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Combining two circuits

Discussion in 'General Electronics Discussion' started by waynebonnici, Sep 25, 2014.

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  1. waynebonnici


    Sep 25, 2014
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Wayne and welcome to Electronics Point :)

    This is a bit more complicated than you expect.

    First, the 555 is probably not the best monostable to use, because if the trigger signal remains active, the monostable will not time out

    Second, if you have a single fixed threshold on the light level, as the average light level passes near the threshold you will get multiple triggerings, because the light level picked up by the LDR will vary constantly and will cross the threshold repeatedly.

    The answer to this problem is to use a circuit called a Schmitt trigger, which has two separate thresholds - a rising threshold, and a falling threshold which is lower than the rising theshold. When the input quantity (ambient light level) rises above the risng threshold, the Schmitt trigger output goes active. Then, the input quantity has to go below the falling threshold before the output will go back OFF again.

    The output of the Schmitt trigger can be used to trigger a monostable.

    A 555 can actually be used as a Schmitt trigger, though you can't control the thresholds independently.

    I can draw you up a circuit using two 555s (or a 556, which is a dual 555) that will do what you want, if you tell me what you want, exactly.

    Do you want an output pulse when the ambient light goes higher than a certain level, or when it goes lower than a certain level?
    chopnhack and waynebonnici like this.
  3. waynebonnici


    Sep 25, 2014
    Thanks for helping! Helped a lot, I need the output when the light level goes down, but no need to help because I know how to make the Schmitt circuit. Thank you
    KrisBlueNZ and chopnhack like this.
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