A simple circuit using a TL072 op-amp and 74LS14 inverter for getting a clear digital trigger signal based on the sound level detected by an electret microphone.
The purpose of this circuit is to get a clear digital signal (as close to rail-rail as possible) based on the sound level picked up by an electret microphone. It is not intended to amplify the audio input, but to provide a trigger once a variable threshold is reached.
I put this circuit together in four basic stages:
- the electret input is amplified with a high-gain inverting amplifier
- an op-amp configured as a variable comparator turns the analog input to a binary output
- an RC filter with a time constant of araound 47ms provides some "de-bouncing" and extends the minimum duration of any triggered input
- a series of inverters pull the signal to as-near rail-rail as possible, and allow tapping the output with the desired polarity (active-low or active high)
Here's a scope trace of a sound triggering the output.
- CH1 (yellow) is the interter output
- CH2 (blue) op-amp output
- CH3 (red) electret output (note the 200mV scale)
