Touch Switch Circuit

Discussion in 'Digital Circuit Design' started by jmg1213, Nov 3, 2017.

  1. jmg1213

    Thread Starter New Member

    Jan 9, 2017
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    I want to us this touch switch circuit in a project. It works fine, but I need to modify it. Currently, when power is removed and then reapplied, the circuit returns to the state it was in when power was removed. For example, I can turn the LED on and off using the two contact points, but if the LED is on and I remove power, the LED will be on when I reconnect the power. I need the LED (and relay) to always have no power to it (them) whenever power to the circuit is restored. What change(s) do I need to make?
     
  2. jmg1213

    Thread Starter New Member

    Jan 9, 2017
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    I should have added. For my project, I will be using momentary push switches instead of some type of touch input.
     
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Connect a capacitor from pin 13 to ground. First guess value 100pF.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    As Albert said but I would go with closer to 10-100nF, depending upon how fast the power comes up.

    Also add a 100Ω resistor in series with that switch so reduce the high peak current from the capacitor, which could otherwise damage the switch contacts.
     
  5. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    If the resistors on those inputs stay at 1MΩ the time constant is getting rather long with much bigger capacitors. I could stretch to 1nF ;)
     
  6. jmg1213

    Thread Starter New Member

    Jan 9, 2017
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    I will try these suggestions over the next couple of days. I may not have any of these capacitors on hand, so I may need to make a trip to pick some up.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That time is just the recovery time until it will respond to another input after the PB is released.
    For example, 1MΩ and 10nF gives a time-constant of only 10ms.
     
  8. jmg1213

    Thread Starter New Member

    Jan 9, 2017
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    I had a 100nF capacitor on hand. I inserted it between pin 13 and ground, and now the circuit operates as I need it. Thanks.
     
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