The above schematic shows a sound activated switch.The resistors R1,R2,R3 and R4 are used as potential divider network which provides a reference voltage which can be varied from 0-100mv.Reference voltage is applied to the inverting input(pin2) of the op-amp.The microphone picks up the sound and converts into electrical signals.When the voltage of the mike is above the reference voltage the difference between the reference voltage and that of the mike's is amplified by the op-amp.The amplified output is taken at the 6th pin of the op-amp.This output is applied to the gate of the silicon controlled rectifier(SCR).As a result the SCR fires.current flows from anode to cathode and to the ground.this completes the circuit and the bulb will glow.To swich off the light you must reset the switch s1.Resistor R5 is used to limit the current to the gate of the SCR.
components used
resistors-R1-5k R2-100k R3-10k R4-1k R5-10k R6-1k op-amp741,electret mike scr-BT136 diode-1N4007 battery-15v switch
