DIY Touch Sensor

This is a short instructable on how to use the Qprox IC (QT113G) as a touch sensor. Using this IC, you can essentially turn any object into a touch switch. This simple circuit may be hooked up to a microcontroller (in this case I used the MAKE controller board).

Step 1: Gather all the necessary parts

1. QT113 - can be ordered at Digikey DigikeyDigikey. Each one costs around $2.
2. 10mF capacitor
3. Wires
4. LED
5. breadboard or perfboard (if you choose to perf, you will obviously need a soldering iron and some solder)

Comments

[leo_oto]

Hi All,

I'd great results with AT42QT1011 ( 6 cm of detection) but i have some issues.
Is it possible to connect the chip to the sensing plate with 1 metre cable and avoid interference?
Which method should i use (gnd shield, driven shield)?

[slopes]

Is this the same Touch switch IC as sold here?

http://www.hobbytronics.co.uk/manufacturer-hobbytronics/touch-switch-ic

[lasermaster3531]

no, that one is actually a little bit better. It has two inputs, so you can control two devices with one IC.

[teche]

if you put a relay where the led is placed you can put 240v put 240volts through it and use it to turn off your lights at home :)

[lasermaster3531]

only problem is that the ICs can only handle 2mA at 5 volts. that isn't enough to power the most anorexic relay, so you will need it to drive a feild-effect transistor (FET) to drive the relay. not too complicated but make sure the FET is there otherwise you may burn the IC out.

[kthunt]

I was looking at Digikey (which seems like about the only source for this line) and Atmel. It looks like they have mostly discontinued this chip set and replaced it with the QT1010-12.

The QT1010 and QT1011 work like the QT113 in basic mode. Only on while touched. The difference is the 1010 has a stuck key timer. These are good for keyboard, switch use.
The QT1012 only operates as an on/off toggle, like for lamps and such.

Also, the default output level has been changed to active-high so you don't have to invert the signal any more.

[alladin712]

I was looking at the QT1012 and it has 6 pins instead of the 8 described above. I'm a complete newb to making circuits. But I wanted to incorporate this idea into mounting an Led on to the bezel of my laptop instead of having one of those awkward usb ones. Any help would be appreciated.... diagrams would be great or even if you could point me in the right direction to a site pertaining to my project. Thanks in advance!

[lasermaster3531]

The newer one is in a much smaller surface-mount package. The pins are really tiny and they don't go through the board. It mounts to the same side as the traces (the other one mounts to the opposite side with the pins going THROUGH the board) If you want the diagrams for the newer ones, Google them and look for the datasheets and application notes. You should be able to figure it out from there. You will also probably need some kind of logic-level FET to handle the current draw of the LED, as the newer ones can only source or sink 2mA.

[ooda55]

This is a realy great project, ive been looking for an easy way to do this for ages, however i cant seem to find that chip anywhere
according to farnell its no longer manufactured, are there any alternatives?

[shadowdude777]

Hi there. I think you can use the newer equivalent, which is the AT42QT1011; you can read more info about the controllers here: http://www.atmel.com/dyn/products/devices.asp?category_id=170&family_id=697&subfamily_id=2109

These seem to be the newer equivalents to those old chips, and they look like they're much better, as, for instance, you only need to apply Vdd to one pin instead of the three in this instructable. It makes for a much neater setup.

[altaholic]

is there any way i would be able to make an on/off latch for the output?

[lasermaster3531]

If you connect both pin 3 and pin 4 to ground it will latch. There Is an 'ible somewhere around the site about building a USB powered proximity detecting led lamp. Just search up touch lamp and you should find it.

[altaholic]

great, thanks a lot. i'll check that out.

[Fletcher]

There are several chips that have inputs sensitive enough to be activated by touch (a 4017 decade counter, which can be used for LED chaser-type projects for example, is one.) I believe you can also achieve the same effect with a darlington pair, but I haven't tried it myself. For those of you interested in having your circuit remain on even after you've stopped touching the lead, Google "complementary latch."

[wiml]

The qprox is nice because it's capacitive. You can put the sense wire behind a small amount of other stuff (veneer, glass, etc.) and it still works. (It's basically a free-running oscillator and some circuitry to detect when the frequency changes due to the added capacitance of somebody's nearby finger.) Another nice chip along these lines is the motorola MC33794, which simultaneously senses 9 electrodes. Good for small keypads (or for targeting airbags according to passenger height, which is what it was designed for).

[Unit042]

"(It's basically a free-running oscillator and some circuitry to detect when the frequency changes due to the added capacitance of somebody's nearby finger.) " So that's how it works? Just a change in capacitance leading to a changing oscillator frequency? This could be done with a 555, put a wire from one end of the timing capacitor, and have a microcontroller time the frequency. If there is a large change in frequency, the microcontroller knows there is a finger touching the wire. (Or, if you like simple analog electronics, like me, the output of the 555 goes through an RC that averages out that voltage, then another RC to smooth it even more. Then, a comparator watches both RC's simultaneously for a significant difference.) Just thinking out loud here...

[koney]

Nice thinkin... have you tryed it?
I han no luck buying QT113 - its not common in our country and dont want to order it from abroad ...
Id like to modify another project (http://www.instructables.com/id/Table-for-Electronic-Dreams/) and make it proximity sensitive - could be pretty cool - what do you think?

[shakespeare1212]

This device works based on the capacitance of your fingers, right? I would not work if you just touched the wire to a wall, or would it?

[laraCat]

can't believe this is the first time I've seen the MAKE controller! very interested in how you set up your output and the software you used. Can you post a link to any info about the NET connect software you used with flash? tx!

[scopevisions]

i want to use it to turn ON... can you help me? nice table man! ;)

[alezito_gdl]

See answer to the comment above :). BTW, you could use the output signal to activate, lets say a relay, so that you could step on and control something bigger than a LED. Nice Instructable!!!

[MANIAC]

could you wire it so the LED turns on when you touch it??

[alezito_gdl]

Looking at the operating voltages, I assume it uses TTL logical levels (0v for a logical "0"-off, and 2.7v to 5v for a logical "1"-on). So, this circuit is basically giving a logical 1 all the time as long as you are not touching the input wire. The easiest solution, (supposing you are using the direct output, in case you are hooking it To to the MAKE board, all you gotta do is to reprogram) is to add a generic logic inverter, such as the 74LS04, though that would represent a lot of wasted space. You could also build a simple inverter using 2 transistors and some resistors. Input signal for the inverter should be the output from the touch sensor circuit. The +5V would be wired to V+. 2N2222 general purpose NPN transistor should do the trick. Those are cheap and easy to get. Hope this answers your questions!. Image @ http://www.play-hookey.com/digital/experiments/images/ttl_inverter_sch.gif

[zen.webb]

Hey, cool article; touch technology is really sweet! While you certainly can do it the way you show I have an alternative method that worked awesome for me, and was much simpler. Hopefully you will find it interesting. You can save on parts and money by actually removing the QT113 IC completely and using your microcontroller (I used an Arduino) to pump charge out from one pin and measure changes in that charge on another pin. The essential technology is the same as you have here, but I was able to do it with just an Arduino, a piece of aluminum foil and a single resistor! More info on capacitive touch sensing with an Arduino: http://www.arduino.cc/playground/Main/CapSense My attempt: http://cs.unk.edu/~webb/2010/03/capacitive-touch-input-device/

[ae86boy]

I am looking to construct a touch - sensitive device that will allow me to start a car.  All i need to do is create a switch capable of handling enough current to flip a relay at 12v dc.  Will a setup like this do that?

Forgive me, I'm not terribly tech-savvy.

[sillyrabbit]

Hi all, looks like a great tutorial but I need help: I'm planning to use this in conjunction with a climbing frame: metal rungs, wooden uprights, to trigger audio when each rung is climbed on. However, I can't seem to purchase the qt110 ANYWHERE (well, I can but it involves buying a huge order which is way out of my budget) I only need about 20. I live in Ireland and my project is due in a couple of months so any suggestions anyone? Any help would be greatly appreciated :)

[djdune]

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=427-1138-1-ND

now could anyone tell me:

i want to use the sensor with the arduino.

will it give me just binary on/off or can i measure the amount of capacitance?

[djdune]

digikey says it's just logic. there are some other ways to read voltage without a IC ( see http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1171076259/0 )

but is there an IC out there that will just output voltage, rather than flipping a switch?

thanks!

[nbru]

Does anyone know if there's a way to de-sensitize the capacitance ability of the qt?
I'd like to be able to paint specific areas of a piece of aluminum so that they cannot send a signal to the sensor if touched...
I've tested it with several kinds of paint, but so far unsuccessfully...

[jack4645]

Could you use this to replace the key on a keyboard? I want to wire up a 10 key USB keypad with something like this to change the selection in a program running on my PC.

[superjames]

Yes, I'm interested on how to do this. It works, but the key is always pressed "down" until touched then the key is up while touched. Is there a way to reverse it? thx

[dunnos]

Maybe using a NOT port? i don't know i just heard of them during physics but it seems like a good solution.

[superjames]

Thanks I believe that is the same thing a a digital hex inverter that someone suggested on another site. Gonna try it out! Thanks for paying attention in Physics class!

[dunnos]

let me see a picture of the finished keyboard, please?

[superjames]

I've actually got it rigged up to a gamepad and it's a friggin mess at this point! But it works, besides that NOT/inverter. I'll try to get a pic if you still want one.

[dunnos]

yes please

[superjames]

Here it is. I got the Hex Inverter working too! I just need to find a better capacitor besides a pile of pennies!

[noctune]

I cant find any QT113 parts in my country. Is there any other similar touch sensor I can use?

I'm trying to make a led cube activated by touch on any face.

[janeoblivian]

so are those QT113s still available? digikey has them listed as "non-stock." where else can one find these things? also, is there a difference between QT113-D, QT113-IS, QT113-DG, etc? digikey has like a whole page of them.

[3!3X]

I got the QT110 and it works fine. It doesn't have an adjustable response time like the original, but 100 usec seems fine to me. I didn't dig in to all the differences, but for the purposes of this project the 110 is good.