Making Capacitive Touch Sensors With Pencil And Paper


There are few things more frustrating than being in the middle of working on a project and realizing that you are missing some crucial component that ties the whole thing together. According to Murphy’s Law, this sort of thing will only happen when parts are completely impossible to procure.

If you’re ever hunting for a touch sensor but can’t get your hands on one, [Alan Chatham’s] tutorial on simple DIY capacitive touch sensors might be just what you need to keep things moving along.

[Alan’s] sensors rely on the conductive properties of graphite, which is easily found in just about any pencil on the market. The sensors are created by simply drawing on a piece of paper with a pencil, then wiring the images or text up to your favorite microcontroller via some paperclips and a couple of resistors.

Paper and pencil might not make for the most durable means of input, but we’re pretty sure that [Alan’s] capacitive touch sensors would be very helpful in a pinch. He doesn’t have video of the sensors in action just yet, though he says he’ll put something together here shortly.

6 thoughts on “Making Capacitive Touch Sensors With Pencil And Paper

  1. Ed: If I’ve got my science right, that’s a resistive touchpad. This is capacitive. They work on different principles, and someone learning embedded systems / circuits / any hardware hacking would benefit from seeing how both work.

    Note in particular that the touchpad requires an anti-static wrist strap that ties the hand to ground, and this capacitive sensor doesn’t require any user attachment to the circuit. They work on different principles; a novice could benefit from doing both to understand what’s involved.

    td;dr Projects aren’t the same.

  2. A great re-invention of a very old idea!

    I first saw this mentioned in 1930’s QST magazines. People used pieces of pencil lead with a wire wrapped around each end as resistors, so it didn’t take long before they started “drawing” resistors and capacitors with pencil lead on paper.

    I played with this as a kid, and found HB (soft) pencils worked the best. I built a “music synthesizer” using neon lamps and RC oscillators all made with pencil on paper. It had a resistive touch keyboard. The capacitors were filled-in areas on each side of the paper. Your fingers were the “patch cords” to create different pitches and waveforms.

    When CMOS came out, I made logic circuits that were drawn on paper with pencil. The chips were just pressed into the paper like thumbtacks! It worked at low speeds (due to trace resistivity), and could not be flexed or you got intermittents; but it worked!

    Capacitive touch sensors certainly don’t require a micro. Just an XOR gate will do. A squarewave is applied to both inputs through separate RC delays. When the RC’s are the same, the XOR gate’s average output is low (with sliver pulses). One of the capacitors is your touch plate; when its capacitance is increased by putting a finger near it, its RC delay increases, and the gate output’s average value goes high.

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