touch sensitivity

Discussion in 'The Projects Forum' started by birdmw, Nov 17, 2008.

  1. birdmw

    Thread Starter New Member

    Nov 17, 2008
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    After getting a feel for how temperature in metals change due to a hand touch, I had a great idea for my Circuits 1 class today. I want to use a differentiating OP AMP (with a very very high gain) and a temp. sensor, to make a touch pad. My local electronics store cant think of a good way to do this :( So, to the forums I go. The restrictions are this: a) it cant be so complex (it is circuits 1), b) I want a device that will receive my momentary spike of voltage, consequently turning "on" or "allowing" another voltage source to do its thing (turn on a fan in my case, that will in turn be auto regulating to the temp of a heat sink).

    In a nutshell:
    I need a switch that flips on/off from a voltage spike.
     
    Last edited: Nov 17, 2008
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    For starters... fine wire thermocouples or very small bead thermistors would probably be best for the fastest sensor response.

    Ken
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    Definitely two good suggestions. There should also be a creative way to use the Vbe drop in diodes/transistors with small packages (sot23). The thermal response may be a little slower, but there could be electrical benefits with the right circuit. Not sure - just a suggestion.
     
  4. birdmw

    Thread Starter New Member

    Nov 17, 2008
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    These are great, thank you very much, we did in fact study Vbe drop in diodes/transistors, but I decided to go with fine wire thermocouples. I am able in lab to produce the voltage spike when i touch the pad. The problem is that I do not know how to make a voltage spike, translate into a voltage by which another device can run. Thanks again for the ideas on the thermals.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Apply the thermocouple output to an op amp and amplify it by some amount. Apply that voltage to a comparator for the switching. The level out of the thermocouple will depend of many factors, and the junction may deteriorate over time due to corrosion.
     
  6. birdmw

    Thread Starter New Member

    Nov 17, 2008
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    Yes, we will most likely use a Schmitt Trigger to act as the comparator because of its "dual threshold action called hysteresis, [which] implies that the Schmitt trigger has some memory." (wikipedia). Thank you for pointing that out.

    But like any trigger, when the differentiators OP AMP stops seeing change in voltage, (from the heat generated from the touch of the user... and the capacitor is charged,) it will also stop outputting voltage. This is where the trigger-switch comes in. Is there such a device that is a switch, that will respond to a voltage jump, in the same way a physical switch, responds to a push of the button?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes, that comparator I mentioned. The old LM311 might do fine.

    Why worry about a difference voltage? Just ground one lead from the thermocouple and use the voltage off the other. You will have a level that is sensitive to room temperature, and will need to be amplified before your Schmitt trigger can sense it.

    On the other hand, the voltage comparator will do fine without the Schmitt trigger. Get a data sheet and you'll see. If you wish, you can add hysteresis to the comparator's response.

    That sensitivity to room temp could be a show stopper. A warm room and a cold finger may never get sensed. I would want to try this out on a breadboard before going into production.
     
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