# Telephone Line Controller

Posted on Dec 15, 2012

A very simple circuit, that will find application in the case where you have a lot of telephones installed on a telephone line and would want to know if somebody of them is open. Thus you will not be off any discussion. Simultaneously it can cut the certain sound from stereo amplifier that are in high volume, the sound of some television or turn on some light the night when it ring the telephone and needs him you raise. 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. Part List R1-2=36Kohm D1....4=1N4002 RL1=12Vdc 2X2 relay R3=100Kohm D5=1N5252 [24V 0.5W Zener] J1-4=2pin connector 2.54mm step R4=100Kohm or 1Mohm *See text D6-7=1N4148 J2=6pin connector 5mm step R5=2.2Mohm D8....11=1N4002 J3=2pin connector 5mm step R6=3.3Kohm D12=Red Led 3 or 5mm F1=Fuse 500mA [5x20mm] R7=1Kohm...

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