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I am trying to build a keyboard sewn into a shirt and since there doesn't seem to be any suitable flat (and thin) pressure switches I will need to DIY one. Here is my idea for a pressure plate viewed from the side:

Pressure plate

The orange parts are conductive layers and the grey ones are some form of elastomers (perhaps rubber?) to push the conductors apart. When the user touches the plate the conductive layers meet and form a circuit.

  1. What is the best material for the conductive layers? Copper?
  2. How many cycles can I expect before the sensor breaks?
  3. Do I need to worry about tinpest, oxidation or similar problems?
  4. Is it better to use high or low voltage? Would 3v work?

My biggest worry is durability since I know that making a good, tough switch is hard. Obviously a hall effect sensor would be the best choice but I don't think it would work on such a small scale.

Another approach is to use conductive fabric like this: http://www.instructables.com/id/Three-Fabric-Buttons/step4/Making-holes/

Would that be more durable compared to my idea?

Or are there any better ways to solve this problem? At first I looked into piezo transducers but they can only detect taps and I need to know if the user is holding down a button.

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  • \$\begingroup\$ Why not use capacitance instead of pressure? Granted its sitting on "skin" which will in itself affect capacitance. \$\endgroup\$ – Ron J. Jun 29 '13 at 14:04
  • \$\begingroup\$ The concept is OK but questions about durability are in the fine detail of the design and it's impossible to say if it will be durable. I think if you take a leaf from membrane switch manufacturers and use silver paint for the contacts and connections you'll be on a better track. If there is any chance of moisture ingress you'll have problems if disimilar metals are used for contacts. \$\endgroup\$ – Andy aka Jun 29 '13 at 15:15
  • \$\begingroup\$ Is this a one-off or a business opportunity? I know of conductive and pressure sensitive electrical fabrics which can do precisely what you are looking for, but the stuff is not available in one-offs. \$\endgroup\$ – Billysugger Jun 29 '13 at 17:24
  • \$\begingroup\$ Billysugger: I know a few guys who like my idea and want to buy one, however I doubt it will be a high volume product. But please do tell the name of the fabric and/or manufacturer and I will look it up. \$\endgroup\$ – shirtkb Jun 29 '13 at 18:49
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Consider a capacitive touch sensing solution instead of a mechanical contact switch if that meets the requirements:

For discrete "keys", pieces of single-sided PCB cut to the right shape and size serve well as capacitive electrodes. For sensing, either use a suitable microcontroller which supports capacitive sensing, or use a touch sensor controller such as the Atmel QTouch series.

An illustration of how this works, from the Atmel QTouch pages:

QTouch

The PCB material, either glass epoxy (FR4) or paper phenolic, serves as the dielectric in the diagram above. The single-sided copper layer would be on the inside of the wearable.

From the question, it appears that a large number of "keys" may be needed. In such case, discrete capacitive touch sensing (one pin per electrode) may be sub-optimal. A matrix touch sensor IC would work better, e.g. Atmel's QMatrix. From the Atmel site:

QMatrix uses a pair of sensing electrodes for each channel. One is an emitting electrode into which a charge consisting of logic pulses is driven in burst mode. The other is a receive electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the panel the field coupling is reduced, and touch is detected.

This works much like the matrix scanning technique used in conventional keyboards.

For up to 48 buttons, the Atmel QT60486 can be used: Up to 48 sense channels, with data communication via SPI and UART.

This capacitive touch approach addresses durability, safety and production concerns in one go. For best results, the exposed copper side of the electrodes could be coated with standard solder resist, or for longer life when exposed to human perspiration and friction, a suitable epoxy coating.


Addressing the bullet point questions:

  1. Copper as on a PCB would work fine for either the capacitive touch solution in this answer, or on the mechanical switch in the question.
  2. That would depend on the copper thickness, the pressure per button press, and any other source of mechanical wear. Of course, for the capacitive touch solution, life is indefinitely higher
  3. You do need to be concerned about oxidation, especially if the actual contact area on the conducting surfaces will be small, as appears the case with the mechanical approach in the question. Again, not a concern for capacitive sensing.
  4. High frequency low voltage would work best - no polarization-related deposit of airborne salts, and yet safe for a wearable. Not relevant for the capacitive solution.
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  • \$\begingroup\$ Great answer. However due to my design the keys need to be spread out so each key would need its own PCB, would that cause any big problems? I should add that I will probably make it some type of chorded keyboard so the key count probably won't be an issue. What I don't like about capacitive touch is the feeling of pressing something hard without any mechanical feedback. Would it be possible to replace the lower conductive layer in my drawing with a capactive surface? Would that solve the durability issue? \$\endgroup\$ – shirtkb Jun 29 '13 at 18:29
  • \$\begingroup\$ Yes, I would just dice up a bare PCB into the required number of rectangles or circles. No problems other than the risk of ripping out one of the many solder connections you would need. For a clicky feel, just put bubble wrap bubbles on each PCB piece, or better yet, on the under side. That way, the sensing electrode to dielectric to finger distance would not get compromised. The distance from finger through dielectric must be a couple of mm, no more. Proximity of skin on inner side is no problem, the QTouch ICs auto-calibrate. \$\endgroup\$ – Anindo Ghosh Jun 29 '13 at 20:59
  • \$\begingroup\$ Thank you for your anwser, I will try your suggestions. But what would happen if I replaced the lower conductive layer in my drawing with a capactive surface? That way the keyboard would also work with gloves and I could change the colour, texture and material of the keys. \$\endgroup\$ – shirtkb Jun 30 '13 at 9:17
  • \$\begingroup\$ @shirtkb Many capacitive key controllers also work in proxility mode, so gloves will work (may need setting sensitivity higher, check datasheet of appropriate part). I'm not sure I see the point of the second layer etc, since the copper layer is the sensing electrode, preferably on the side closest to the finger, and you need just the one. I suspect you might have become married to your initial concept, a bit :-) \$\endgroup\$ – Anindo Ghosh Jun 30 '13 at 9:25
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For a DIY approach you may want to consider going to a surplus computer store and picking up a few old computer keyboards. Most keyboards made in the recent 10 years or so are made of a matrix that works almost exactly like you describe the setup that you want to make. In the case of the computer keyboard there are three plastic layers, two with conductive traces on facing sides and one that has holes in it. The one with holes acts as a separator to keep the two conductive layers from touching. When the two outer layers are squeezed together over the holes in the separator layer the conductors touch and complete the circuit.

This assembly is very thin and may be workable for what you have in mind. It is possible to enlarge the holes of the separator sheet to allow for the contacts to work with less pressure. You can also cut the plastic pieces up to make your own switch layout but it may be difficult to attach wires to the plastic sheet traces.

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  • \$\begingroup\$ Using parts from old keyboards is somthing I want to avoid. But yes that is basicly my idea. Though I have not found any information on how the durability issue gets solved by keyboard makers. \$\endgroup\$ – shirtkb Jun 29 '13 at 18:34

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