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Touch activated circuits

Discussion in 'General Electronics Discussion' started by Nala, Aug 31, 2017.

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  1. Nala

    Nala

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    1
    Feb 21, 2015
    Hi There

    I am using an online electronics simulator called Yenka and I want to simulate a circuit that has a touch input to trigger the circuit. Does anyone know how I simulate the 'touch'
     
  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Depends on how the touch is sensed. As a potential difference, capacitance change or resistance change?
    Welcome to EP!
     
  3. Nala

    Nala

    24
    1
    Feb 21, 2015
    Its a circuit using a 555 IC with a wire from terminal 3 on the IC and a finger touching it triggers the output to a LED.
     
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    Can you post the schematic of your circuit, using the 'Upload a file' button?
     
  5. Nala

    Nala

    24
    1
    Feb 21, 2015
    I have attached schematic thanks
     

    Attached Files:

  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Pin 3 is the output on a standard 555. Touching it will not trigger the 555. You probably mean pin 2, do you?

    To trigger the 555 you need to create a low going input voltage pulse on the trigger input. An isolated finger touch will not be able to do so. What may trigger the 555 is a connection to the 555' ground via the body's resistance and/or capacitance. This requires that circuit ground is somehow connected to your body, e.g. by using a sensor consisting of two metallic parts, one connected to pin 2, the other to ground.
    Use a switch and a series resistor to simulate the trigger in this case.


    Or you rely on the small rf voltage that your body picks up to trigger the 555. This works with many CMOS ICs, but the 555 requires a trigger current of 0.5 µA (0.9 µA max). If this is your triffer method, use an LMC555 (CMOS version) whichrequires a trigger current of only 10 pA.
    Use a switch and a small rf voltage source to simulate the trigger in this case.

    Ceterum censeo: show us your circuit diagram and the expected trigger method.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You posting the schematic happened while I was writing my sermon.
    You seem to rely on the second method I described. Therefore a dmall ac source and a switch would imitate the touch sensor.

    Note that this is not a physically accurate model of the reality. I don't know how you could simulate a real world sensor without some extensive modeling of all stray components (capacitors, resistances etc.). You'd have to develop a model for the sensor by itself,
    It is probably easier to forgo the simulation of a real world sensor and simply use a switch/pushbutton to ground to simulate the trigger pulse - at least if you are not interested mainly in the trigger function but in the rest of the circuit.
     
  8. Nala

    Nala

    24
    1
    Feb 21, 2015
    Thank you the information is very useful.

    I originally used a 555 circuit with a push button input as you described to trigger an output to another IC circuit that lit 4 led's in sequence to avoid 'bouncing' that I was previously getting from a simple push switch.

    I am now trying to use a human touch instead of the push switch and wanted to adjust the values to achieve the same result, as I am getting some bounce from the output at the moment from this circuit. This is why I wanted to simulate the circuit.

    Yes I did mean pin 2
     
  9. Alec_t

    Alec_t

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    Jul 7, 2015
    A voltage source with a series resistance of <100meg works fine as a trigger on pin 2, using the default 555 model in LTspice. But the model is an idealised one. I'm not familiar with Yenka and haven't a clue how accurate its models are.
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

    11,310
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    Nov 17, 2011
    You will not be able to simulate the bouncing. This effect is caused by the physical contact acting erronously during opening and closing.
    You could use a low pass filter to suppress the effect, but this will not go well with your tocuh sensor design.
    Or use an amplifier to decouple the touch sensor from the elecrtonics, then a filter and a Schmitt-Trigger to create a clean inpt signal to the 555' trigger input. This page describes this and other options.
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    You can easily simulate bouncing in LTspice. Why not in Yenka?
     
  12. BobK

    BobK

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    Jan 5, 2010
    Yes, you could simulate it in LTSPICE by using a PWL voltage source.

    Bob
     
  13. Terry01

    Terry01

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    Jul 5, 2017
    Woohoo! This is a tiny bit off topic buuut....I spotted the mistake off pin 3 being the output pin straight away as I've been using the 555 lately. That was very nearly my first contribution with any meaning to the site. All be it a "pico" contribution. I must be learning something after all. Oh well...that's my piece of useless information for today. :)
     
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