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Infrared LED - Transmitting Multiple Bits

  1. Nov 24, 2007 #1
    Hi all, quick question here.

    As part of a sophomore seminar class, I have this little task to complete.

    I currently want to send a series of 4 bits through an IR LED that will reach a GaAs photo transistor. What I'm doing is using a shift register to load in the binary value (via a parallel link) to the shift register. Then I will clock the chip and output the same value serially. Now, my question is, how do I interface the shift register and my transmitting LED circuit...?

    For my LED circuit, I'm thinking about something like (schematic drawing... these parts are in series)... Vdd --- IR LED --- Resistor --- GND.

    I'm not too sure how to interface the two.
  2. jcsd
  3. Nov 24, 2007 #2


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    How about: shift register output---resistor---led---ground?
  4. Nov 24, 2007 #3

    When I clock the shift register it will be spitting out the bits and providing voltages for that... will the phototransistor be able to read it that quickly? i know phototransistors have relatively slow response times compared to photodiodes...

    but anyway, assuming that it works (I will have to check it out on Tuesday when I have access to my lab), I have a design that may or may not work for the ir receiver...

    what I have is a phototransistor, base is grounded, collector is connected to resistor, connected to voltage source, and the emitter is grounded. I will be measuring the voltage just at the collector... now i'm thinking about using a decoder to find out what pulses were sent by the LED?
  5. Nov 24, 2007 #4


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    One note here about driving the led with the output of your shift register. You need to check the registers specs to see if it has enough drive (current out) to turn on the LED. If not, then you can add a driving transistor.

    "will the phototransistor be able to read it that quickly?"

    Depends on how fast you are clocking the shift register.

    "what I have is a phototransistor, base is grounded..."

    The base of a phototransistor should not be grounded. This is the region where the light strikes which will cause a conduction from collector to emitter.

    EDIT: Here is a http://www.fairchildsemi.com/an/AN/AN-3005.pdf" [Broken] on phototransistors that might be useful.
    Last edited by a moderator: May 3, 2017
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