Greetings All, I have an electronic thermometer that uses a piezo speaker for an audio alert when the temperature reaches a certain point. I would like to use the output to the piezo device to actuate a relay. I think the output is probably an AC signal. There are two wires, one red and the other black. So does this mean that the output is polarized? If the output is DC could a transistor be used as the switch to actuate the relay? And if it is AC would a full wave rectifier and filter capacitor driving a transistor work? Is there a better way? A simpler way? Thanks, Eric R Snow
I have an electronic thermometer that uses a piezo speaker for an
It will perobably be AC. (remove the piezo and see if it works with DC (some do) use the thermometer battery.
if it's AC use capacitors between the thermometer circuit and the rectifier, this way the relay and the thermometer can both be run from the same power source with short-circuiting one of the piezo wires.
Firstly, the signal depends on the piezo device itself.
Two types -
Transducer -
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Buzzer -
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If it is a piezo transducer then you need to feed a signal of the frequency you want to sound (typically a square wave).
However, it is a buzzer, then you just feed a DC voltage becuase the unit has the electronics to turn it into a sqaure wave.
Either way, these signals will be very low current and not enough to power a relay coil.
You will need to add a transistor stage to drive the relay coil.
If it is a transducer, its not true AC - just DC pulsed on an off at the desired frequency. So a simple RC circuit will probably be sufficient prior to the transistor.
Hi, Eric. I'm going to assume your thermometer has a 3V battery -- if it's different, it will change the way you look at this problem.
Most small piezo beepers are driven directly by two logic outputs from the microcontroller -- one on each side of the piezo element. To turn on the beeper, the red is made logic high (3V) while the black is logic low (0V). It then reverses polarity 6000 times a second or so, depending on the resonant frequency of the element. To the element, it looks like a 6V peak-to-peak square wave, which should be more than enough to drive many small elements.
You can use a small bridge rectifier to give you a 4.6VDC "ON" signal, which should be plenty to drive a darlington transistor. That should be able to turn on a reasonably-sized relay (opf course, you're assuming the thermometer battery isn't connected to anything else here, and you're using another power supply to drive the relay).
You can use a TIP110 TO-220 NPN darlington to drive up to 2A of relay coil current.
A simpler way (assuming a manual reset button is OK with you) is to use a sensitive gate SCR in place of the darlington. That will mean you can lose the 1uF filter cap:
This is pretty simple. If minimum component count is important, you can replace the resistors and silicon with a solid state relay that operates on a 3VDC input signal (you'd still need the 1uF filter cap). However, that will load down the thermometer battery more than these setups (the darlington only uses 1/3mA or so of battery current
-- the SCR about twice that). A SSR input will use several mA at least.
If you have a 1.5V supply, it becomes a little more complicated, because you only have a 3Vp.p. square wave. You'll then have to use schottky diodes and possibly a voltage doubler, especially if you want to drive a logic level MOSFET instead of a darlington transistor or SCR.
This avoids the bridge and double-diode drops that attend it and it takes some milliseconds of time to pull C1's voltage down. When the ringing stops, C1 gradually charges again. I figured these values for about 3kHz and 50% duty. (D1 gives C2 somewhere to send those electrons when Q1 isn't accepting them through its base.) Works for either 1.5V or 3V supplies and doesn't mind the polarity used.
Greetings Chris, Thanks for the detailed how-to. The thermometer is powered by 3 volts but I am replacing the battery with a wall wart. Since I have solid state relay, an OPTP22 brand I think, that's what I'll use. Cheers, Eric
The OP should note Mr. Kirwan's is a simpler circuit, and should work well, too. As he mentioned he's using a SSR, this circuit should easily drive the input.
Also, there's a small error in my SCR circuit above. The reset switch can either interrupt power to the circuit, or be placed between the SCR anode and the relay.
Thank You Jasen, Nigel and Jonathon. Your posts were all helpful. Your posts are what makes usenet great. People from all over the world helping strangers. And where I live I have nobody close by who can come over and show me how to do anything electronic. Cheers, Eric R Snow
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