Where in this thread was that ever mentioned?

What was mentioned?

 

I feel that just a basic circuit like this is worth starting with unless I'm missing the obvious, sorry about my drawing!

Just noticed but from that drawing it appears you are trying to power the relay coil directly through the OC. That's not ever going to work as the relay coils draws appx 75ma.

 

Go get the relay module with the OC input and try this circuit I posted earlier for pete's sake.

 

What was mentioned?

What you said in the post above my question: That the sounder needed to be isolated from ground. As far as I can tell, that is an assumption you made.

 

That is correct sir.

 

Hi Sghioto,
Thank you, you had it exactly right!
I followed your circuit with the 10K but nothing so changed it to 47K and I have a triggered relay!!! Perfect!
The relay draws 3ma while idle which, no doubt, will reduce battery life slightly but I can address that if it becomes a problem.
This has been my third request for help on this forum and each time I have had amazing effort from its members, solved issues and learnt so much. I will, no doubt, will find something new to ask for help with and know this is the place to come.
Thank you again and everyone else who contributed.
Peter

 

I followed your circuit with the 10K but nothing so changed it to 47K and I have a triggered relay!!!

Are you sure? With 29V and a 47K resistor, the max current is 0.6mA.

 

I followed your circuit with the 10K but nothing so changed it to 47K and I have a triggered relay!!!

I think you meant to say a 4.7K resistor.

The relay draws 3ma while idle which, no doubt, will reduce battery life slightly

That's from the LED that indicates the module has power. You can de-solder the LED or resistor associated with it for zero idle current.

 

This is a photo of the inside of the siren unit. Does this match your alarm?
Read everything you see on sounder.
Should be possible to connect directly across the sounder to access a voltage to drive a mosfet or relay circuit.
View attachment 299583

I just saw this thread this morning. The text that we see part of on the back of that black sounder package will tell us exactly what the voltage feeding it is, and then it will be simple to creat a circuit to interface it with.

 

I just saw this thread this morning.

Did you read post #86?

 

Hi Sghioto,
Thank you, you had it exactly right!
I followed your circuit with the 10K but nothing so changed it to 47K and I have a triggered relay!!! Perfect!
The relay draws 3ma while idle which, no doubt, will reduce battery life slightly but I can address that if it becomes a problem.
This has been my third request for help on this forum and each time I have had amazing effort from its members, solved issues and learnt so much. I will, no doubt, will find something new to ask for help with and know this is the place to come.
Thank you again and everyone else who contributed.
Peter

 

Sorry, typo, yes 4.7k.
That’s a good idea, it’s not seen, I’ll de-solder it.
thank you again.
Peter

 

Did you read post #86?

Yes, and while it seems that a suggestion has worked, evidently I should have followed the thread to find the correct response. Ther seem to have been a few guesses along the way.