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]

18 thoughts on “Touch sensors: overview, theory, and construction

  1. “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. 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. @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.

  4. 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).

  5. 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… :)

  6. @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.

  7. 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.

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