Question about caps

Discussion in 'General Electronics Chat' started by jpitz31, Jan 31, 2011.

  1. jpitz31

    Thread Starter Active Member

    Oct 24, 2007
    Hello All I am working on a problem in the Make Electronics book. Chapter 5 page 286. Enclosed is a schematic of a circuit that uses a 555 timer to rotate a stepper motor.

    I understand how the rc circuit works on pins 6 and 7. The cap and resistor is used to determine the length of time the output pin stays high (pin 3)

    But I am having trouble understanding how the cap between pin 3 on one 555 timer is being used to trigger pin 2 on the next 555 timer, to go low.

    Since positive current is routed to all pin 2's, when pin 3 goes low after being turned off on the first 555, how does the cap between pin 3, on the first 555, cause pin 2 on the next 555 to go low. Does the positive current flow to the cap instead of pin 2? How does charging and discharging between the cap cause pin 2 to go low? Having trouble picturing this in my mind.


  2. mbohuntr

    Active Member

    Apr 6, 2009
    It's my understanding that "edge triggering" utilizes a capacitor's ability to pass AC that causes it to momentarily drop voltage in response to a rapid change in voltage on the other pole. Bill and others will have a much better explanation, but I'm a long way from that point.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    The voltage across a capacitor cannot change instantaneously. Charging a capacitor requires...guess what...charge: the integral of current over time. As the voltage at one end of a coupling capacitor is abruptly pulled down by the 555, the voltage at the other end of the capacitor must also fall.

    The resistor at the input of the next stage will tend to pull the voltage back up, but this takes time. Provided that the fall time of the driving voltage is short compared to the time constant of the coupling (R times C), the result will be a momentary low at the input.
  4. jpitz31

    Thread Starter Active Member

    Oct 24, 2007
    Thanks Adjuster and mbohuntr for the info on coupling capacitors.