Sound Activated Relay, #2

Parts List:
R1 = 1K                             C1,C2 = 0.1 uF, ceramic
R2 = 100K                              C3 = 1 uF, 16V or better, electrolytic 
R3 = 220K                              C4 = 10 uF, 16V or better, electrolytic
R4 = 25K potentiometer
R5 = 2700 ohm (2K7)

D1 = 1N914, signal diode             MIC1 = Electret mike, 2-wires
D2 = 1N4004, general purpose diode    Re1 = Relay, suit to your needs
U1 = LM741, op-amp, 8-pin
Q1 = 2N2222, NPN transistor
This relay remains dormant until the op-amp activates upon sound via the electret-microphone. Only tested with 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 coming off pin 6 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.

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.
R5 is a 'minimum' bias protector when the potmeter is adjusted to '0' ohms and protects the transistor.
Diode D3 is mounted over the relay coil to absorb sparks when the relay opens. Cathode goes to '+'.

Copyright © 2003 - Tony van Roon