Easy touch capacitance

posted Oct 17th 2009 7:24am by Jakob Griffith
filed under: news, peripherals hacks

[Humberto] from NerdKits is one of our favorite tipsters. We like how he can take a concept that seems so extremely complicated, in this case touch capacitance sensing, and present it in a clear and concise manner thats impossible to not love. As previously mentioned the most recent NerdKits hack is on touch capacitance; by using a resistor capacitor pair and some clever switch timing, anyone is able to detect the presence of a human limb. Now who’s going to be the first to adapt this concept further in their own hack?

Comments [15]

tagged: touch

15 Responses to Easy touch capacitance

Rui says:

October 17, 2009 at 8:02 am

Years ago, i found how to replace a computer on/off switch with a capacitive surface.
I already wanted to apply that same schematic to another project, but never found that webpage again not even information regarding it…

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Chuck says:

October 17, 2009 at 9:22 am

A lot of it went over my head.

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Tomasito says:

October 17, 2009 at 9:25 am

This is older than transistors.

Really, I saw this on a Popular Mechanics from back 50′s :P

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Matt says:

October 17, 2009 at 10:13 am

Pretty cool, but there are ICs that do this much more simply. QTouch for example.

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Free Xbox Live Points says:

October 17, 2009 at 10:53 am

So this is a Halloween hack a day!

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emilio says:

October 17, 2009 at 11:47 am

bravo! good video, nice explanation of the basic concept. a block-level summary of the code would have been nice for folks that aren’t coders or that would like to try it in another language. also, a cookie sheet would be a far more durable electrode plane for that candy bowl.

gosh, someone is gonna go over the top with this, like having dummy corpses fly at kids when they just want a piece of candy… i can’t wait.

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Humberto says:

October 17, 2009 at 1:16 pm

@emilio. Thanks for the kind words. I’ll be sure to provide a high level diagram of code for future projects if folks think it would be helpful. We tend to spend more time explaining the electronics, as that is what we find people are often most confused about. Thanks for the suggestion.

We actually tried a cookie sheet, but we didn’t have any around without a non-stick teflon coating; so we could not get a good electrical contact to the metal. Tin foil proved to the best commonly available household material we could find.

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therian says:

October 17, 2009 at 1:48 pm

@Matt”Pretty cool, but there are ICs that do this much more simply”
By simpler you mean go to only shop but Ic pay for delivery wait, wait ,wait finally get IC make PBC for it and read data-sheet how to use it. It so simpler than getting single resistor.

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samthefreeman says:

October 17, 2009 at 7:30 pm

Liked the demo but why complicate it to that level?

I never had to do something like that, but I think I could do it in a simpler way, probably without a micro-controller too. How about this:

1- Create a simple filter with the said capacitor as it’s main component (either LPF or HPF).

2- Use any mean that suits you to generate a time varying voltage signal at the desired frequency, this frequency according to design is either blocked or permitted when no hand has been inserted (again either LPF or HPF)

3- Depending on your filter design, you can detect the drop or rise in peak voltage amplitude and then have your circuit trigger when peak voltage amplitude either drops or rises (according to design) above or below a chosen reference peak voltage

What do you guys think? Doesn’t that save time on coding and fiddling with an MCU?

I like playing around with MCU’s as much as the next guy, but when you can do it with a simple circuit why add MCU and code complexity to it?

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therian says:

October 17, 2009 at 8:24 pm

@samthefreeman “I like playing around with MCU’s as much as the next guy, but when you can do it with a simple circuit why add MCU and code complexity to it”
because more than half people here have no idea how to make transistor oscillator and filter

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Georgio says:

October 17, 2009 at 8:28 pm

Thanks Humberto for another clear concise explanation of the wonderful world of ions.

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Wwhat says:

October 18, 2009 at 8:22 am

Nice to see some classical basic electronics used, and in a way that explains more complex variations too.
Sometimes I get a bit sad about people making whole projects with arduinos and such and having no clue about any basic electronics, not that it’s bad to use arduinos but you should not throw out basic usable stuff and use expensive and ridiculous overkill when a 2 cent FET can do something.

@Tomasito Yeah it is old, basic electronics, but whole generations grow up not bothering learning anything about it :[, and this way they get introduced.

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Tim says:

October 18, 2009 at 9:48 am

Excellent video. The effect on the scope seemed very subtle but it looks like it works ok.

Also those QTouch chips are a) quite expensive, and b) only come in annoyingly tiny packages.

Setting up an RC oscillator and filter would be another option. Definitely not as simple though.

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PocketBrain says:

October 18, 2009 at 9:51 am

I could do that analog. Use a relaxation oscillator that drives two RC filters; use diodes to separate them. Tune them so that the one with the fixed C discharges more slowly when the sensor cap is not interrupted. Use a comparator to determine when the sensor’s circuit is discharging more slowly, use the output to power LEDs or switch an LED driver. You can also play with a little resistor on the charging input of the fixed RC circuit and get some PWM from that.