Telephone Hold Line

The UNIVERSAL HOLD circuit detects the dual tone, multi frequency (DTMF) tone that is generated when the # key is pressed and then activates a circuit that partially loads the telephone line so that the central office thinks a phone is still off hook even after it is hung up. When any phone is again picked up, the hold function is canceled.
Telephone Hold Line - schematic

Figure 1 shows the schematic diagram of the UNIVERSAL HOLD circuit. The telephone line is connected to the hold components through bridge rectifier BR so the input is not polarity sensitive and the input lines may be connected either way. BR always connects the positive side of the line to zener diode D2 and the negative side of the line to IC2. If L1 is positive, current flows into the circuit through bridge diode A and back out through diode D into L2. If L2 is positive, current flows in through diode C and out through diode B into L1. The telephone line also connects to tone decoder IC4 through C4 and T1. Optoisolator IC1 contains a silicon controlled rectifier (SCR) having a characteristic that makes it continue to conduct current, or latch on, once turned on by current flowing through an optically coupled LED within the IC. The SCR conduction continues even after the LED current is removed, providing enough anode current is available to sustain it. When the SCR anode voltage is removed and conduction stops, it will not resume when voltage is re-applied until again triggered by LED current. This characteristic makes the SCR optoisolator ideal for a line hold application. When all phones are hung up (on-hook) the voltage across the telephone line is about 48 volts. When a phone is picked up (off-hook) it places a load on the line and a current of about 20 milliamps flows through the phone which...

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