I've been working on a project for my brother, the photographer
to build a sound-based trigger for a flash unit. I've put together this circuit with no luck so far:
I'm fairly sure that my problem has to do with the amplifier part of the circuit coming from the microphone. I have a Propscope (thanks, Parallax!), and have been using it for troubleshooting, and I'm very pleased that it has been so useful. So far I've verified:
- that when I apply power, the 1 uF capacitor properly charges up.
- that when the input to the MOC 3020 optoisolator is triggered, the flash fires reliably.
I have a small electret microphone from Sparkfun, and I had hoped to find a test point in the circuit where I could verify (using the Propscope) that the signal from the mic is being properly amplified before going into that second BC 547 transistor. I put the scope ground to circuit ground and then the probe to the base of that transistor, and made sound into the microphone, but saw nothing on the screen. It seems to me that if things are working properly in the amplifier part of the circuit, I should get a voltage there that changes with the sound level into the microphone.
I also have that same microphone mounted onto a small amplifier board (also from Sparkfun). When I put the scope on the output of that board, I do see waveforms as expected. I hooked that output to the base of that second BC 547 in the circuit. When I turn the circuit power on, the flash triggers after a few seconds, sound or no sound. Here's what I believe is happening: When I just hook up the output of the mic/amp board to the scope, I see waveforms around a roughly 2V offset. I suspect that the 2V that's always there is itself enough to trigger the transistor, and therefore when the 1 uF capacitor builds up enough charge to fire the circuit, it fires right away, regardless of sound level.
That would be good news, I think, as it would suggest that everything in the original circuit except the amplification of the microphone is working fine. It also suggests that I can replace the amplification part of that circuit with the amplified breakout board mic, if I can get rid of that 2V offset.
How am I doing so far?
So it occurs to me to hook up both channels of the Propscope, one to a variable voltage (using a potentiometer/voltage divider circuit) input to the base of the second transistor (the one that triggers the circuit) and the other channel to the capacitor input to the MOC 3020, which builds up as the capacitor charges and then drops off to zero when the circuit fires. That way I can see what the voltage threshold for firing that transistor is, and use the potentiometer/voltage divider to bring the output of the microphone/amp to a point below that but close enough that sounds into the mic trigger the circuit.
Make sense? Or am I hopelessly confused?