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]

19 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.

  8. Consider two spherical conductors. One of the conductors, the smaller sphere has an excess of charge = Q, and the other is uncharged.
    If we connect the spheres together a large amount of the excess charge will go to the larger sphere (the “earth”).
    Since for all practical purposes, the earth is by far the largest conducting sphere possible this explains why when we ground something by connecting to the earth all excess charge gets absorbed by the earth and the potential of the two spheres will become very small.

    In a touch switch, this process is at work when we touch the switch pad.

    A touch switch comprising a pnp transistor Q1, a load resistor connected to the emitter and a small copper plate which we call the “touch pad” connected to the base of Q1. The transistor is supplied 12VDC with the collector connected to the negative and the load resistor to the positive rail.

    When the switch pad is left untouched, then the potential difference between the emitter and the collector will be 12V and the potential difference between the emitter and the base will be zero (this I admit is debatable), since the base is not connected anywhere, which means the difference of potential between the base and the collector is 12V in a direction that reverse biases the collector-base junction.

    With these conditions, there is no current through the load resistor and the transistor.
    When the touch pad is touched by one’s bare finger, then, initially the salt water conducting film on the surface of the skin, allows the deficiency of electrons on the wire to be neutralized by supplying electrons from the skin.

    The potential of the system comprising the body and the wire is lowered. The lowering in potential will be substantial considering the large surface area of the person touching the pad which is assumed to be small.
    The power supply will immediately sense the drop in potential and supply a tiny current to restore the loss in potential at the wire and body interface and make the potential difference between the base and the emitter again zero.

    The tiny current that caused the potential of the person to be raised to the potential of the wire immediately after the momentary drop by depositing positive charges on the person’s body, will then cause it to spread all over the body and a state of static equilibrium is reached.

    When the finger is withdrawn from touch pad the excess charge remains on the body and will neutralize slowly by ionization processes in a short while. It should be noted that no matter how the pad is touched i.e whether briefly or a prolonged touch and then release, the effect is the same on the profile of the voltage VEC.

    The transistor Q1 will conduct when the potential of the base is lowered because the emitter-base junction is forward biased, and a current will flow in the resistor connected to the emitter briefly. The transistor Q1 is chosen to be high-gain because the base-current will be in the very low microamp range.

    The signal obtained at the emitter is further processed by coupling the emitter voltage to trigger a flip-flop.

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