A reading of 29VAC on most multimeters is actually 200V or more since most multimeters are made to accurately measure 50Hz or 60Hz. The sounder probably is driven with and produces 3kHz.
Many multimeters do not accurately measure a sinewave or a squarewave. An oscilloscope like I said will measure them fairly accurately.

 

That will work for the 5 volt relay module. Can you post a close up HD photo of the pc board inside the siren unit?
There maybe better sources to use to activate the relay module.

 

A reading of 29VAC on most multimeters is actually 200V or more since most multimeters are made to accurately measure 50Hz or 60Hz. The sounder probably is driven with and produces 3kHz.
Many multimeters do not accurately measure a sinewave or a squarewave. An oscilloscope like I said will measure them fairly accurately.

Thank you for your help and comments
I think I’ll try a 2 diode, half wave rectifier and see what voltage I get. Hopefully it will be enough to power the 5v normal relay otherwise I’ll go for a 4 diode bridge and see if that works.

 

If the sounder is piezo then its driving current is too low to drive a 5V mechanical relay coil.
A Mosfet can drive the relay if you attenuate its gate voltage enough.

 

That will work for the 5 volt relay module. Can you post a close up HD photo of the pc board inside the siren unit?
There maybe better sources to use to activate the relay module.

Here’s a picture of the board:

 

Can't really tell from the photo.
1/2 wave rectifier should only need 1 diode. Pretty sure the relay module is going to load the sounder too much.
Is this circuit you have in mind?

 

Can't really tell from the photo.
1/2 wave rectifier should only need 1 diode. Pretty sure the relay module is going to load the sounder too much.
Is this circuit you have in mind?
View attachment 299703

Yes, I don’t want any induction feedback when it stops so that looks good what value R & C do you think I need?

 

Yes, I don’t want any induction feedback when it stops so that looks good what value R & C do you think I need?

I believe the optocoupler relay wouldn’t load the sounder too much

 

I believe the optocoupler relay wouldn’t load the sounder too much

Based on what?

 

That relay module draws 75 ma when activated. It doesn't make any difference if you use the 6 volt battery to power the relay module or voltage from the sounder as the current required is the same and that is supplied by the battery. I'll draw up the circuit I think is needed here shortly.

 

Based on what?

It only needs to light a LED without it flickering

 

It only needs to light a LED without it flickering

Why are we talking about a relay module?

 

That relay module draws 75 ma when activated. It doesn't make any difference if you use the 6 volt battery to power the relay module or voltage from the sounder as the current required is the same and that is supplied by the battery. I'll draw up the circuit I think is needed here shortly.

That would be very good of you, thank you.

 

Why are we talking about a relay module?

Sorry, I’ve been back and forth with a normal one and a optocoupled one. From here on I’ll work with the opto version

 

Sorry, I’ve been back and forth with a normal one and a optocoupled one. From here on I’ll work with the opto version

I believe the optocoupled relay triggers at just 5ma which I hope isn’t too much for the sounder

 

I'll draw up the circuit I think is needed here shortly.

 

If you can power opto-coupler, you can power the LED directly. The opto input IS an LED.

 

If you can power opto-coupler, you can power the LED directly. The opto input IS an LED.

Correct but the opto in the relay module is not isolated. It's referenced to the negative supply.

 

Correct but the opto in the relay module is not isolated. It's referenced to the negative supply.

 

Correct but the opto in the relay module is not isolated. It's referenced to the negative supply.

Thank you for your replies.
Tomorrow I’ll try the suggestions and advise how I got on.
Peter