Put that phone on hold! (telephone hold circuit)(Cover Story)

If you have touch-tone telephone service, you can now put a call on hold from any phone in your house by plugging this simple device into any telephone jack. The universal hold circuit works with any phone that has a keypad with a # key. To put a call on hold, press the # key and hang the phone up. A timer extends the #-key function while you hang up phones that have a keypad built into the handset.

The universal hold circuit first detects the dual-tone multifrequency (DTMF) signal that is generated when the # key is pressed. It 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. The hold circuit remains active for five seconds after the # key is released so the key does not have to be held down while the phone is being hung up. When any phone is again picked up, the hold function is canceled.

How it works

Figure 1 shows the schematic diagram of the universal hold' circuit. The telephone line is connected to the hold components through bridge rectifier BR1 so that the input is not polarity sensitive. The positive side …