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What am I doing wrong (555 timer as a one-shot)?

  1. Nov 27, 2006 #1
    What am I doing wrong (555 timer as a one-shot)??

    I'm trying to use a 555 timer to produce a quick pulse whenever a switch is switched a certain way. I have it set up exactly as the data sheet describes (attached picture) and am using a 47 micro Farad Cap as C1 and a 1k ohm resistor. I have the switch connected to "Trigger" (pin 2)...but when I test it out...The pulse is lasting indefinently...and i do mean a long time. it should only be lasting a fraction of a second.

    I thought it might be a bad IC so I used the other one I had...got the same result. So I got desperate and tied the switch to where Vcc is instead of 5V...it actually worked like a one shot! I know this isn't the correct way to handle an IC...so WHAT could be going on???

    Attached Files:

  2. jcsd
  3. Nov 28, 2006 #2


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  4. Nov 28, 2006 #3
    no...i'm not using any transistors...i simply have a switch pumping out 5 volts when connected
  5. Nov 29, 2006 #4
    Your connection of the 555 timer is for a simple monostable operation. If you have it connected as you show it in the diagram then the output from pin 3 should go high (to Vcc) when pin 2 is grounded or made to be less than 1/3 of whatever Vcc is. The time your output from pin 3 is high should be equal to 1.1 X 1000 x 0.000047 seconds, approximately 53 milliseconds.

    Are you sure you are triggering the number 2 pin to go low, less than 1/3 Vcc?
  6. Nov 29, 2006 #5
    It would help me to see how you have the rest of the circuit connected.
  7. Nov 29, 2006 #6
    well I have a switch connected to pin 2, and pin 3 is connected to an active HIGH clear on a counter. everything else is connected as shown...I even checked and rechecked a few more times to make sure of it.

    flickering the switch should get it to pulse going one direction or the other...but i'm getting a constant high on the output, i only want a small pulse long enough to trigger a reset on the counter ( a 1 shot).

    I'm serious...connecting the switch to Vcc actually made it work...so I'm going to stick with that and hope my teacher doesn't notice...if he does i'll probably be back here asking for more help
  8. Nov 30, 2006 #7


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    Pin 2 should not be left floating if I recall the inner workings of the 555 correctly. Use a pull-up resistor to Vcc on pin 2 and switch it to ground when you want to start a cycle.
  9. Nov 30, 2006 #8
    is this what you're describing?

    Attached Files:

  10. Nov 30, 2006 #9
    I believe Averagesupernova wants you to connect the resistor to Vcc and then to pin 2. The switch then is connected between pin 2 and ground. When the switch is open, pin 2 will have a voltage on it that is close to Vcc and definetely higher than 1/3 Vcc. When you close the switch, this will cause pin two to become grounded and thus less than 1/3 Vcc and the output of Pin 3 will then go high.
    Last edited: Nov 30, 2006
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