Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: John McHarry Newsgroups: comp.dcom.telecom Subject: Re: Need a Light to Indicate Phone *Has Rung* (Not *is Ringing*) Message-ID: <1913@accuvax.nwu.edu> Date: 6 Dec 89 18:36:56 GMT Sender: news@accuvax.nwu.edu Organization: TELECOM Digest Lines: 33 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 9, Issue 558, message 3 of 4 I haven't had a chance to build this yet, but: Most US subscriber loops have about -52v on the ring and ground on the tip when on hook. When off hook, the voltage drops to something less than 8-9 volts. Ringing is accomplished by superimposing a nominal 86v rms on the -52v ring-tip voltage. An indicating device that turned on at something greater than 52v would accomplish the task, if it could be kept on by an external power source or by the 52v on hook voltage, the latter requiring very low current draw to avoid appearing off hook. As I recall, a device that can do this is the old NE-2 neon bulb. It requires something over 80? volts to turn on, but can be kept lit by well under 1 ma. The bulb, in series with a 100k resistor should be put in series with a diode good for at least 200v. Put about a 2 micro farad cap across the resistor and lamp part of the circuit. Hang the whole thing across tip and ring with the cathode end of the diode hooked to ring. If the lamp is so sensitive as to light with just the 52v, a 100-200k resistor across the lamp should solve the problem. The diode and cap are in the circuit to keep the positive cycle of the ringing voltage and short interruptions of the 52v from extinguishing the lamp. Taking a phone off hook should automatically extinguish the lamp by interrupting the -52v long enough for the cap to discharge. As I said, I have yet to build this, but it should work. It may be subject to some falsing due to lightening, etc. Also, it should be checked for immunity to 'dial tap'. It will indicate an incoming call attempt, but not whether there is voice mail, of course. (The usual disclaimers)**2 and do be careful, a telephone line can bite like a light socket...remember the apocryphal dog. *************************************************************** * John McHarry (703)883-6100 M21198@MWVM.MITRE.ORG * ***************************************************************