swapping piezo for electret mic?

[Delinquent]

Another circuit I need modified! **broken link removed**

This is the circuit, using a piezo element to trigger a flash (well, supposedly, personally having no luck getting it to work at the moment!)

Regardless, the action I want to capture makes very low end noises, which the piezo would be no good at capturing anyway, so I want to think about swapping in a electret mic I have in place of the piezo. Could anyone tell me what else I will need to add / remove from the circuit to make that work?

Many thanks!

 

[moffy]

I believe that an electret mic is just a piezo device. As long as it is unpowered, it should be a drop in replacement. You might consider some filtering (simple resistor/capacitor) to make the mic more selective. There is always the question of sensitivity or gain, is it sensitive enough to do what you want?

 

[MikeMl]

An electret mic requires DC bias, and the signal is capacitively-coupled to the input of the first amplification stage.

I'm not surprised that the posted circuit doesn't work.

 

[audioguru]

An electret mic has an output level much lower than a piezo transducer so it will need a preamp circuit in addition to a bias voltage.

 

[Delinquent]

Mike, can I ask why you say you are not surprised it doesn't work? I know people have got it working with a bit of 'tinkering' but not having the skills to know where to tinker I'm at a bit of a disadvantage! (also I can't easily get hold of some of the exact components so have had to try and understand datasheets to swap them!)

 

[audioguru]

The posted circuit has a 68k resistor supplying base current to the transistor. The base voltage is about 0.65V.
A piezo mic is a capacitor that does not conduct so the 0.65V remains biasing the transistor and does not affect the piezo mic.

But an electret mic draws current for its Jfet inside so it "steals" much of the base current away from the transistor and the resulting 0.6V or less base voltage is not enough voltage for an electret mic to work properly.

The electret mic should have its own biasing resistor to the positive supply, use 10k ohms. Then its signal should have a series coupling capacitor of maybe 0.33uf to the base of the transistor. Without having a preamp then a very loud sound might trigger the SCR.