Telephone Spy Conversation Recording Circuit

Telephone Spy Conversation Recording Circuit

Circuit Project Description

The circuit aims to create a recording scheme not only for spying but for business purposes where conversations are noted for future reference.

Terminology 4N28 – a general purpose optocoupler that consists of a gallium arsenide infrared emitting diode driving a silicon phototransistor in a 6-pin dual in-line package used in microprocessor inputs, digital logic inputs, and power supply regulators due to its features such as high isolation voltage between input and output, operating temperature up to +110ºC, and available in wide-lead spacing4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumptionLM7812 – 3-terminal 1A positive voltage regulator with short circuit protection, thermal overload protection, output transistor safe operating area protection, output current up to 1A, and output voltages from 5V to 24VLM317T – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V and requires only two external resistors to set the output voltage due to its internal current limiting, thermal shutdown and safe area compensation, making it essentially blow-out proof Circuit Explanation

The operation of the circuit starts as the handset is raised where simultaneous recording occurs on both sides of either of the line. Along with the circuit is a tape recorder which is driven by a voltage and regularly functions with every use of the telephone. An external power pack can provide a 16 V DC supply in its output. The circuit is driven by the IC4 together with the relay having 12 V while IC5 drives the tape recorder with the required voltage of 3 V due to the presence of trimmer TR1 regulation. A heatsink is placed on IC4 to prevent excess temperature. The voltage in the IC5 input in lower levels is provided across R17 & R18, which also maintains the heat dissipation of IC5. The values of the resistor can be modified depending on the needs.

The connector J1 & J2 provides the input and output for the telephones lines. The circuit can be isolated from the lines with the use of switch S1 & S2 to avoid excitation of the circuit in every event of raising the handset. The isolation of lines other circuit is handled by IC1 to IC3, which works with IC2 to drive the relay RL1 & RL2. The voltage command of the tape recorder comes from the contacts of the relays. If the tape recorder is taking its supply from batteries, the circuit will have to be modified due to the remote in input. During the ringing pulses, the circuit is prevented from stimulation from capacitors C1 to C6. A visual indication of which line is being used is shown by LED D4 to D9 while the transformer T1 handles the audio of the telephone.

Part List

R17-18=1.2ohm 5W [see text]
C1-6=10uF 25V
C2-3=470nF 250V MKT 5%
C4=820pF 100V ceramic
C5=680nF 100V MKT 5%
C7-8-11-13=100nF 100V MKT 5%
C9=470uF 25V
C10=47uF 25V
TR1=4.7Kohm trimmer
C12-14=10uF 25V
IC1-3=4N28 [optocupler]
D1-6=   ——–
D2-7=20V 1W zener *[see text]
D4-9=LED RED 3mm
T1-2=TEL. Transformer 600R/2X300R
J1-2=4pin connector – 2.54mm step
J3-4=2pin connector – 2.54mm step
S1-2=Mini Switch 2X2
S3=Mini Switch 1X2


The phone spy conversation recording system is suitable for recording calls clearly on most telephones with detachable handsets, which includes most digital telephone extensions and analogue telephones. They are also used in telephone surveillance but should not be used for unauthorized recording because certain laws and regulations should be followed while using it in a legal and lawful manner.

Sorry, comments are closed!