Trigger 12 volt relay from output of countdown timer

Thread Starter

bosko47

Joined Mar 24, 2016
23
I'm trying to trigger a relay using the piezo buzzer output of a countdown timer to put 12v across an external bell.
Timer.jpg Timer inside.jpg

I have used the this relay driver circuit in the past to do a similar thing with a wireless door bell, but the doorbell receiver was 5v and I used the speaker output as the driving signal. The countdown timer is only using a 1.5v button battery.
Relay driver.jpg
 

DNA Robotics

Joined Jun 13, 2014
614
Can you get a measurable voltage to the piezo?
It looks like there is a 1K resistor missing on the + input of your relay board.
Or is that the 1K resistor between the capacitor and the 4N28 optocoupler?
If the signal to the piezo is low voltage, maybe use a 220 ohm resistor in place of the 1K.
 
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Thread Starter

bosko47

Joined Mar 24, 2016
23
I get a pulsing voltage to the piezo, starts at about 1v then drops down to about 0.2v

I've got a wire link where the 1k resistor should be as I was going to try some lower value resistors as you've suggested, but I'm researching some other methods first. My main concern is to not try and pull too much current from the button battery powering the timer. I really just want a circuit that senses the output from the timer without loading it down.
 

AnalogKid

Joined Aug 1, 2013
9,492
There is an good chance that your circuit in post #1 will work with minor modifications. But first, more data.

1. If you connect a 1.5 V battery to the beeper directly, does it beep?

2. Can you determine with a meter if either of the two beeper connections on the pc board are tied directly to either Vbat or GND?

ak
 

Thread Starter

bosko47

Joined Mar 24, 2016
23
Finally got some spare time to have another look at this. I can't get the piezo to do anything by putting 1.5v across it. One of the piezo connections on the timer board is tied to Vbat.

I was able to get the relay chattering with the timer output, put straight, no 1k resistor, into the circuit inputs. I played around with some input resistors, up to 1k, but got the same relay chattering result.
 

AnalogKid

Joined Aug 1, 2013
9,492
No beep means that your piezo device is a bare piezo element, not an complete beeper with the oscillator built in. That means the timer is sending a square wave to the beeper. That means that the relay chattering is the coil trying to keep up with the pulses. It wants relatively clean DC, and is unhappy.

What you need is a circuit that either converts the timer square wave signal to clean DC, or one that senses the signal and switches DC from the battery to the relay. More correctly, it switches the ground end of the relay coil because one of the beeper contacts already is Vbat.

If the batt voltage were higher this could be done with nothing more than one diode and one capacitor, but there will be too much voltage drop across the diode, leaving not-enough voltage across the coil.

That leaves the detector/driver approach. Wally has a version of this in post #5, but I think it can be more simple. No schematics today, so here is the text:

Beeper output signal
Series resistor
Capacitor to ground
NPN transistor base
Emitter to ground
Collector to relay coil
Other end of relay coil to Vbat
Small diode across relay coil (1N914, 1N4148, etc.)

ak
 

Thread Starter

bosko47

Joined Mar 24, 2016
23
Went with this design. Seems to do the job. 1.8mA current draw in stand-by, 120mA with relay activated. Only 1.1uA from the timer required to trigger the circuit.

circuit_1.jpg
 
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