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Guide QT110 Touch-Sensitive Button Guide

Discussion in 'Modding' started by gongzero, 16 Oct 2005.

  1. gongzero

    gongzero What's a Dremel?

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    I've been lurking these forums for the last year getting info and ideas for a project I want to understake. The wealth of knowledge is incredible and, as a "thank you", I decided to post up this guide. It's my first so please feel free to offer up any constructive criticism.

    Part of my project will make use of touch-sensitive buttons. Electrodes will be placed behind a plexiglas panel to activate these buttons. I mostly see questions about power switches, so I'll cater to this, but the QT110 will ground its output on activation (active-low) so you can use it for any button that does this.

    The QT110 by QProx (datasheet here, PDF) is actually a very simple IC to implement in a circuit. Better yet, it's CHEAP! All it really needs to function is a 10nF capacitor (non-electrolytic per their instructions), an electrode, and a 2.5-5VDC power supply.

    In this guide, I'll add some other options supported by the QT110 that might be of interest to the PC modder.

    First, the parts list:

    1 x Breadboard (Radio Shack p/n 276-150)
    1 x QT110 IC (Digikey p/n 427-1000-ND)
    1 x IC socket (Digikey p/n A24802-ND)
    1 x Piezo sounder (Digikey p/n P9910-ND)
    1 x 475kOhm resistor (Digikey p/n 475KXBK)

    You'll also need some wire, and (obviously) some soldering equipment. For wire, I use 20-gauge solid-core wire because it fits nicely into the holes in the breadboard. It's a little less flexible than braided-core, but I find it easier to solder and trim.

    First, we set up the signal path around the breadboard:

    [​IMG]

    For the most part, I use lead clippings from resistors to make jumpers. Don't worry, I know it looks sparse, but it'll make sense as we go along.
     
    Last edited: 13 Feb 2008
  2. gongzero

    gongzero What's a Dremel?

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    I've hit "submit" rather than "preview" as I intended, so bear with me while I type up the rest of my guide. :sigh:

    Next, we're going to add the IC socket, and leads going to power and ground. For a PC power switch, you can have the ground lead go to the power switch header along with your output lead (check your motherboard's pinout to connect it correctly). This particular circuit will activate a DVD-ROM tray eject button, so the other end of these leads go to a standard molex connector -- the red goes to red (+5VDC) and black goes go black (ground).

    [​IMG]

    Make sure to orient the IC socket with the notch facing down. You'll be orienting the QT110 chip the same way, with pin 1 at the bottom right in the image.

    The little "jumper" you see at the left straddling the power / ground lead is only there to hold the wires in place. You don't need to include it if you don't want to, but it helps keep things nice and neat.
     
    Last edited: 13 Feb 2008
  3. gongzero

    gongzero What's a Dremel?

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    Next, we're adding leads for output (pin 2 -- green wire), the electrode (the green wire below the resistor), and a snazzy LED (the red and black wires leaving from the top of the board).

    [​IMG]

    Also note the short black wire next to the IC socket connecting pins 1 and 3.

    I'm going to assume you know how to wire up a resistor/LED combination. If not, there are great guides on the boards -- in fact the electronics forum contains a great little sticky with a step by step guide.

    The source voltage is 5V. The chip will source 1mA. The LED will be on unless you "press the button", i.e., the circuit is active -- perfect for backlighting a touch-sensitive area on a front panel, wouldn't you say?
     
    Last edited: 13 Feb 2008
  4. gongzero

    gongzero What's a Dremel?

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    Here's an annotated version of the last photo.

    [​IMG]

    I've marked the corner pins for the IC (1,4,5,8), and the various leads. Also, we have Cs and Rs marked.

    Cs is supposed to be a 10nF (.01uF) non-electrolytic capacitor. Instead, I'm adding a small ceramic piezo sounder. The part # I've given in the first post fits the QT110 specs -- 3.5-4.5kHz peak out, 10nF capacitance. It will output a 75ms "beep" every time you press the button, adding some useful tactile feedback to our button.

    However, as you can read for yourself on the spec sheet, when you apply power to the circuit, you'll hear a clicking coming from the sounder unless you add the Rs resistor in the photo. Keen eyes will see that while I spec'd a 475kOhm resistor (per QProx's recommendation), I've only used a 100kOhm resistor. Why? It's what I had lying around, and it does a good job of muting those clicks.
     
    Last edited: 13 Feb 2008
  5. gongzero

    gongzero What's a Dremel?

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    Here's the final photo of our fully assembled circuit:

    [​IMG]

    Note the orientation of the notch on the chip, as I stated earlier. Also, note that the piezo sounder is non-polarized -- doesn't matter which way you hook it up across the to long middle busses.

    The size of the sounder (about the size of a quarter) is the reason for the size of the breadboard (a little smaller than a business card). If you were to replace it with a capacitor (for example, Digikey's p/n P3848-ND), you could very easily fit 3 such circuits on one board with some clever jumpering. Of course, you wouldn't get the snazzy beeps, either.

    Also, if you do forego the sounder, you won't need Rs either. The circuit will work just fine without it.

    All that remains to be done is to connect your output, power, ground, and electrode leads. For the first three, to hook up a power switch, I did this:

    - ground (black) and output (green, top) go to the motherboard power-switch header. For my circuit, I clipped the leads from the power button on a now-dead case and soldered them to the breadboard so that I had a connector already attached, without requiring any crimping.

    - power (red) is tied to +5VSB on the ATX connector. However, to make it easy to move between PC cases, I bought an $8 ATX power connector extension like this, traced the +5VSB wire (usually the purple one coming from your PSU), and removed the corresponding pin on the extension (you'll need a molex pin remover for this). I then cut back some of the wire sheath, soldered the red wire from the QT110 circuit to the exposed core, and slipped some heatshrink tubing over the lot of it. Pop the pin back into the ATX connector, and you're done. Easy peasy.

    I've tested the circuit by touching the bare wire on the output lead and it works fine. I won't go into electrode design because what will work for one person won't work for another, depending on what you're trying to project the sense field through (plexiglas, wood, plastic, glass), its thickness, etc. Obviously, don't connect it to your metal PC case or you'll probably turn your entire PC into a big power button. ;)

    Connect the motherboard header, connect the ATX extension between the motherboard and the ATX power connector coming from your PSU, run the wire to the electrode, and you're done!

    Hope this helps.
     
    Last edited: 13 Feb 2008
  6. One~Zero

    One~Zero What's a Dremel?

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    Thanks for posting an actual guide. Have seen many different circuits, but no one has written anything up for implementing (that I can recall). Good Job! :clap:
     
  7. gongzero

    gongzero What's a Dremel?

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    Thanks One~Zero -- that's what prompted the guide. I've seen a whole mess of threads asking about the QT110, and while everyone is quite helpful in offering up information, I thought that putting everything --including part numbers-- into one guide would help out the electronics n00b (like myself).

    It's easy! The only electronics work I've ever done prior to this is building a little CMoy headphone amp and sheathing my PSU cables. If I can do it (and make it work), then anyone can. :)
     
  8. alter_ego

    alter_ego What's a Dremel?

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    i love you so much, ive been wanting one of these for ages
     
  9. coorz

    coorz Miffed

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    Actually i find the guide hard to follow; just text only! You should post the schematic as well.
    I've built the switch on a small breadboard strip 2x1 cm and just heatshrunk the bunch!
     
  10. Sea Shadow

    Sea Shadow aka "Panda"

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    Thanks for the guide, this looks really usefull!
     
  11. gongzero

    gongzero What's a Dremel?

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    coorz,

    My schematic is largely based on yours, which is largely based on what is in the QT110 spec sheet, but you're right, it's an oversight on my part to have not included it! :duh:

    [​IMG]

    CS is the piezo sounder in my schematic. The part I've spec'd is 10nF; if you need to increase the sensitivity you can add a non-electrolytic cap (5-20nF) in parallel with the sounder to do so. The RS resistor is to "mute" the clicks that the voltage step across the sounder create.

    The LED is tied to the OUT pin (pin 2) and the Vss pin (pin 8, ground). RL will vary, of course, depending on the LED you choose.
     
    Last edited: 13 Feb 2008
  12. gongzero

    gongzero What's a Dremel?

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    In the schematic above I've really set each lead coming off the circuit apart from the rest to (hopefully) avoid confusion. Here's a close-up of the power, ground, and output leads in my own circuit so people can get an idea of what I did to physically connect these wires.

    [​IMG]

    The +5VSB lead traces back to pin 1, Vdd. I spliced it to the +5VSB line in this ATX power extension (NOTE: in my extension, it's a white wire; according to ATX standards, it's the PURPLE wire coming off your PSU, so make sure you match them up right!!!).

    The GND lead traces back to pin 8, Vss; the FP_BUT lead traces back to pin 2, OUTPUT. As you can see, I hacked off the "button" end of an old power button lead and soldered it to the appropriate pins on the circuit so that I've already got the right connector ready to be slipped onto the front panel power button header on the motherboard.

    Hope this helps clear things up!
     
    Last edited: 13 Feb 2008
  13. sdat1333

    sdat1333 What's a Dremel?

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    hey, thanks for the nice guide
    i was linked to it from EOCF
    anyways i just have a couple of questions:
    I plan on building two of thes for my next case mod (One for power switch, the other for opening my optical drive.
    So how would i go about connecting it for the CD drive switch?

    Also i wanna put the switch to open the DVD drive right behind the acrylic panel thats stealthing it, how do i use the acrylic as an electrode?
    Thanks very much
     
  14. gongzero

    gongzero What's a Dremel?

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    You'll want to use ZapWizard's excellent "Rewired DVD Eject and LED" guide found here:

    http://forums.bit-tech.net/showthread.php?t=23443

    You just need to connect the QT110's output lead to the rewired eject switch lead.

    You can't use acrylic as an electrode -- it's not conductive. You'd need to put an electrode behind your acrylic panel and somehow connect that to the QT110's electrode lead. THe difficulty here lies in the fact that you're electrode will be moving back and forth as with the drive tray, so connecting it without getting tangled wires will require some thinking.

    Also, you'll want to make sure that you make the electrode just barely sensitive enough to activate the circuit from behind the acrylic panel, but not so sensitive that it's accidentally tripped by proximity (i.e., someone putting a disc into the drive tray). Experiment with higher Cs values.

    Hope this helps.
     
  15. sdat1333

    sdat1333 What's a Dremel?

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    Oh didnt even think about the cables bein a problem
    yah too hard to deal with then
    but i still want to have a touch button, ill make it as a little piece of alluminum next to it.

    alright so i still hook the CD drive switch up the same way as a motherboard one

    so i can adjust the sensitivity by adjusting what level capacitor is there?

    EDIT: alright so basically when you touch the electrode, the output wire is grounded?
     
    Last edited: 9 Nov 2005
  16. mattthegamer463

    mattthegamer463 What's a Dremel?

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    im confused. what is it doing? its a button... thats as far as i can figure out. you say its touch sensitive, but then again isnt every button touch sensitive? is it amount of pressure, or when your finger goes near it or what? it looks like a small speaker, is that what it is? please tell me what it does, it looks like something i want to make.
     
  17. coorz

    coorz Miffed

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  18. mattthegamer463

    mattthegamer463 What's a Dremel?

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    OMG! does this IC cost $100 US!?! thats way too much for me to blow on a button.
     
  19. sdat1333

    sdat1333 What's a Dremel?

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  20. coorz

    coorz Miffed

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    :hehe:

    That depends if you buy the QT110-H (active high) or QT110-L (active Low) version.
     
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