Instructables
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).
 
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Step 1: Gather all the necessary parts

Picture of 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)

Step 2: Build your circuit

Picture of Build your circuit
wire up your circuit according to the following diagram:

pin 1: Power (3.3-5V)
pin 2: Output (in this case I used a red LED)
pin 3: Ground
pin 4: Power
pin 5: Power
pin 6-7: 10 micro Farad capacitor. These pins also connect to the input wire.
pin 8: Ground

Step 3: Input wire

Strip the tip of the wire and embed it into or behind an object or the surface which you want to act as a touch switch. when connected to a conductive metal sheet, or mesh this works really well. When doing multiple switches, it is important to make sure the surfaces are surrounded by ground, so as to protect each distinct touch surface.

You can see how the light turns on and off when my finger touches the wire.

Step 4: Connect to a microcontroller

In this case I used the MAKE controller - connected the output wire from Step 2 (which was an LED there) into the 4th Input pin of the controller board. When touched, the pin goes low, otherwise its state is high.
In this example, I used the NET connect software to create a flash animation that changes its shape according to a user's touch.

Good Luck!
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leo_oto2 years ago
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)?
jasonamri leo_oto5 months ago

try a shielded wire like: http://www.radioshack.com/product/index.jsp?produc...

source: http://makezine.com/projects/A-Touchless-3D-Tracking-Interface/

slopes3 years ago
Is this the same Touch switch IC as sold here?

http://www.hobbytronics.co.uk/manufacturer-hobbytronics/touch-switch-ic
no, that one is actually a little bit better. It has two inputs, so you can control two devices with one IC.
teche3 years ago
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 :)
Solid State Relay.jpg
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.
kthunt3 years ago
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.
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!
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.
ooda553 years ago
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?
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.
altaholic4 years ago
is there any way i would be able to make an on/off latch for the output?
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.
great, thanks a lot. i'll check that out.
Fletcher7 years ago
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 Fletcher7 years ago
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 wiml5 years ago
"(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 Unit0424 years ago
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?
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?
push_reset4 years ago
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!
i want to use it to turn ON... can you help me? nice table man! ;)
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!!!
MANIAC4 years ago
could you wire it so the LED turns on when you touch it??
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
jasonwebb4 years ago
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/
ae86boy4 years ago
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.
sillyrabbit5 years ago
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 :)
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 djdune4 years ago
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!
jonathryn7 years ago
Is there a way you could toggle the sensor off and on?
giladlotan (author)  jonathryn7 years ago
sure. you can add a regular switch in between the power source (battery or power adapter) and the chip. When you toggle it on, the chip receives power and it works, and when you turn the switch off, there is no power to the circuit, and nothing works...
I think I may have been unclear--perhaps what I meant to ask is whether there is a way to toggle the output on/off with a single touch. I see that the LED is normally on, and that it turns off with a touch. Absent the touch, the LED turns on again. Is there a way to connect the chip in such a way that when you touch the sensor, the LED/output turns off and stays off, and then when you touch it again it turns on and stays on? The application I have in mind would be to use a number of these as selector switches, as with the old Bang & Olufsen receivers, which used slick touch-sensitive selectors to choose the audio source and volume.
giladlotan (author)  jonathryn7 years ago
ahhh. I understand. You can simply do this by adding a microcontroller (for example PIC or Arduino) where you can program this logic. The microcontroller will have an input coming from the qprox (where the LED is placed currently) and an output LED. You can then write a simple program that listens to the input pin. Whenever it "sees" a long enough touch, then it tuns on the LED (output pin). The next time is "sees" a touch, it turns the light off, etc... This is the way I would go about it. I think there might be a simpler method, using logic gates and other IC transistors. Maybe someone else on this list will know better. Let us know how this goes!
3!3X giladlotan6 years ago
It's actually even easier than that. By running pins 3 and 4 to ground instead of V+, you get a toggle mode.

The product info sheet for the IC has a few other tips, like adding a little piezo clicker for some satisfying feedback. That's a QT110 feature, but I couldn't get fewer than several hundred QT113 from DigiKey....

Download here: QT110
 Could you clarify?  A diagram would be awesome, if you could!
what you want is called a "flip flop circuit". upon further inspection of the Q-Prox product line, QT118 has a built in "flip flop function". it would require no other circuitry to complete the task you require. hope this helps.
Just use a transistor as an inverter logic gate. Here is a web page with a schematic diagram: http://www.spsu.edu/cs/faculty/bbrown/web_lectures/transistors/
Or you could use this web page to make a more complicated inverter:
http://www.allaboutcircuits.com/vol_4/chpt_3/2.html
And if you're really crazy, you can wire up some motors and switches to this: http://goldfish.ikaruga.co.uk/not.html
nbru5 years ago
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...


jack46456 years ago
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.
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
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