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IR LED's for a touch screen. HELP!!

Discussion in 'LEDs and Optoelectronics' started by st. ginger, Jun 13, 2011.

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  1. st. ginger

    st. ginger

    Jun 13, 2011
    Hi there.

    Basically, i need help with IR LED's. how they work, how i can put them together and how i can power them.

    i need to connect 120 IR LEDs together so that they give a continuous, strong light. and how do i then power the whole thing. preferably it would be connected to the mains somehow, but i have NO IDEA how this works. I've heard you need resistors and you need them in parrallel and all sorts. the most advanced electronics i've ever done is wiring a guitar, so i'm a bit out of my depth.

    I am looking to make a homebrew touch screen for my computer using an FTIR system, where there is a glass/acyrlic sheet flooded with Infra-Red light, under which is an LCD matrix, all built into the top of a box with light at the bottom and an IR camera plugged into the computer that tracks your finger on the glass. when you touch the glass, the IR light (which is being held inside the glass like a fibre optic cable) reflects off your finger and down into the box, to be picked up by the camera and interpreted by the software in the computer.

    I've got everything worked out, except how to make the LED array. I will be using either a 19" 16:9 screen, or a 17" 4:3 screen, but either way it will need about 2 leds per inch on all sides. i can't remember the exact amount off the top of my head, but its about 100-120 leds total.
    They are supposed to be connected together and mounted in a rail which is attached to the side of the glass sheet, so that as little light escapes as possible. This means that the glass is flooded with the maximum amount of light possible, giving the strongest result for the camera to pick up.

    So... how do i do it. and can anyone give me advice on where to buy components in the UK. As cheaply as possible, as i am making this on a boot string, being a penniless student and all :D

    Thanks so much in advance,

  2. st. ginger

    st. ginger

    Jun 13, 2011
    oh, also...
    I'm an artist, so I've been thinking about the options for making a stylus for this setup. there's obviously the option to just have a stick that reflects a big enough amount of IR light to make a mark, but thats a bit innaccurate and basic.

    I'm envisaging some kind of pen containing a battery at the back end and an IR LED at the tip on a spring, so that when you touch it to the surface it makes a connection and it shines a light down into the camera. it would give a much stronger signal than a reflection, but i'm wondering whether the angle of pen would mean the signal would be distorted, giving a wonky image when you're drawing.

    I'm not looking for a big answer to this one, just if you have any advice. the big question is above. thanks again!
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Check out this thread.

    If you don't get all you need from then please ask.

    I would be very hesitant in suggesting anything mains powered unless it is via a commercially purchased power supply. I'm presuming you have no previous experience with circuits running at mains potential.
  4. donkey


    Feb 26, 2011
    hi just a quick thought. i am assuming that the touchscreen will be connected via a usb port for input signals. when doing your calculations remember that usb's supply 5volts. the current on usb 1 and 2 is 0.5A and usb is 0.9A. there are usb cable that double the voltage if you need it and others that can double the current(from what i have heard) in case you need to do that. you could also use a usb powered hub that is not connected to your computer

    as a computer is a valuable object (even experimental ones) my advice is if you have a few dollars get a usb transformer(like the ipod charger, but there are others) that cost bout $20 aus to test it first as you would spend alot of time and money repairing your usb on your computer if you were to have a short.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Of course the other option is a USB touchscreen interface. It may be cheaper and easier to get working.

    Something like this.
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