Sound Activated Relay, #1

Parts List:
Rx = 10K    C_bias,C1,C2 = 0.1uF, ceramic            Q1 = 2N2222,2N3904, etc.
R1 = 1K             C3 = 1uF, electrolytic        IC1 = 741 op-amp
R2 = 100K        C4,Cx = 10uF, electrolytic       MIC = Electret Mike, 2-wire
R3 = 100K           Ry = Relay
R4 = 25K         D1,D2 = 1N914,1N4148,NTE519  
R5 = 2K7            D3 = 1N4001

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 22uF. Adjust if necessary. C_bias 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 volt alkaline batteries.