Hello,
I am trying to use the Audiomoth with an external piezo contact microphone (CM-01B from TE Connectivity). I have hard wired the microphone up to the solder pads as per the documentation but I cannot see any signal. I know that the microphone works as I have connected it to other pieces of audio equipment. The internal microphone works ok and I can see a 3V dc offset when measuring between the tip and ground pads.
Is there anything else that I can try? Could it be a compatibility issue with the mic and the input electronics of the Audiomoth?
Thanks for your help
Thanks a lot for your support. I have fixed the problem. foodle
Hi Jonathan, The AudioMoth input expects some load from the JFET amplifier inside a typical external electret microphone. The piezo contact microphone has no amplifier so it does not correctly bias the input circuit of the AudioMoth. This is why you see a 3V DC offset which is too close to the supply voltage for the amplifier to work. The input needs to be set between 0.5 and 2.5V to constitute a suitable input signal. You can fix this by putting a 2K7 resistor in parallel with the piezo contact microphone. You will then see the full signal from the microphone, and the DC offset of the input will be about 1.5V. This is a quick solution but isn't ideal as the capacitance of the piezo sensor, typically about 20nF, and the 2K7 resistor will form a high pass filter with a cutoff frequency of about 3kHz so you will lose a lot of the low-frequency signal. Large impacts on the piezo sensor can also generate large voltages which might damage the AudioMoth amplifier. A better solution is to use an additional amplifier. We use Aquarian PA1 amplifiers for this - https://www.aquarianaudio.com/pa1.html. This amplifier is essentially the same sort of JFET amplifier found inside electret capsule microphones. It provides a small amount of gain but crucially prevents the loss of low-frequency signals. It will work with all bare piezo contact microphones. Alex