Here is a simple circuit called listing bug using the most popular op-amp IC 741. This circuit can be used for spying because by using this circuit we can hear any conversation held in any other room or we can say by using this circuit one can hear the conversation held in another room.
Circuit Description of Listening Bug Using 741
The circuit listening bug comprises an op-amp IC 741, two amplifier transistors, a loudspeaker and a microphone. A microphone is a transducer which converts audio signal to voltage in range of milli which further need amplification. A long wire from microphone is connected to inverting input (pin 2) of IC1 through resistor R2 and capacitor C2. The output of IC1 from pin 6 is given to base of transistor T1 and T2. These transistors are used here to amplify the output of IC1 to desire level. The output of this transistor from collector is given to loudspeaker for audio output. Capacitor C4 is used to filter the amplified signal from transistor and also block DC signal if any available.
PARTS LIST OF LISTENING BUGS USING OP-AMP 741
Resistor (all ¼-watt, ± 5% Carbon)
|R1 = 3.3 KΩR2 = 4.7 KΩ
R3, R4 = 10 KΩ
R5 = 2 MΩ
C1 = 100 µF/10V
C2 = 0.047 µF
C3 = 10 µF/10V
C4 = 250 µF/10V
|IC1 = LM741T1 = AC127 or AC176 or ACA87
T2 = AC188 or AC128 or AC153
|MIC1 = Condenser microphoneLS1 = 4Ω loudspeaker|