pin 4 to reset the device but it has no effect.

Am wondering if the first timer in the 556 is fryed?

 

[colin55]

To turn off an SCR you have to reduce the current through the "anode-to-cathode" path to zero. Even if the current is very small, some SCR's will remain "on." That's why you may need to use a transistor or FET. These devices turn off when the current into the base (or "gate" for a FET) reduce to zero.