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Need help with circuit

Discussion in 'General Electronics Discussion' started by Jonny, Jun 15, 2010.

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  1. Jonny

    Jonny

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    May 31, 2010
    Found this one online, will copy the text, then make my notes:

    Simplest touch sensor circuit

    The illustration on the left shows a simple touch sensor circuit. It will light up the LED when a person gets in contact with the wire or metal connection to the transistor base. This touch circuit is cheap and easy to construct. All you need is a npn transistor, a resistor, a LED, a metal contact surface and of course the wire connection. It is so simple.

    NOTE: Interface the circuit will requires a bit further improvement to the design. Do not connect the ground reference to the earth. The LED will either not light up or will be very dim.

    [​IMG]

    I am using a 9 volt battery. (I am also a newbie). Using a 2222a transistor, wich wire goes to what? Do I run the transistor on the positive or the negative? I am assuming that the "base" connector goes to the plate I want to use as a switch, but I cannot get this goofy thing to work. Any advice?
     
  2. jackorocko

    jackorocko

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    1
    Apr 4, 2010
    Well the most obvious question I have is... Did you hook up the LED in the correct orientation?

    Depending on the type of LED you have, with a 9 volt battery, the series resistor might need to be of a lesser value to drive the LED to it's brightest state.

    You hook the battery up the exact way it is in the schematic, the collector to the positive side of the battery and the emitter to the negative side of the battery.
     
  3. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    I suspect this circuit will not work with a battery as the power source. What is needed is a power supply that will provide parasitic capacitance back to the power mains through the step-down transformer. When you touch the base plate your body becomes a virtual capacitor to ground which develops an AC voltage on the base through the parasitic capacitance. This also explains why making an earth ground will attenuate the AC voltage available through the parasitic capacitance. Put an oscilloscope on that LED when lit and I'll bet you see 60 Hz current pulses.
     
  4. Jonny

    Jonny

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    May 31, 2010
    Thank you for the responses guys! Jackorocko- my led has a built in resistor with pos and neg leads, so it should be in the orientation, but I misunderstood the diagram. I will try it as you described. Laplace- WOW. Sorry, but I am way to new at this! About all that I understood about that was that it may not work with a battery. Now I won't feel so bad if I can't get it to work. As for the rest, I have a LOT to learn!!

    Thanks again!
     
  5. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    http://en.wikipedia.org/wiki/Parasitic_capacitance
    I didn't know what parasitic capacitance was either, but could the above be why this circuit works? Just guessing here, but wouldn't you touching the base make you act as a resistance activating the transistor? Laplace may be the more correct, I am just trying to understand as well.
     
  6. Jonny

    Jonny

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    May 31, 2010
    Okay, to show my complete ignorance here, if I am understanding you Jack:
    If I hook the pos of the battery to the collector, and the neg of the battery to the emitter, what do I do with the leads from the LED? If I hook them to the same spot, it finishes the circuit and the "Base" does nothing. Does that make sense? I have a pos and a neg lead from the 9 volt, and the same from the LED. Add in a wire from the "touch plate", that makes 5 wires. My transistor has 3 pins. I am getting more confused than when I started!! LOL

    Thanks for any help guys!
     
  7. jackorocko

    jackorocko

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    1
    Apr 4, 2010
    You misunderstood what I was saying, i'll take the blame for that. I should have been more precise.

    You hook the positive part of the battery to the positive lead on the LED, then hook the negative lead from the LED to the collector. The schematic shows the LED is in series. Then keep the negative part of the battery hooked to the emitter. The base of the transistor will be hooked up to the touch plate.
     
    Last edited: Jun 15, 2010
  8. Jonny

    Jonny

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    May 31, 2010
    Ok, thanks very much for the detail! Unfortunately, it still doesn't work. If I jumper between the cllector and the emitter, it lights up, so I know the battery and the LED are good. It may just be the circuit won't work this way. Do you know any other cheap/easy ways to make a touch switch? What I want is a box with an LED and a momentary touch switch. Sounds easy, but apparently little knowledge/little funds makes everything complicated!
     
  9. florinanghel

    florinanghel

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    Jun 14, 2010
    I don't think it's that expensive to make a simple infrared touch switch. After all, you only have to use a 1x1 infrared grid... which isn't even a grid. You could even use regular LEDs instead of infrared ones, but you'll then have to deal with the interferences.

    I'd give you more details, but I'm not an expert on this. All I know, though, is that this way you won't have a current passing through your body. :)
     
  10. Jakebeck15

    Jakebeck15

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    Jun 24, 2010
    ^Agreed...
     
  11. blocka

    blocka

    18
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    Jul 7, 2010
    i think at the very least to get this working you will need much higher gain from the transistor. I would use a darlington pair
     
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