555/556 reset question

Discussion in 'General Electronics Chat' started by jrdoner, Jul 3, 2013.

  1. jrdoner

    Thread Starter New Member

    Feb 21, 2012
    I am trying to trigger a 555 by first delaying its activity using the RESET pin. I begin by charging a capacitor from 0 v., and apply that value to the RESET pin. Eventually, the RESET pin considers itself as HIGH, and the timer goes about its business. In a simulation, the changeover point seems to be about 2/3 of supply voltage.

    I have read several data sheets and can't any explanation of the low/high transition point on the RESET pin. The sheets usually give a specified range for what LOW is, but does not deal with the HIGH transition.

    Does anybody out there know at what voltage a 555/556 RESET pin would normally transit to chip in active status? I suppose it is also a function of chip technology.

    Thanks in advance for any help with this.
  2. Ramussons

    Active Member

    May 3, 2013
    All Digital Inputs have a Low Threshold below which Logic is 0; a High Threshold above which Logic is 1. Between the two thresholds is a range that CANNOT be defined as Lo or Hi for the reason, that for a particular voltage in that "grey" area, some samples of the same component may respond to it being HI, others LO.

  3. LDC3

    Active Member

    Apr 27, 2013
    Since the Reset line is connected to the base of a PNP transistor (according to TI's datasheet), a voltage 0.7V above the low voltage level should allow the timer to work. But for best results, use a voltage as close to Vcc as you can.
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Yep agree with ldc3, the minimum low threshold is 0.7V for reset signal.
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Using a large cap, 10μF ?, in shunt with reset works for powerup reset, but adding an IC output for reset allows more versitility. Still need a hi value pullup resistor with cap.
  6. elec_mech

    Senior Member

    Nov 12, 2008
    This site looks useful for hooking up and determining an ideal R and C value.