Making Microphones

It is a surprisingly good microphone

When I’m attacking a problem I sometimes like to have several fronts open at the same time. I’m presently trying to get speech input into my Red Telephone. The phone has a carbon granule microphone which changes in resistance in response to sound. This won’t work with the microphone input in the phone. I could either build a circuit to make the carbon microphone work with a “normal” microphone input or I could find a different microphone to put in the phone. I decided to do both. Starting with microphone replacement.

I got the microphone above with a stereo system from way back. It helps the amplifier adjust itself to the acoustics of the room its working in. The stereo is long gone, but I kept the microphone because well, you never know when you might need one. It works fine with the USB audio input I’m using with the Pi and it even fits inside the handset of the phone. But there is a problem. The handset is connected to the phone using a lovely red curly cable. This cable is not screened at all because old phone cables don’t need screening. Unfortunately, microphone cables do. So although I got an audio signal from the microphone I also got an awful lot of hum caused by the cable picking up mains radiation from the devices in the house. Oh well, on to tomorrow.