How to trigger an SCR with sound?

Question

I'm trying to make an audio flash trigger. It works triggers the flash with a small solar cell as a photo slave but not with sound from amplified mic via headphone jack. The SCR I'm using is MCR8SM. Any ideas?
Thanks!

Answer 1

You probably just don't have enough signal drive going into the gate. Add a transistor amplifier stage, and you should be good to go. You need to make sure your audio peaks are over 1 volt peak with the gate connected. If you don't get this, you don't have enough audio drive to trigger.



Since you are dealing with audio signals, having access to an oscilloscope is handy for troubleshooting.

Answer 2

You could try a small step up transformer with the microphone. They are generally available from stores like Radio Shack, that connect into the cord of the microphone. Once you have enough voltage to fire the SCR it will of course cause the flash to operate. It shouldn't, however, cause the SCR to remain on because there will not be enough "hold on" current to keep the SCR turned on, which I don't believe you want any way.

Answer 3

I don't think you should try to trigger an SCR gate directly with audio frequency. Too messy and unpredictable....

I would "emulate" the solar cell function (and are you sure it is a "real" solar cell and not just a LDR?) with a dedicated microphone amplifier and a logic circuit that ensures you only get one trigger pulse. Something like this:

http://www.electronic-circuits-diagrams.com/remotecontrolsimages/remotecontrolsckt4.shtml