555 monotable with resetting timer?

Discussion in 'General Electronics Chat' started by MikeA, Jan 20, 2013.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Hi folks, I'm trying to build a timer circuit that will be triggered a signal from a motion sensor. I'm having trouble finding any circuit designs that would allow me to have a timer, but allow the timer to be reset every time motion is detected.

    Having the timer reset is kind of important, as most 555 circuits I've seen do not respond to new triggers until the timer completes. So if the timer controls the lights, the room can potentially go dark with someone in it.

    What ideally should happen is that every time motion is detected, the timer period should restart. How can I make that happen? :D
     
  2. crutschow

    Expert

    Mar 14, 2008
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    MikeA likes this.
  3. MikeA

    Thread Starter Member

    Jan 20, 2013
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    That's brilliantly simple! I was thinking of exactly the same thing, that the timing capacitor needs to be discharged, then the 555 triggered again. I'll give that one-shot circuit a shot! :D (google somehow could not come up with that circuit)

    On the same note, the motion sensor that I'm using, outputs a 5v pulse when motion is triggered. Since I need to bring trigger to ground in the 555 circuit, I need to invert the output obviously. I can do that with a transistor. But is there a more elegant solution?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    I just Googled "retriggerable one-shot".

    Can't think of a better way then just adding an NPN inverter in place of the "trigger switch" to get the desired reset polarity.

    You could also use two NPNs such as a 2N2222, one for the trigger and one for the capacitor dump. In that case the capacitor dump NPN would also be controlled by your 5V signal (through a separate base resistor). That would actually work slightly better since the capacitor discharge would be more complete. (The PNP can only discharge to about 0.6V whereas the NPN should discharge to its saturation voltage of <0.1V.)
     
  5. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Did I get it right? :D
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Yes. But you don't need the resistors to ground unless you are operating the circuit at a high temperature and need to sink any base leakage current.
     
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