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Super simple help needed - IR receiver

  1. Nov 5, 2005 #1
    I've been looking all over the Internet for what I thought would be an easy thing to find instructions on. Unfortunatley, anything that comes close either complicates the matter with extra features or assumes I know a lot more about electronics than I do....

    I need to build a very simple circuit: When it detects an infrared signal, it lights up an LED which stays lit for as long as the IR signal is active.

    The problem is that it needs to not drain it's battery much when it is subjected to ambient light, including some sunlight through stained-glass windows. (This is going to go into a church and will serve as a stand-up signal for the choir.)

    Everything I have found online is either too sensitive to ambient light, or has decoding chips for complex signals. I just need something very simple.

    If I had just a components list of stuff that I could get at Radio Shack or similar places, I'm sure I could easily figure out how it works and build it.

    Does anybody have any suggestions on where I could start? (I used to think I was a little proficient with electronics. In researching this, and seeing the "simple" circuits offered up on the Internet, I've come to realize that I'm much more clueless than I ever imagined.) :(

    Any pointers would be greatly appreciated.

  2. jcsd
  3. Nov 5, 2005 #2


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    Its a full blown IR repeater circuit, but you only really need a few parts, but the parts you don't need are cheap anyways...

    The TSOP1738 chip ends in a 38 because its looking for a 38KHz IR remote control signal, so it may not work for you. But it does have a filter on it to try to prevent sunlight/flourescent light interference and outputs only the pulses with the 38KHz carrier removed.

    You can get an equivalent chip to replace the TSOP1738 as instead something like the Vishay TSOP4838 (or other form factors or frequencies with slightly different part numbers) from Mouser.com for just a little over a dollar. The rest of the parts will still amount to less than $10.

    If you don't have a 38KHz carrier signal, then your options change, these chips are designed to reject non-modulated signals.
  4. Nov 5, 2005 #3
    Closer, btu still hoping for simpler

    Thanks for that link - it helped me understand a little better about what I'm attempting.

    I'm having a hard time, though, figuring out what can be removed from it and what can't.

    If the TSOP1738 is built to reject unmodulated signals, then does that imply that I can do away with the 555 and almost everything else?

    If I just take a battery, a TSOP1738, and a red LED and wire them together with appropriate resistors to bring the voltage down, will the led flash when the TSOP1738 is hit by a 38kHz IR signal?

    To make it last as long as possible between battery changes, I'm thinking of using a nine volt battery. So, would I go from battery positive, through an adequately large resistor, to the high side of the TSOP1738, then from the low side of the TSOP1738 to the battery negative and from the TSOP1738 output through the LED to the battery negative?

    And if that would work, then how big of a resistor would I need to throw in there between the 9V battery and the TSOP1738? (And, would other resistors or components be required to make it work?)

  5. Nov 5, 2005 #4


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    Basic Infra Red Tx & Rcv

    How about this reference. They show basic circuits for an infrared transmitter and reciever (look towards bottom of that webpage).. They say its range is about 4 meters but could be increased up to 15m with additional components.
  6. Nov 6, 2005 #5


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    Marshall - yes, you are correct that you could eliminate the 555 because all it does is re-modulate the signal with a 38KHz carrier. The IR receiver chip simply outputs demodulated pulses because IR remote controls send out a specific series of pulses for the different commands.

    Based on the form factor, its the TSOP1238 that is pretty much the equivalent, the 4838 is a miniature version. No real difference besides form factor, 4838 has more 'cool' factor because of its small size.


    The output current is only 5mA max and if I recall correctly the output is held high so it makes sense to throw a transistor on the output to really light the LED when you send pulses.

    Given that the voltage on 9V batteries changes so much as it discharges (in my experience anyways) I'd just run a 7805 regulator and be done with it.

    Battery + and - to respective 7805 pins.

    7805 output to 4838 Vss and 4838 - to batt -

    4838 output to 3.3K resistor to base of transistor with 390 ohm current limiting resistor in series with LED, maybe change resistor to fit your specific LED.

    You'd want to test the output of the 4838 to see if its high or low to and use the appropriate PNP or NPN transistor like maybe a BC327 instead of a BC337.

    If you wanted to experiment with something like Ouabache linked to, you could also use one of the BP103 equivalent phototransistors like a QSD123 that has an output current of 16mA.

    So then it'd be battery + to QSD123 collector and QSD123 emitter to LED anode and LED cathode to battery - and you're done. Part count of 2 but the narrow angle of reception and inability to reject ambient light present in that narrow range may be a problem. 2 is a pretty low number though and you could twist the leads together and test in seconds!
  7. Nov 7, 2005 #6
    Thanks Ouabache and Cliff_J!

    I think I've got all the info I need now. (What a cool forum!)

    Wayne (aka Marshall)
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