One way interphone

Back in 2008, I had to make one way interphone for one of our clients. There were three requirements:

  1. It has to be very low cost.
  2. It has to be very sensitive (client can speak from distance up to 2,5m from microphone).
  3. There shouldn’t  be too much noise.

Regarding first requirement, cheapest way to make this device is to buy some computer speakers and computer microphone, and connect them to microphone preamplifier. With computer speakers, you get amplifier, speakers, and power supply, and you can buy it for 4-6 euros!!! So, I went to computer store and found cheapest speakers on shelf, if was A4 Tech AS-6. After disassembling it, only thing to do was to find 12V DC power supply on amplifier board that I can use for my preamplifier, and it was easy.

I don’t know too much about analog electronics, so at first I try to find schematic for microphone preamplifier on internet, but i wasn’t lucky. There are some good schematics on the net, but sensitivity for those preamplifiers I found was bad. I remembered that my friend built some spying bug with mic preamplifier and FM transmitter in high school, so I called him to send me this. Bingo! There was solution for my problem. He made great preamplifier I can use for my project. i made few changes, instead of few resistors I used potentiometers, so I can adjust sensitivity, amplification and attenuation.

As you can see from schematic above, there are 3 potentiometers:

  • RV1 – adjusting sensitivity of microphone
  • RV2 – adjusting amplification
  • RV3 – adjusting attenuation of signal so there is no distortion on speakers (for AS-6 it was 200-300mV on input according to technical specification).

Additionally, I soldered one switch in parallel with mic, it acts as a mute switch.  It must short microphone ends, if you just disconnect microphone, it would be muted, but you’ll get too much noise on speakers.

At the end, I got great one way interphone, and customer was very satisfied.

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