How to reset an astable 555?

Discussion in 'The Projects Forum' started by hrs, May 21, 2016.

  1. hrs

    Thread Starter Member

    Jun 13, 2014

    In an ongoing project I've made a solid state relay for muting audio speakers because of turn-off thump. The large filter capacitors of the power supply will be by-passed with a full wave rectifier to be able to detect turn-off before the main supply voltage collapses. Please refer to the attached diagram. This by-pass should yield about 26 VDC.

    I want to use this 26 VDC signal to switch on/off the 555 that drives the relay. But how do I pull the RESET pin below 0.7 V? With a 2N3906 I can get it to work in a simulation if the base resistor value is low enough (<1k Ω). But since the voltage drop across the 2N3906 is around the RESET threshold value this will probably not be The Right Way.

    Swapping the 2N3906 with a p-channel enhancement MOSFET makes things worse which I don't understand. LTSpice has the BSS84, I have some BS250s. They seem close enough. The RESET voltage drops to only ~2.4 V. If the Rds_on is about 10 Ω and R3 is 10k Ω, why is V(n002) not 15*10/10k = 0.015 V?

    Is it a problem to drive the base/gate with a voltage much higher than the collector/source voltage? I think Vcbo does not apply here as the junction is reverse biased.

    If you have a suggestion for a better way to do this, I'm open to suggestion! But it would need to work with the existing 555 circuit.

  2. crutschow


    Mar 14, 2008
    To pull the Reset to ground so you use an N-MOSFET.
    A P-MOSFET won't work, since it can't pull the source any lower than a threshold voltage above ground, as you've found.

    Note that you can label a node with any word you want using the
    (F4) option, making it easy to identify the voltages in the plots.
    hrs likes this.
  3. #12


    Nov 30, 2010
    There are npn transistors with low VceSat like a 2N2222 which stays below 0.2 volts up to 200 ma.
    You just don't know about them yet.;)
    hrs likes this.
  4. Colin55


    Aug 27, 2015
    Basically the reset pin has 950mV on it when not connected to anything due to the components inside the 555.
    It will deliver 350uA due to this condition. You no not need a pull-up on this line.
    To create a 555 reset, the line must see a voltage below 500mV.
    Most NPN transistors will produce 350mV or less C-E and can be used.
    hrs likes this.
  5. hrs

    Thread Starter Member

    Jun 13, 2014
    I tried some n-devices before but it didn't work somehow :oops:
    Now it works perfectly with either nmos or npn! Thanks!