Automatic telephone recording circuit

  
The device is a dc switch that is normally on via the forward biasing of Ql via R3. Ql now clamps Q2 into a forward state by biasing its complement well into a saturated state via R4. The dc switch is turned off via a negative voltage above that of the zener (Dl). This voltage is usually about 48 and is the on-hook value of the phone line. This negative voltage overrides the effect of R3 and keeps the circuit `off.` When the phone is off the hook, the 48 volts drops to 10 volts, that is below the zener voltage of Dl and R3 now turns the circuit on.
Automatic telephone recording circuit - schematic

The audio signal is via attenuator resistor Rl and dc isolating capacitors Cl, C2. The device is a high impedance switch that isolates the recording controlled device from the phone line via some relatively simple electronic circuitry. It requires no battery and obtains power for operating via the remote jack that in most recorders is a source of 6 volts. When clamped to ground it initiates recorder operation. The unit interfaces with most portable cassette recorders providing they contain a remote control jack.




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