Audio signal actiated relay , This Relay remains dormant until the op-amp activates upon sound via the electret-microphone. I only tested the 2-wire type. The input stage is a regular off-the-shelf 741 operational amplifier and connected as a non-inverting follower audio amplifier.
- Gain is approximately 100 which you can raise by increasing the value of R2.
- The amplified signal is rectified and filtered via C3,D1/D2, and R3 to an acceptable DC level D1 and D2 can be any signal diode like 1N914, 1N4148 or the NTE519.
- Cx can be anything with a value of 6.8 to 22µF. Adjust if necessary. Cbias is to DC-couple the mic to the input of the 741. Q1, the 2N2222 or 2N3904 is a general purpose NPN transistor and is not critical. The NTE123A will work too.
- Potentiometer R4 is used to set the audio level to a desired sensitivity value to activate the relay via transistor Q1. Diode D3 is mounted over the relay coil to absorb sparks when the relay opens.
- Cathode goes to ‘+’.
- The op-amp configuration in this particular drawing needs a dual voltage power-supply which can be made from two 9 volts alkaline batteries.