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Introducing Double M Design Inc.

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Hello fellow members

I have been seeking a forum which likes to discuss new product development as this is what my company has been doing for the past 39 years in Toronto, Ontario, Canada. I have been involved in projects ranging from stealth aircraft coatings to electric wheelchairs to electronic controls to industrial automation to consumer goods. It's been a long and rewarding career. I am now semi-retired but keeping my hand in the games by developing products for a select group of clients.

Currently, I am working on a new desk lamp control for one of Canada's largest office furniture manufacturers. Among other things, this desk lamp is turned on and off with a capacitance sensor which is nothing more than a copper pad on the PCB and a sensor chip from Azotec; IQS127D-00000-TSR. This sensor has 2 modes; proximity and touch. Due to the processing time, I am using the touch mode. Over the copper pad is a plastic overlay made of ABS and about 0.050" thick. When this overlay is in physical contact with the copper pad on the PCB, the sensor works fine but the minute there is even an 0.005" gap between, the sensor fails to pick up the touch of a finger. I have tried using solder paste as a "glue" to fill any gap between the plastic and PCB and, although it works, the paste dries out in time and I'm concerned that it will eventually give me an air gap resulting in the sensor failure.

I have recently concocted my own material using a mixture of copper powder and pure silicone. This seems to work but, when I try to measure the capacitance of the copper loaded silicon, it comes up as zero. Same with the conductance. I read an article that not all copper powders are the same and that, for conductive inks, a flattened particle works but spherical particles do not.

If anyone has comments or experience on this topic, I would like to hear from them. I'm sure that as time goes on and I am working on other projects, I will be able to share and consult with the members of this forum.


Michael Marr, P. Eng.
I have a fried that has extensive touch experience and he can sense through 1/2" of wood.
What have you programmed the threshold to?
Do some research on the "touch sensor" geometry. I believe you need an interdigitated pad design, not a blob of copper. Much like a conductive elastomer contact in a common remote control. You might also want to selective gold plate that contact for relaibility. Oxides might change the properties slightly.

We have both know how to print overlays with a laser printer. e.g. adhesive white and a layer of adhesive clear.
Letter size is about $3.00 USD in materials.
Have you tied something as simple as an adhesive to prevent the gap? Or a piece of thin double-backed tape? I don't think you need a conductive material.
Not sure how an interdigitated pad design will be an improvement over a copper plate. Both have a small, specific capacitance and the sensor chip is looking for a change in capacitance. A human has a much greater capacitance than the pad so that when a human finger comes within specific proximity to the pad, the capacitance changes dramatically. In fact, the copper pad on our PCB is covered with solder mask so it cannot oxidize. It is of a similar size to the pad on several Arduino capacitive touch sensors. The chip that we are using is self calibrating to the resting capacitance and re-calibrates whenever power is interrupted.
The idea of using some double sided tape OR just plain silicone without any copper loading is on my list to try. I'm waiting for the production run of the PCB assemblies which will give me 10 or 20 boards to play with. It would be great if the silicone worked as it will absolutely fill any gap and, once cured, hold tight to both the plastic and PCB surfaces. I'll report my findings back to this string of comments.
How about a small piece of self-adhesive copper foil screening tape (a type with conductive adhesive), stuck to the touch area and the end folded back on itself to also stick to the PCB.

That should retain both contacts even if there is no pressure between the two parts.

Examples - you can get this or similar types in various widths:

The copper foil with conductive adhesive backing is also a good idea. Now I have several things to try once the boards are produced. First run is 250 boards for my client and 10 or so for me. All of the suggestions are very much appreciated. For me, its a matter of finding a "production" friendly solution that can be handled by the pcb assembly house staff.

Just as a comment (and my soap box ranting) I am building this product in Canada. I know I could have saved 30% -40% and put it in my pocket by building in China but I really would rather see Canadians employed for this. As a point of regret, I spent 11 years with Honeywell moving production from Canada and the US to Tiangin, China to put even more profit in the Honeywell coffers. Anyone who was similarly employed did not do North America a service in the long term.
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