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Using speaker signal to actuate switch?

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 24, 2007.

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  1. Eric R Snow

    Eric R Snow Guest

    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
     
  2. Jasen

    Jasen Guest

    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.




    Is there a better way? A simpler
    |
    something like this perhaps +----+
    | |
    / |
    || |/ |
    ----||--+-->|---------+---+-[100]----| |
    || | | | |\| / 2N2222A
    +--|<----. | | ~\ |/
    || | | === --|
    ----||--+-->|----|----' | 100nF |\|
    || | | | ~\
    +--|<----+--------+------------------+
    22nF 1N914
     
  3. Firstly, the signal depends on the piezo device itself.

    Two types -

    Transducer -
    http://www.maplin.co.uk/Module.aspx?TabID=1&DOY=24m6&ModuleNo=3202&criteria=

    Buzzer -
    http://www.maplin.co.uk/Module.aspx?ModuleNo=3211&criteria=piezo&doy=24m6

    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.

    Cheers,

    Nigel
     
  4. Chris

    Chris Guest

    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).

    Here's one way to solve the problem (view in fixed font or M$
    Notepad):
    |
    | .---o------.
    | | | |
    | | | RY1 |
    | 1N4001| C| |
    | - C| |
    | ^ C| |
    | | | |
    | '---o |
    | | |
    | | |
    | .---o |
    | .---------. | | +| 12VDC
    | Red | | ___ |/ | ---
    | o-------o------o~ +o----o---|___|-o-----| | -
    | | | | | 10K | |> | |
    | | | Bridge | | .-. | |/ |
    | ----o---- |Rectifier| +|1uF 10K| | o-| |
    | .-------. | | --- | | |> |
    | BZ1| | | | --- '-' | |
    | '-------' | | | | | |
    | ----o---- | | | | | |
    | | | | | | | |
    | o-------o------o~ -o----o---------o-----------o------'
    | Black | |
    | '---------'
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    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:

    |
    | .---o------.
    | | | |
    | | | RY1 |
    | 1N4001| C| |
    | - C| |
    | ^ C| |
    | | | |
    | '---o |
    | | |
    | | |
    | | |
    | .---------. C106B V +| 12VDC
    | Red | | ___ - ---
    | o-------o------o~ +o---|___|-o----------/| -
    | | | | 470 | | |
    | | | Bridge | .-. | |
    | ----o---- |Rectifier| 10K| | | |
    | .-------. | | | | | |
    | BZ1| | | | '-' | |
    | '-------' | | | | |
    | ----o---- | | | | |
    | | | | | | |
    | o-------o------o~ -o---------o-----------o------'
    | Black | |
    | '---------'
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    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.

    Good luck
    Chris
     
  5. Hi, Chris. I was thinking more along these lines:
    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.

    Jon
     
  6. Eric R Snow

    Eric R Snow Guest

    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
     
  7. Chris

    Chris Guest

    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.

    Cheers
    Chris
     
  8. Eric R Snow

    Eric R Snow Guest

    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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