Simple Touch Sensor And Other Lessons

[HankDavis] sent along this link to a video showing a tutorial on how to make touch sensors using a “darlington pair”. In the video we are taken through the idea and how we’re going to construct it.  [Thad]explains in detail how this works in simple terms and illustrates it clearly. Unfortunately they don’t show an actual constructed system, but this is so simple you could toss it together quickly and see for yourself. This is a great lesson on how to get a simple touch sensor into your projects. This video appears to be one of a series of class visuals, and you can find several others on youtube under this account.


21 thoughts on “Simple Touch Sensor And Other Lessons

  1. Hello everyone, thanks for the positive remarks. These videos were created for a CS class at Georgia Tech. The class is called “The Art of Prototyping Intelligent Appliances…or just enough electrical and mechanical engineering to make you dangerous.”

    Class website is

    The class has been taught multiple times with the current incarnation hosted

    Please check out some of the really great final projects completed by project teams.

  2. can you do anything with this besides light the led?

    anyone have ideas on how to interface this so that it would provide a high signal to a cmos chip pin (like 4066)?

    I just had this exact circuit on my breadboard twp days ago and could get the led to light fine, but had trouble using it to control a 4066 input pin.

    any advice would be appriciated

    the other controlled by the 40106 looks promising too. thanks @Gert

  3. @sariel

    thanks but i am looking for a super simple low part count solution… so no relay…

    using 2 transistors, a resistor, a relay and a diode across the relay is just plain excessive to drive a 4066 control pin high. — might as well use the arduino.

    i’ll probably just end up using a lowly momentary pushbutton switch. yawn.

    i thought that I was just overlooking a simple way to have a high signal interface with the cmos from the schematic in the video….

  4. @slipp
    Just pull out the LED and connect the 1K resistor directly to the transistor.
    Then connect the chip to the same place (transistor’s collector)

    This will give you a low signal at the chip when there’s a finger.

    If you need a high signal, you need to redo the circuit upside down with pnp transistors, or add an inverter transistor (or logic gate)
    For an inverter transistor, connect the transistor’s base instead of the chip, connect the emitter to ground, and connect the collector to Vcc with another 1K resistor; Then connect the chip to the collector of this transistor. (anyone know something better for drawings?)

  5. @Rob

    I would say that the 100k resistor is so that a tiny current is passed to the base of the first transistor in the cascade. If you didn’t have this you would pass a bigger current, possibly damaging the transistors and it might feel like it’s zapping you when you bridge the terminals with your finger.


    Not everyone has access to radio shack books from 25 years ago. I’m studying my BEng and I found this really useful.

  6. Well, I built this the other day. Imagoes my disappointment when I measured two volts leaking through the transistor even with nothing on the contacts. I thought that maybe I damaged the transistor while soldering, but I don’t know. Thoughts?

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