one more smoke alarm question

[firealarmfreak]

Hey, I just got a cheap 7 dollar smoke detector from wal-mart and I was trying make it activate a relay instead of the piezo when smoke was sensed.

There was 3 wires going from the smoke alarm board to the piezo and I snipped them and connected two of them to the relay coil and when i pressed the test button, the relay just made a weird pulsing noise, but never switched, but when i used an led, it worked fine.

The relay worked fine as long as i touched my thumb on this piece of metal from one of the wires (there was 3, 1 did not work, so i used the other that worked), however when the other two were connected, i had to put my thumb on the one that did not work (the metal part where the wire was soldered) to make the relay click on and off.

Any ideas? do i need a transistor? Im using a 5 VDC SPDT relay, and the smoke alarm is a 9 VDC detector.

Chris.

 

[Boncuk]

Hi Chris,

obviously your relay pulls too much current from the circuit. If the buzzer is a self contained device with built in oscillator circuit the output of the circuit should change state in any direction - high or low - which you should check just in front of the buzzer against ground.

If the change is from low to high you can add a small NPN power transistor like a 2N2222 to switch the relay. Use an adequate base resistor of 2K7 to 5K6 (experimenting) to saturate the transistor with the output active. (Can be determined without measuring by listening to the relay click.) Don't forget to connect a free wheeling diode across the relay.

Boncuk

 

[jpanhalt]

Here is a smoke alarm circuit from the Internet:

**broken link removed**

I realize it uses a speaker and not a piezo. The point to be made is that that circuit will not pass DC current to activate a DC relay. Your circuit may have a similar oscillator/piezo driver. In order to help you more, we need to know a bit more about the circuit in your smoke detector.

Can you post a picture of the circuit board? Can you find a schematic of it? I suspect it is made in China, so there may be some information in the owner's manual. What is the coil current rating of the relay you are trying to use? Any additional information would be helpful.

John

 

[audioguru]

The piezo transducer has 3 wires. Signal, ground and feedback. It is driven from a chip or a transistor on the main circuit board.

The relay coil might be able to be turned on by the transistor if it is connected properly.
The high voltage created by the relay coil when it turns off will probably fry the transistor if a reversed diode is not connected across it.

 

[Hero999]

The circuit is very simple - just one transistor.

 

[firealarmfreak]

can someone make me a simple diagarm on how to connect it, so the smoke alarm activates a transister which activates the relay... i have been told that works, but i dont know how to do the smoke-transister-relay circuit.

Chris.