Telephone recorder

Automatically records all phone conversations

Introduction

This circuit automatically records all incoming and outgoing phone conversations with an ordinary
cassette or tape recorder. The circuit below distills the audio signal from the phone line and automatically
starts/stops the cassette recorder.

Circuit description

Connections A/B are connected to the phone line; the (AC) audio signal will pass through C1/R3 and TR1 to conn 1
which is connected to the record input of your cassette recorder. TR1 is necessary for electrical separation of the
phone line and the recorder. D6/D7 protect the recorder input against high voltages e.g. when the phone rings!
*Transformer TR1 is an 1:1 transformer; I used a small mains filter transformer from a TV set, but any 1:1 transformer will
do as phone audio is not hi fi anyway!

The circuit built with rectifier D1-4, zener D5 and optocoupler IC2 detects if the phone is on or off hook:

• On hook the phone line voltage is about 40 Volts DC (Dutch phone system, I think it is the same for most countries)
• Off hook, the phone line voltage drops to 5 to 6 Volts DC; the audio signal is an AC signal on top of this DC voltage.

So off hook, opto IC2 is conducting pulling down node "A".
On hook the voltage drops below the zener voltage of D5 and opto IC2 will stop conducting, R4 will pull up node "A".

The schmitt trigger NAND gates build two one shots: IC1 D will pulse the play/rec relay when node "A" goes high
(off hook start of conversation).
IC1B inverts the signal on node "A"; consequently IC1A will pulse the stop relay when node "A" goes low
(on hook, end of conversation)

The connections play/rec and stop must be wired in parallel with the play/rec button and the stop button of your
cassette recorder. The recorder must be equipped with a servo drive!

Another way to start stop the recorder is to use a relay which switches on/off the motor of your cassette recorder:
The circuit can easily be adapted to work this way: short C2 and use the play/rec really to switch the recorder motor.
The stop relay, D1, T2, C2, R4, IC1A and IC1B will become obsolete in this case.

Telephone recorder schematic:

Picture of the somewhat messy, though fully functional experimental setup:

→ Download specifications of key components at the  Component specs page

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