IR LEDs and Receiver module

Discussion in 'General Electronics Chat' started by hspalm, Sep 12, 2010.

  1. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    Hello
    I have desoldered some receiver modules from a couple of old VHS players and so. One receiver module is called TFMS5360 which I am focusing on, the others are similar, but have no visible name on it. The module sinks current through my red control LED in a fine blinking pattern which is also visible on my scope when pushing a TV remote control button. Which is fun. But then I bought my own IR LEDS to try and make some remote projects, but it has already failed in the learning process; when my IR LED is switched on the red control lamp only blinks weakly 1-3 times (barely visible), about 500us long pulses. Then it stops blinking at all after about 2-3ms, while I am still holding the IR LED button down!

    Reading the datasheet for the receiver module I see there are specific modules for different carrier frequencies. This is still a bit vague for me; does it mean my pulses need to be at this frequency? Will it not work as a phototransistor? My infrared LEDs are wavelength 850nm, from ledshoppe.com. I could have chosen the 940nm ones, but I just didn't know which difference it would make at that time. Could this be my problem? I am using the wrong LED type?
     
  2. MartinLarsson

    Member

    Feb 25, 2009
    17
    0
    Such IR-receiver only accepts a certain carrier frequency as you said. So if you blink the IR LED, at 36kHz in your case, the receiver will output LOW. You can't use it as a phototransistor though and I doubt the wavelength matters as long as it's IR.

    You could simply make an oscillating circuit with a 555 timer and place the switch and IR led in series on the output. Or you can get a micro-controller and send actual data.

    If you're interested in IR protocols, signals, etc. take a look at http://www.sbprojects.com/knowledge/ir/ir.htm. I think you want to look at the RC-5 and RC-6 protocol which Philips are using, since they use a 36kHz carrier frequency.
     
  3. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
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    Thank you so much! I will read through the website.
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    One thing not mentioned in that website is that many IR receivers have some logic in them. I started designing with one and later found I had overlooked the fact that it requires specific resting periods in the pulse stream. I had to redesign the circuit and even though I finally got it to work I still don't like it.

    For instance read through page 5 of this document for a very popular IR receiver module:
    http://www.vishay.com/docs/82090/tsop48xx.pdf
     
  5. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
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    Okay, I have read and understood the website, which actually just added to my understanding of the carrier frequency (which you just learned me).

    But on page 5 of the datasheet posted just above me, I don't see what the specific resting period of this receiver is? And is it supposed to occur between the pulse train? Which can be of almost any length?
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Partial copy/paste of what I'm talking about:

    Minimum burst length 10 cycles/burst

    After each burst of length 10 to 70 cycles
    a minimum gap time is required of ≥ 12 cycles

    For bursts greater than 70 cycles
    a minimum gap time in the data stream is needed of > 4 x burst length
     
  7. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    Hmm, I just have to check up on my english here. A light burst is when a device is emitting light, in this case infrared light. When you say cycles, a cycle is a light burst of 2.7e-5 seconds at 36khz? So 70 cycles equals 1.94ms before it needs to rest for at least 12*2.7e-5 seconds?
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    No, the above has to do with the data stream the receiver is expecting to receive, not the fundamental 36 KHz frequency.
     
  9. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
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    Okay, but then a light burst must have a miximum length? If they only count cycles, I don't see how that can be representative to the receivers "stress".
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    IR remote control uses an ultrasonic carrier (36kHz for yours) to avoid interference from sunlight and lights.

    The receiver reduces its gain unless it receives bursts of 36kHz pulses. Each burst must have a certain number of 36kHz pulses (read its datasheet since the TSOP IR receivers are different) and have a certain rest period (no pulses) duration between bursts.
    It does this to avoid interference from compact fluorescent light bulbs that produce continuous 36kHz pulses.

    Then the output goes low for each burst of 36kHz pulses and goes high for each rest period.
     
  11. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    Is it not strange they make the carrier frequency at 36kHz when such a common light source as the compact fluorescent light bulb produce continuous pulses of IR light at the same frequency? I will check up upon the datasheets for these devices. The truth is I do not know which of the TSOP devices I have, as nothing is written on it. I think I will leave the salvaged parts and buy new ones with known carrier frequency for experimenting.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    There is no common frequency produced by all these different fluorescent lamps, (and other interference producing devices) over the years they've just found that IR transceiver circuits attain the best rejection from most of the common sources if operated between 30K - 50K as the carrier frequency.
     
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