555 Bistable Random Power-Up State

Discussion in 'The Projects Forum' started by apqo1, Oct 16, 2013.

  1. apqo1

    Thread Starter Active Member

    Oct 5, 2008
    I'm working on a bistable latch circuit to switch a MOSFET on/off with a momentary push-button switch. I've experimented with every BJT and FET-based "soft latch" circuit I can find; some don't work at all and others are intermittent or difficult to debounce.

    I finally settled on the attached 555-based circuit. It works beautifully while powered, and I can't make it misbehave no matter how hard I try to bounce the switch. However, there's one problem: it sometimes starts in the "on" state when power is applied.

    I've tried increasing decoupling cap C2 to as high as 220μF. This improves it a little, but if power is removed for more than ~15 seconds, it will still sometimes start up in the "on" state.

    How do I guarantee that the circuit starts in the "off" state when power is applied, regardless of how long it's been removed?

    Last edited: Nov 25, 2013
  2. #12


    Nov 30, 2010
    Use a resistor and capacitor to delay the reset pin coming up to "high" during power up.
    KJ6EAD and apqo1 like this.
  3. apqo1

    Thread Starter Active Member

    Oct 5, 2008
    Well, duh! Thanks, #12.

    The 555 datasheet says RESET voltage is 0.7V typical. I put 1MΩ/0.1μF on RESET for ~5 millisecond delay reaching 0.7V at power-up.

    Works perfectly! New schematic attached.

    EDIT 1: Substituted 100kΩ/1μF to achieve the same RESET delay with parts already used elsewhere in the circuit.

    EDIT 2: Added scope capture of the result. CH1 (
    yellow) is Vin; CH2 (blue) is RESET. Nice to see the math work perfectly: 0.7V in 5.2 msec.
    Last edited: Nov 25, 2013
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Just using a cap on pin 5 (which is almost mandatory for 555 use anyway to reduce noise issues) could have got you the same result.
    apqo1 and #12 like this.