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556 Trigger Voltage Question

Discussion in 'Electronic Design' started by Terry, Oct 24, 2004.

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  1. Terry

    Terry Guest

    Hacked a motion detector for a Halloween prop. Shows 24 volts when I
    measure the two wires I soldered on the detector circuit board. When
    the detector detects someone, the voltage drops to zero.

    Put together a standard non-retriggerable timer circuit using a diode
    off the second output to control the non-triggerable duration per
    responses to a previous message. Running this circuit at 9 volts.

    When I fasten the positive from the detector directly to the triggers
    and the negative to the common ground of my circuit, it reads 24 volts
    until something detected and then drops to only 7.5 volts. This is not
    enough to set off the timers.

    Not being an electrical genius, can someone explain to me why the drop
    to 7.5 volts instead of 0 when I hook the detector to my circuit?
    What's the best way to hook up this detector to the circuit to trigger
    the timers?

    Thank you for any help.

  2. Can't say without seeing your schematic. In particular, you've not
    stated what supply voltage you're using for the timer circuit. I'm
    assuming not 24V, which would be beyond the 555's capability.

    Possibly you're not edge-connecting the trigger(s)? IOW, using a
    capacitor to isolate the DC voltage level and just pass the negative
    edges to the 555 trigger(s). A simple example is shown here:

    If correctly configured, a brief 24V to 0V pulse from your motion
    detector should satisfactorily trigger a 555 mono using a lower supply
    (e.g. 9V). BTW, to be on the safe side, I'd also connect a diode from
    the 555 trigger(s) to the +ve supply. (I assume you *have* connected
    the grounds of both sections?)

    If you post/publish your schematic, we can probably be more helpful.
  3. Fred Bloggs

    Fred Bloggs Guest

    You don't think it's important to tell us what you have hanging off the
    555 TRIG input besides that diode? Whatever it is, it's loading the
    motion detector output with too much current. If you are using the
    standard 555 , then the diode is all that you need- you can run the
    24VDC line directly through the diode to pin 2 TRIG of the IC and
    nothing else (no pullup to Vcc required)- it makes no difference what
    power supply you use for the 555 just so long as it is less than 24V.
    This will cause the 555 monostable to output a pulse that is the longer
    of the motion detector and the 555 RC network. You do notice that the
    diode has a *band* on it and this is the cathode which connects to the
    motion detector.
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