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Message Waiting LED on Simple Office Phone?
Q. The telephones in our office are simple phones, like the "2500" plain old touch-tone telephone, but they have one additional feature: a "message waiting" LED that flashes when the PBX tells the phone that there is a message stored in the PBX. What is the signal that drives this LED and what is the schematic of the LED circuit to detect this signal in the telephone? I'd like to add an LED to a non-LED phone and want to wire it the same way that it is the "normal" telephones, rather than "re-invent the wheel".
A. If it's a 2500 style set then chances are that the signal that lights the lamp is high-voltage pulsating dc -- pulsing at the rate that the lamp turns on and off. As I recall, it's about 130Vdc. The old style message-waiting lamps were neon bulbs. I think there is supposed to be a current-limiting resistor in the line equipment. The nominal voltage of the line (when onhook, and no message-waiting signal) is probably between 40 and 53Vdc. So, a circuit that should work is: * Bridge rectifier -- connect the two "a.c." leads to the two leads of the phone line. * Zener diode (about 70V, 1W should do) -- connect the cathode to the (+) lead of the bridge rectifier. This zener will keep your LED from being lit by the onhook line voltage, but will allow current to flow when the message-waiting signal is applied (and also, when the phone rings). * Resistor (about 10K, 1/4W) (the smaller this is, the brighter the LED will be) -- connect one end to the anode of the zener. * LED (a blue one would look cool) -- connect the anode to the free end of the resistor, and the cathode to the (-) lead of the bridge rectifier. If it's a modern "message waiting" feature.. when you pick up the phone, if a message is waiting, you have a special dial-tone for a second or 2...then it goes back to a regular dial-tone. What those phone do is..: after each phone call you make.. then will wait 30 seconds or so, then pick up the line, and "listen" to that specific dial-tone. if it's present they turn on the LED, or a circuitry that flash an LED... if it's absent, it will be turned off..
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