| A very simple circuit, Simultaneously it can cut the
sound from stereo amplifier that are in high volume, the sound of some
television or turn on some at night when it rings. Exists a pair of free
contacts of RL1 that connects to the J2, which you can use connecting there
any appliance you want. The telephone line connected in the J1, with what
polarity you wants. When the telephone is closed then the line voltage is
roughly, 48-50Vdc. This voltage turn on the photodiode and this, the
transistor of IC1, which it simultaneously isolates, the circuit from the
telephone line. The phototransistor in IC1 are now in situation ON, the
input of IC2A are LOW [L] and output HIGH [H]. Ignoring for little the
circuit of delay D6, R4, R5, C1, the IC2B input, are also this HIGH hence
the output are LOW, transistor Q1 are OFF and the RL1 are deactivate. When
the telephone earphone is raised, then the telephone voltage line fall in
6-10Vdc. All the previous situation is reversed also the RL1, turn on. The
telephones that use for dial choice, disk or pulse system, can they open and
close the RL1 at the duration of choice. With delay network, that exist
between in gates IC2A and in the IC2B, we delay the situation changes in the
input of IC2B, ensuring thus stability in the operation of RL1. If the
R4=100K then the RL1 is activated when the telephone ring or when the
earphone is raised. On the contrary if the R4=1M, then the RL1 is activated
only when the earphone is raised. The circuit supply becomes with a simple
regulation circuit, in + 12V. |
