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Monostable 555 Timer Problems

  1. Apr 17, 2007 #1
    So I will start off saying that I am not a genius when it comes to this stuff quite yet, but i have done several EE classes(slow but its going) and tinker around for fun.

    But for some reason this 555 Timer(i have a 556 Dual Timer) is just not working and i cant tell if its a faulty chip or what.

    I am hooking up this circuit from figure-11 mid way down the page to create a monostable output for a circuit i am working on.

    http://www.uoguelph.ca/~antoon/gadgets/555/555.html [Broken]

    Now from what i understand what should happen is when i get it all hooked up it should begin charging the capacitor to ~2/3rds of Vc as well as put the output high, then inside the timer it should flip back and thus drain the capacitor back to zero and turning off the output.

    From what i can tell using those values it should last approximately 7 seconds. But this is what happens.

    I setup my multimeter to watch across the capacitor, and short the trigger very quickly thus starting the charge process and turning on the output & LED. I watch it climb and climb until 2/3rds then nothing happens, the voltage SLOWLY starts to drain out of the capacitor but the output stays on. I am sure if i left it on long enough the capacitor would fully drain and turn off the circuit but we're talking atleast 5 minutes to do so.

    What could i possibly be doing wrong? I have tried both sides of my 556 chip(that i got from Radioshack) and they both do the same thing. Forcing reset low does immediately drain the cap and turn off the output, but for some reason the stupid chip never does it itself.

    I have gotten some info that the 555 may not like me reading across the capacitor with my MM but thats how i originally had it and only started doing so in hopes of finding the problem.

    Any ideas?
    Last edited by a moderator: May 2, 2017
  2. jcsd
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