VoIP Telphone ringer Klaxon

Thread Starter


Joined 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.





Joined 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.