Touch Sensing (Capacitive, Proximity, others?)

All,

I am working on an embedded application where I want the device to turn on (microcontroller wakes up) when the user touches the device with their hand . We want to conserve power and don't want to have to poll the touch sensin g device to know when to wake up. Then after this step we want to continue checking to be sure the user continues touching the device.

Here are the two requirements:

1.) The product should turn on when the user touches the product.

2.) After the device is fully powered on the microcontroller will keep chec king to make sure the user is still making contact with the system. When t he user stops touching the system the microcontroller will sense this and c hange states and go back to sleep.

What are some options for this?

1.) Proximity Sensor?

2.) Capacitive Sensor?

3.) Anything else?

Thank you for any comments on this.

Reply to
electronicsman2016
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What is the device like physically? What does it do? How is it powered?

Must it stay on with static contact?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Detect 60Hz power line hummm pickup when they touch a contact that turns the users into an antenna. It's easy enough with an audio amplifier, but you don't want the continuous power drain. So, you get to try it with passive parts, which I suspect will be marginal. After that fails, perhaps just one hi-Z input low power amplifier or opamp, which should be sufficient to drive a detector of some sorts. Be sure to debounce the switch and do something to keep RF from triggering the device. Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

If you touch your finger to an oscilloscope probe, you get a big signal, and it is appealing to think that would work, BUT that only happens because the 'scope has a ground lead plugged into the wall. A capacitive 'touch switch' needs to be polled, basically generates its own AC tp sense with. You might consider tilt switching, or even warm-hand-sensing, but those are noisy at best (a cold hand isn't un-heard-of).

Two good options: leave the device always ON, and just power-down its least used functional parts. Or, require the user to lift a cover, and make it convenient to close the cover when the device is no longer in use.

Mostly, an item like a TV remote can poll its buttons so slowly that the battery can last many months.

So, t

Reply to
whit3rd

A microcontroller with touch sense controller

It will run at micro amp current

For example the TI cc2640. You get Bluetooth functionality included if you may want that

Or just choose a cheaper version micro with sense controller

Regards

Klaus

Reply to
Klaus Kragelund

My Dell monitor has a nifty proximity sensor in the control button LED's, I believe it looks for LED light reflections. I never found out exactly how it works.

Cheers

Reply to
Martin Riddle

Research Cypress PSoC products. IIRC, they have touch sensing such that it will wake on touch or proximity.

I am using one to sense water level in a PVC pipe that has been running for 2 years on two AA cells. The cells are still at 1.5V ea. So, I expect the batteries to last another 5 years if the alarm stays silent.

I have no connection to Cypress other than as a happy customer.

Reply to
John S

Description is very close to what most "smart" phones do.

Reply to
Robert Baer

I have used the Cypress parts before and also touch sensing. They used to b e priced higher than a "normal" microcontroller, and they still are somewha t. The 4000 series is cheap, but capsensing is offered by a lot of microcon trollers, and you can roll your own using just a resistor if the microcontr oller has no inbuilt support for cap sensing

You should probable look into the microcontroller series you are used to, a nd select a part that has capsense

Cheers

Klaus

Reply to
klaus.kragelund

Yes, the older versions were higher priced than the CY8C4000 series which are ARM processors and lower power. It is true that capacitance /proximity sensing is offered by others, however, the Cypress application can auto-tune both the touch and proximity parameters. I prefer that rather than fiddling around with parameters on my own.

If you want to roll your own, you can get an 8-pin ARM from them for about $.60 in single qty. If you want to get the job done rapidly, you can get one with built-in capacitance/proximity sensing and auto-tuning for about $1.60 in single qty.

I all depends on your evaluation of your needs, as you know.

Reply to
John S

An LTC1540 comparator might do what you need. It has a built-in voltage ref, 2 to 11V supply, and a 300 nA operating current. You might want to use a low voltage drop diode in series with the sensing element to make sure you don't drive the input negative. If you use a contacting plate for the sensor you might want to limit the max voltage input also. I used one of these with a pizeo-on-plastic sensor to make a tap-to-wake feature on a hand-held device a couple of years ago.

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Reply to
ChesterW

with a matrix keypad polling is not needed. set an an interupt on each column input and drive all the rows, then sleep, when input is seen wake up and scan the matrix

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Reply to
Jasen Betts

A push-button switch? You don't get much lower power - this can power up your uC which can then set an output to hold the supply on with a MOSFET.

A variant of the above with a microswitch activated by who knows what, the weight of a hand on some hinged part?

Cheers

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Syd
Reply to
Syd Rumpo

In a "vandalism proof" (street environment) embedded application, often a "Piezo switch" is used to wake up the sleeping controller.

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Reply to
Rob

Thanks for all the posts everyone!

Reply to
mrhenkvisser

What do you guys think of this part for proximity sensing?

ISL29029

Datasheet:

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1.) For the proximity sensing I just will need one external IR LED?

2.) I thought I was going to need a IR transmitter and then also an IR photodiode (receiver) to measure the reflected light. Or is the single IR LED doing the transmission and receiving?

Thanks for your thoughts.

Reply to
mrhenkvisser

You need an external IR LED for the transmitter (see figure 5). The internal on chip LED handles the reception

Cheers

Klaus

Reply to
klaus.kragelund

nal on chip LED handles the reception

1.) I don't have a physical part for reference so I am missing something he re. Is there a cutout on the package of the part (i.e. a window) for the IR to be brought into the IC for the reception? 2.) The area that I will be doing the proximity sensing will probably not b e immediately next to the PCB with the proximity sensing IC. I can bring o ut an external IR LED on the device but if the reception / window is on the part I am not sure how to make this work?

Ideas?

Thanks.

Reply to
mrhenkvisser

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