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0.5 second monostable pulse??

Discussion in 'General Electronics Discussion' started by William van den Bogert, Apr 12, 2014.

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  1. William van den Bogert

    William van den Bogert

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    Apr 12, 2014
    Here is a circuit I built in iCircuit, and it does exactly what I want it to do. It creates a single 0.5 second pulse when the button is pressed. No matter if I hold it down or just tap it. Here is the circuit schem:

    http://imgur.com/LqFj7Z4

    But even though I have checked my connections about ten times now, It still won't work in real life. In real life, it only creates a single pulse when I tap the button, but when I hold it, it basically just holds the light on also. I can't figure this out. Whether physics is wrong, or iCircuit is wrong (I don't think its physics). If anyone can help me with my circuit, that would be great.

    (On iCircuit the GND of the 555 timer is automatic that is why its not pictured.)
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi William and welcome to Electronics Point :)

    That circuit should behave the way it does in the real world. A 555 connected as a monostable will hold its output active while its trigger input is held low.

    I don't know why the behaviour is different in iCircuit. Perhaps there's something odd about the simulation of the pushbutton - perhaps it only ever closes briefly in the simulator.

    To fix the problem, you need to insert an R-C circuit between the pushbutton and pin 2 of the 555.

    Leave the 10k resistor connected to the pushbutton, and break the connection to pin 2. Connect a capacitor across that break - any value from 1 nF (0.001 µF) to 100 nF (0.1 µF) is fine - and connect another 10k resistor from pin 2 to the positive supply.

    The way this works is that while the pushbutton is held down, the capacitor charges up through the second 10k resistor, and pin 2 goes high. This allows the 555 to time out and turn its output OFF.

    You may have a problem with the 555 triggering again when you release the button. This is caused by contact bounce. Google will take you to articles that explain it.

    Edit: BTW you don't need the diode between the output and the positive supply, and you can replace the two resistors in series with a single resistor.
     
  3. William van den Bogert

    William van den Bogert

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    Apr 12, 2014
    Thank you so much, I had tried putting a capacitor between that connection, but I didn't have the whole RC circuit! Your solution worked like a charm!
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Cool :)
     
  5. neon

    neon

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    Oct 21, 2006
    LOOKS LIKE THE IC IS LM555 AND GUESS WHAT THE OUTPUT CAN CONDUCT BOTH WAY the pull up diode causes a diode to power short. remove the diode then it will work,
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    The diode won't cause a problem. Its cathode is connected to the positive supply rail. If it were round the other way, there would be a problem.
     
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