High Gain Spy Listening Circuit

High Gain Spy Listening Circuit - Dual Microphone

Chip l
Standard LM386 Audio Chip. Place A Jumper Wire Between Pins 2 and 4. The Pin Connections are indicated in the Schematic

This is a standard 8 Ohm mono earphone or headset

ON/Off Switch:
Any, On / Off Switch Will Do - connecting the + side of the 9 volt battery to the circuit

9 Volts D.C. Connect the + side of the battery to the circuit, the Negative side to ground

M1, M2:
This Unit incorporates Two standard Radio Shack PART NUMBER # 090 Electret Microphone elements. Actually, two of these elements in parallel produces double the input gain. More than two produces a loss - so stick with the schematic.

Turn the On/Off Switch On. Adjust R11 for a confortable noise level. This is an extremely quite (Internal Noise Level) amplifier and open for a great deal of experimination such as - used with parabolic dishes, etc.
Resistors: 1/4 Watt

R1 = 4.7K
R3 = 10K
R4 = 2.2K
R5 = 10K
R6 = 47K
R7 = 22K
R8 = 10K
R9 = 1K
R10 = 100 Ohm
R11 = 10K Adjustable Pot ( Volume Control)

Capacitors: Disc Or Electrolytic
Electrolytic Shown with +/-
All Low Voltage

C1 = .1
C2 = .01
C3 = .01
C4 = .01
C5 = .01
C6 = .1
C7 = .1
C8 = .1
C9 = 47 MFD
C10 = .01
C11 = 1 MFD 16V
C12 = .01
C13 = 220 MFD
C14 = 100 MFD

FET1, FET2 = MPF-102

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High Gain Spy Listening Circuit - Dual Microphone
This High Gain listening circuit is extremely sensitive and very powerful. It is used in spy or surveillance applications for gathering a very weak audio signal and amplifying those small signals to a high level. The audio is boosted by two low noise FETS and then fed into an LM386 Amplifier circuit. Once amplified the audio is fed into a set of 8 ohm earphones. The gain of the amplifier is controlled by R11, a 10K pot. All components are listed below.