Subject: Re: Message Waiting LED on Simple Office Phone?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sun, 08 Sep 2002 22:23:48 GMT
NNTP-Posting-Date: Sun, 08 Sep 2002 15:23:48 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
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.
So, a circuit that should work is:
* Bridge rectifier -- connect the two "a.c." leads to the two leads of the
* 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.
Hope this helps.
"Bob Neidorff" wrote in message
> 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".
> Thanks for your help.