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help understanding the monster I created :-)

Discussion in 'Electronic Basics' started by Tom Horsley, Jan 20, 2005.

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  1. Tom Horsley

    Tom Horsley Guest

    I've been working on a silly gadget using the cut & paste bits
    of other circuits technique combined with a large does of
    ignorance :).

    The gadget details can be found here:

    With my breadboarded circuit, I have been observing a
    phenomena that my tiny little brain can't figure out.

    When I expose the cadmium sulphide photoresistor to light,
    my relay triggers once (briefly) as expected and desired.

    However, fairly often (not all the time), when I put the
    lens cap back over the resistor and block off the light,
    the circuit will trigger the relay again.

    The only theory I have (with no access to a scope that
    can record whats happening at different points and show
    me) is that the photoresistor resistance actually bounces
    around some as it gets dark rather than simply smoothly
    going back up. Could that be it? Or has the ignorance part
    of my design process merely led me to do something
    silly along the way?

    Any ideas for improvements to my design or explainations
    of this puzzling behavior gratefully accepted. (I doubt
    it is gravity waves though :).

    (Perhaps chuck the whole thing and detect incoming calls
    via the headset interface is a better plan - but I'd have
    to find out about interfacing to cell phone headsets
    for that to work...).
  2. Andrew Holme

    Andrew Holme Guest

    Yes, that's correct. The LM339 is very sensitive: it responds to very
    small differences between the inputs. As the photocell resistance
    passes through the critical threshold, the slightest "bounce" is enough
    to momentarily push it back below the threshold. To overcome this, you
    should add a little hysteresis by connecting a very large resistor (at
    least 100k, maybe even 1M) between the output and the non-inverting (+)
    input. The on-off and off-on transitions will then occur at slightly
    different points e.g. like a thermostat.
  3. Jamie

    Jamie Guest

    i just looked at the circuit you have there. from what i can
    see, puting a small cap from the output of the OP-amp to the
    - input could increase the response time thus absorbing the
    quick pulses you may be getting.
  4. To overcome this, you should add a little hysteresis by connecting a very
    Thanks! That did indeed do the trick. In another Doh! moment, I notice that
    the web page I point to with info on comparators even describes the reasons
    I'd want to add hysteresis, but I never made the connection :).

    the comparator page I mention is at:
    --email: icbm: Delray Beach, FL |
    <URL:> Free Software and Politics <<==+
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