Dev Gualtieri
Electronic Design
A simple assembly of Teflon and Kapton forms a switch that uses the generation of static electricity (triboelectricity) by basic motion to generate a voltage spike, which then triggers a circuit.
Triboelectricity, a form of electricity known for millennia, is the static electricity that pulls a spark from your finger to a switchplate on a winter day when the air is dry and the humidity is low. It's also the reason why most integrated circuits are stored in anti-static foams and bags.
It’s generated when certain combinations of materials are rubbed together. The quantity of electric charge that can be generated from rubbing depends on how well the materials can generate or accept electrons. The table ranks the triboelectricity of common materials. The ones which are widely separated in this table, such as acrylic and Teflon, will generate the most electrical charge when rubbed.
| Table. | Triboelectric materials. | ||||||||||||||||||||||||||
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The triboelectric effect can be used in a simple switch (Fig. 1). A metal tine wrapped with Teflon tape is sandwiched between pieces of copper-clad epoxy circuit board stock and covered with Kapton tape (a polyimide film developed by DuPont in the late 1960s that remains stable across an extremely wide temperature range, from −270 to +400 °C). The surfaces are in tight contact with each other, and are joined by a blob of silicone adhesive. In this case, Teflon and Kapton are used as the triboelectric couple, but other materials can be used.
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| Figure 1. | Construction of the triboelectric switch shows the layered implementation, which allows for rubbing and thus generation of high voltages. |
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The silicone adhesive allows a rubbing notion when the assembly is tapped from above or on end, as shown by the arrows. The generated charges are collected by the metal tine and the copper surfaces of the circuit board. When discharged into a large resistance, impressively high voltages can be generated (Fig. 2). (The author's demonstration switch was four inches long and ¾-inch wide, but smaller switches are possible.)
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| Figure 2. | The peak voltage of the pulse obtained by tapping the switch depends on the force and speed of the finger tap, and the loading resistance (the probe impedance was 10 MΩ). |
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You can use such switches to activate CMOS digital circuitry (Fig. 3), where two triboelectric switches control a set-reset flip-flop and control an LED. The diodes prevent high voltages from damaging the input transistors of the CD4013 integrated circuit.
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| Figure 3. | The demonstration circuit uses the switch to activate a simple LED; standard 1N4148 diodes are used as clamps to prevent high-voltage spikes from damaging the flip-flop IC. |
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