LL on/off for 555 PWM?

Discussion in 'General Electronics Chat' started by elRey, Sep 16, 2009.

  1. elRey

    Thread Starter Active Member

    Feb 23, 2009
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    Using this circuit provided by Bill:

    I will set a duty and frquency. What I need now is a way to turn the PWM on or off via logic high or low. I will have a 0-5v signal. When that signal reaches, say 2.2v, I want the PWM to turn on. And when the signal is below 2.2v the PWM turns off.

    Again, thanks in advance.
    Rey
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The easiest way is with pin 4 on the 555, which is a reset pin for the 555 (basically a digital input).
     
  3. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    Great. So, I need the wave from the 555 to go to V+ input of the comparator to make it work like I want. When 555 reset is high, 555 output is low. If that went tp V- input, the output of circuit would be high. But I need it to be low. ... I need to switch my comparator input on my circuit.

    Thanks again Bill!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    When the 555 reset (pin 5) is connected to Vcc (high), the 555 timer will run.
    When the reset pin is held low (0v), the 555 will not run, and pin 3 will be low.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Thanks Wookie. This is true.

    If the logic state of the comparitor is the problem switch the comparitors inputs. Problem solved.
     
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