infared phototransistor issues

WhelzornWhelzorn Posts: 256
edited 2004-10-13 - 21:38:29 in Robotics
I have these infared phototransistors from the local radioshack, and I am having a problem with them detecting all light, and not just infared. They work great, except for that. The problem is, obviously, that it can accept a certain frequency from another (Im having two robots communicate by the way) because all they see is a solid stream of incoming light. I just want to figure out how I'm going to filter out all light but infrared? If anyone has any ideas, I love to hear it!

Thank you,


  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-10-13 - 13:42:42
    Most IR transmissions (TV remotes, etc.) use IR that is modulated at around 40 kHz; this can be filtered on the receiving end. You might want to be an IR demodulator and save yourself some trouble.

    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • allanlane5allanlane5 Posts: 3,815
    edited 2004-10-13 - 15:00:16
    Yes, Radio Shack also sells an 'IR Demodulator'. This is a three-wire gizmo with a little 'bump' on the front. This is an active device. It expects +5 on one wire, Gnd on another, and produces a signal out on the middle wire. The signal is normally high, but goes low when a certain frequency of modulated IR light hits it. That's the part you really want to use.

    Each 'Demodulator' model number supports a certain frequency. Typical frequencies are 36 KHz, 38 KHz, 40 Khz, and 42 Khz. I believe the Radio Shack one is 38 or 40 Khz. The device 'holds' the signal long enough so that a BS2 which sent a pulse through an IR-LED with a "FREQOUT IRPin, Width, 32000 " would be able to read an echo back -- thus allowing its use for collision detection (there's an app-note on this on the Parallax site).

    The nice thing about the demodulator is that it does filter out most light, since most light is not modulated at 38 Khz. I don't know about direct sunlight, though. A nice protocol uses a 1 mSec pulse for a "0", and a 2 mSec pulse for a "1", as these values can be easily generated by a BS2 and detected by a second BS2 (using FREQOUT and PULSIN).
  • WhelzornWhelzorn Posts: 256
    edited 2004-10-13 - 16:04:46
    so what your saying is that its not a "filter" i need,·so to speak, but I need to modulate the frequency on the emitter, and demodulate it on the phototransistor end? my local radioshack does not carry this "demodulator", and I'd have to order it, so I was wondering if there is any possible way to use the "infared" phototransistor I have. If not, I will just have to use a demodulator.

    thanks again,

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-10-13 - 16:24:26

    ·· You could use the Phototransistor you have, if you build the Demodulator circuitry yourself.· Before the 3-pin IR Demodulator device was available, Radio Shack sold the 555 and the 567 ICs...The 555 Timer was used to modulate the IR beam, and the 567 was used to decode it (Through an op-amp, 741, also available at Rad Shack).· I believe these schematics are still available through the Mini-Engineer's notebooks Radio Shack used to carry (They still should).

    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:··
    Personal Page:···
    Designs Page:···
  • WhelzornWhelzorn Posts: 256
    edited 2004-10-13 - 21:38:29
    I finally got my hands on one of those 38khz reciever modules from the shack, and have some hookup questions. The back shows 3 pins:
    |     |
    | (O) |
     | | | 
     | | |
     1 2 3  

    1: Out
    2: Gnd
    3: Vcc

    I just need some guidance on how to hook this thing up to the basic stamp, if anyone knows

    edit:: ok, scratch that, its simpler than I thought [noparse];)[/noparse]
    thanks again for everyones help

    Post Edited (Whelzorn) : 10/14/2004 1:21:26 AM GMT
Sign In or Register to comment.