# Electronic circuit question - sonic flash trigger

I’m working on a sound flash trigger, and I found this circuit: http://courses.ncssm.edu/hsi/pacsci/text.html. I’m having some trouble getting it to work, so I want to first make sure I understand how it works. As far as I can tell, initially, current flows through the two 5 Kohm resistors to the transistor’s collector. Current also flows through the 68 Kohm base resistor and switches the transistor on. Current flows through the transistor. When the pizeo trigger is activated, current is directed through it and away from the base, switching off the transistor. Current then moves through the SCR gate instead of the transistor, activating the flash.

Does decreasing the value of the variable resistor increase sensitivity? By permitting more current to reach that portion of the circuit, do smaller changes in the collector-to-emitter current make it easier to push the gate current above the threshold value?

It’s been years since I’ve done anything with electronics, but I’d guess that initially no current is going through the transistor. When the piezo electric mic is activated, it allows current to flow through the transistor, grounding the gate of the SCR and allowing it to fire.

It does look like the 5K resistor is supposed to be a potentiometer, so I’d guess that is a sensitivity control.

Actually, the more I look at the circuit, the more I think your explanation is right. Although having a circuit where even a small current flows all the time seems unnecessarily hard on the batteries.

So…given that I’ve demonstrated by complete unfitness to answer the question, I’m going to slink out of this thread and go do some remedial reading on transistor circuits…

You’re more or less correct, only it’s not current through the piezo element which turns off the transistor, it’s the voltage generated across it, which acts in opposition to the voltage applied through the 68k resistor, resulting in a lower base voltage. This doesn’t turn off the transistor, but decreases the collector current, which raises the SCR gate voltage above its trigger threshold

Increasing the value of the 5k pot will increase the sensitivity, because you’re applying a larger voltage to the gate, so it’s closer to its trigger voltage.

Thanks. I got it working now. The SCR was faulty. I started by disconnecting the base entirely, which did not trigger the circuit. Then, I directly connected the first 5k resistor to the gate, which still did not trigger the flash. Connecting the gate directly to the 9v supply did, though. Either I accidentally damaged the SCR earlier or it was already damaged. They are CR02AM SCRs I pulled out of an old thyristor flash. The flash wasn’t working properly, which might have been related to the damaged SCR I was first using.