Touch sensors: overview, theory, and construction

This collection of touch sensor information should be of interest to anyone who liked the simple touch sensor post from Thursday. That was a resistive touch sensor and is covered in detail along with AC hum sensors that trigger based on induced current from power lines around you, and capacitive touch switches like we’ve seen in past hacks. Each different concept is discussed and clearly illustrated like the slide above. [Giorgos Lazaridis] has also put together individual posts that build and demonstrate the circuits. We’ve embedded his resistive sensor demo video after the break and linked to all three example circuits.

Resistive touch sensor video:

[Thanks Giorgos and Ben]

Comments

  1. andar_b says:

    “AC hum sensors that trigger based on induced current from power lines around you”

    Interesting…so these shouldn’t be used in mobile devices, just in case the user is in the middle of nowhere?

  2. Marco says:

    It says so right on that page:
    “First of all, in order to operate, there must be power lines near by. And i do not mean right above the body, but in the near.”

    For a recent project I was trying to find out if the user is (sufficiently) grounded- that’s easy enough if the appliance itself has a connection to ground, but if it’s battery-operated.. how would that work out?

  3. dooglehead says:

    I enjoyed these tutorials. I have been wondering why I heard a 60 hz buzz when I touched the plug of an amplified speaker.

  4. Jake says:

    @dooglehead

    I hope you aren’t serious.

  5. Phil says:

    Für alle die sich für eine Interessante Darstellung eines Touchscreens interessieren. Die Sendung mit der Maus gab da auch ein super Video dazu her _) http://www.ardmediathek.de/ard/servlet/content/3517136?documentId=6099692

  6. space says:

    @andar_b The device doesn’t need to be near power lines. It could make it’s own ac electric field and use capacitive coupling. Given the high gain and input impedance of darlington transistors it wouldn’t need to use much power too.

    @Marco it is the problem with reference voltage. Since the device is battery operated it is capacitively coupled to the ground or ac voltage source (depends on position relative to the ground/sources). You’ll need to make an experiment.

  7. David S says:

    I can confirm that touch based devices will work sans power lines… in the middle of a desert.

  8. o says:

    So, what they’re saying is that if you followed their circuit exactly, your touch sensitive device would not function in the cold silence of space?

  9. o says:

    I’d prefer the device to make its own AC field

  10. Gert says:

    Let’s take this one step further.
    At my gym they have this machine with two meta bars.
    You grip each bar with one hand and it can measure your fat level and other stuff. Or so it claims.

    Does anyone know if this really works and how they do this?

    I know it sound like the meter from a certain crazy cult (i don’t name em because they do not deserve it).

  11. The presence of active AC power line is only needed for the AC Hum touch sensor. All other techniques can operate in the middle of the desert.

  12. Max says:

    Ok, anyone who has ever touched an oscilloscope probe has actually seen that “induced 50/60Hz” sinusoid. But has anyone actually tried the same thing in the middle of nowhere…? Is that sinusoid still there…? I’m honestly curious… :)

  13. Rodrigo Cerezo says:

    Greeat post!!!

  14. Tanntraad says:

    @Max
    I think that a power line sinusoidal hum would could be seen by an oscilloscope from any spot on earth. A metal pole of some length (antenna) would definitely pick it up, so I think it would come down to the sensitivity of the oscilloscope and the position of the body.

  15. john says:

    @Max It seems that to test this, we dont’ need to take an oscilloscope to a desert, just go to europe and see if the frequency changes to 50 Hz?

  16. NatureTM says:

    I was wondering why I get a signal when I touch my scope leads! Really glad I saw this.

  17. nathan says:

    why is everybody talking about using this in a desert? why / how would you be using a mains powered device using touch sensors in a desert? these wouldn’t be used on a mobile device, especially if it is for survival. simple switches would be used for reliability.

  18. Mouhamad says:

    You guys are funny. If you touch oscilloscope probes in the dessert or in outer-space, this means that you are near an AC power source and the signal and the switch will work. :-).

    Nice circuit.

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