VoIP Telphone ringer Klaxon

Discussion in 'The Projects Forum' started by Virgil, May 4, 2007.

  1. Virgil

    Thread Starter New Member

    May 4, 2007
    The long and short of it is that I'm trying to get a 240 Vac Klaxon to sound when a Cisco VoIP phone rings. We've looked at all sorts of solutions which I'll not bore you all with - what we're looking at, at the moment, is:

    we have a 1.8v square wave that comes into two LED's on the top of the unit, I can intercept them and present them on the back of the phone via a 3.5mm jack-plug arrangement.

    These LED's flash when the phone rings, at about a 1:2 duty-cycle - the plan was to push this through a rectifier and then a transister hooked to a local 12v DC power supply, this in turn would turn on a dc-controlled 240 Vac solid state relay. All sounds good, yes? Attached is a rough circuit of what I had in mind...

    Then came the killer blow, the LED's will also light if the user has a voicemail message (i.e. 1.8v dc instead of the squarewave) - in my mind, this means that the klaxon will sound constantly when the voicemail light is lit (nice for getting them to answer their voicemail I admit, but I don't think they'll like it!) since the 1.8v dc will go straight through the rectifier bridge and hold the transistor/relay open... ...at least that's what I think, to be honest it's been a while since I last worked with rectifiers.

    Does anyone have any suggestions for solving this, I guess I could use something like a small transformer to de-couple it, but with the voltages I've got I don' t know whether that will work...

    Please don't suggest that I stop these phones having voicemail - they're generally large, hairy engineer-types and I don't want to even go there...

    Any help appreciated.


  2. thingmaker3

    Retired Moderator

    May 16, 2005
    What if you trigger a monostable with the square wave and AND the two together to run your SSR?


    Or you could use a NAND and a 556, & use the second timer as a pulse-stretcher.