Bi-Directional Telephone Line


Posted on Apr 14, 2012

This self-build project can simulate a telephone call / connection between any two local telephone devices. This for a fraction of the outlay of some commercial units, many with cost-increasing features that usually are not required. The circuit is centred on two cascaded binary counters clocked by a 555 astable multivibrator. Several of the counters` outputs provide the frequencies, tones and time delays required. A pair of operational amplifiers detect on / off hook conditions at both ends of the simulated line. To detect call answering, a third op-amp monitors the ringing current. R/S latches and logic gates provide event storage and signal control. Discrete circuits provide the voltage multiplication and DC to AC conversion required for the ringing signal. A pair of miniature relays, switch each end of the simulated line between connection and ringing.


Bi-Directional Telephone Line
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