40106 based monostable problems

Discussion in 'The Projects Forum' started by JingleJoe, Apr 20, 2012.

  1. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    I'm making a circuit using monostables to delay a pulse, which is about 1ms long; it activates the first monostable which is approximately 2ms long, which then activates another monostable with an output pulse length of 1ms again. This delays the first 1ms pulse by about 2ms which is plenty for what I want.

    Simple, right? Wrong. First one of these I built oscillated and wobbled so badly I just started again.
    This one works considerably better but I still get erratic oscillation, adding a pull down resistor to the input of the inverter after the first monostable helped, but not enough, I still get bursts of oscillation after some input pulses. The oscillations seem to come from directly after the first stage of the delay circuit, the 2ms monostable.

    Could this be down to the crappy ceramic caps ringing? Would perhaps polyester work better? Or maybe my scope is introducing some interferance which causes it? Or is there anywhere in my circuit which could use another pull down/pull up resistor?

    Tech notes:
    Vs: 12 V
    Chip: 40106 CMOS inverting schmitt trigger buffer
    Timing relationship of pulses shown on attached diagram.
  2. MrChips


    Oct 2, 2009
    Use a 4528 or 4538 dual monostable.
  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    You do not have a true monostable, only a "half monostable" or edge detector.

    For the half monostable to work correctly, the input pulse must remains longer than the half monostable output time. i.e. the input pulse is say 5ms and the output is 2ms, not the other way round.
  4. Bernard


    Aug 7, 2008
    Use a pulse stretcher for first stage [ integrator ], diferentite trailing edge & invert. Adj. 220k for desired delay.
    Hit wrong button
  5. Bernard


    Aug 7, 2008
    Sorry- lost thumbnail