Message Waiting LED on Simple Office Phone?

Edwards Finance > Phones

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.

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. FYI -- the nominal voltage of the line (when onhook, and no message-waiting signal) is probably between 40 and 53Vdc. 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.. I had a similar requirement for home use, as raised in news:a2nfutkb27qcg68sgmk2nktto63qvr3i8m@4ax.com A little later, on 30th Nov 2001, in 'Detect 'phone line status' in message news:e65f0u0ktisfdikjh12fka957mlij2u4g7@4ax.com I see I rashly wrote: "Breadboarding today, and will post result later." Nearly a year on, the circuit I was developing remains only about half complete, as I shortly afterwards got distracted with something else. But the approach I summarised in that last thread still seems valid. (If/when I master PICs, that could become another contender.)

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