Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

IR transmitter and recievers

  1. Jun 17, 2004 #1

    mrjeffy321

    User Avatar
    Science Advisor

    Hello, I recently got some IR transmitter LED's and reciever LED with the ide of using them to make somewhat cheap remote control.

    whern I started to build it, I realized that I didnt know much about what was going on and how I should put it together, so I experimented with it a lot to figure out what was happening where, and I have come to the conclusion that the transmitter LED is just like a normal LED, just that it emmits IR waved insted of visible light, and that the reciever "LED" (it isnt really a light emmiting diode, but it looks like one, so I will continue to use LED) works something like a switch, when it recived a wave within the range of frequencies it can detect, it opens up the switch and allows current to pass through.

    Am I correct so far? if i am not, then please correct me.

    now, I have also noticed that the light from the light bulb in the lamp above it also triggers it to open the switch, how can I avoid this? it seems that wherever I go, the light sets it off, but for instance, in the TV remote control reciever, it isnt as effected by light, does it use something diferent, or does it filter out something?
    how can I make it so only when I send my signal, it will activate, and not just background IR that it picks up?

    perhaps the TV's reciever is ust like mine, in that is is always being triggered, but it can tell the diference between the background "noise" and the signal sent by the remote.
     
  2. jcsd
  3. Jun 18, 2004 #2
    If your receiver is a photodiode it isn't really a switch - it absorbs light and produces a current, which is used in the circuit attached to the diode. Visible light has more energy than IR, so it will produce a photocurrent, too.

    Your TV remote receiver may be filtered, but more likely the reason is that the signal is modulated, and the receiver circuit is set to only respond to the modulated signals. So a DC signal doesn't do anything to it.
     
  4. Jun 18, 2004 #3

    mrjeffy321

    User Avatar
    Science Advisor

    as it says on the packaging, the detector is a "photo transistor"

    as one of my tests, I hooked a battery up in series with a volt meter and the IR detector, then in near darkness measured the voltage through the circut, it came to about .03 volts (because it wasnt total darkness, and then I turned on the IR LED and the voltage increased upto very near the max voltage of the battery, then I shut off the IR LED and turned on the lamp, and again, the voltage measured was very near to the max voltage on the battery. this lead me to believe it opporated like a switch.

    also, it says on the packaging, that the detector spectral band width range is from 620 - 980 nm, and the IR LED emmits 940 nm waves. visible light turns out to be about 700 nm, so it too falls into the IR detecors range.

    so how would I go about modulating the IR transmition, and/or make it so the detector will only pick out the modulated waves.
     
  5. Jun 20, 2004 #4

    Cliff_J

    User Avatar
    Science Advisor

    If you pick up an IR receiver like Radio Shack 276-0137 then it will have a lens cover on it to filter out most visible light (while allowing IR), and it will also de-modulate an IR signal at like 38KHz and output the pulses directly. Simple 3 wire hookup, use a 5V feed.

    For sending the modulated signal, good old 555 will do the trick.

    Here's some more stuff to look over, may be of assistance as the extenders function as both a demodulator and modulator to send the signal:
    http://www.mitedu.freeserve.co.uk/Circuits/Interface/candi.htm

    Cliff
     
  6. Jun 20, 2004 #5

    mrjeffy321

    User Avatar
    Science Advisor

    I have Radio shack product number 276-142, it came with 1 IR emitter and I IR detector.

    I dont know if this comes off in my posts, but I am not an electrical expert in circuits, I can recognize A few things in circuit diagrams and how things should work, but once it gets cluttered up with a bunch of stuff, muti-parrallel and series paths through, I get totaly confused. and logit chips, weh, they blow my mind on how they work.
    so when you say, "For sending the modulated signal, good old 555 will do the trick." I have no idea on earth what you are talking about.

    righ now I am trying to do the project in 2 stages:
    stage 1 - get a very simple remote control thing working, on and off, send the signal to the reciever and have it do something.
    stage 2 - possible send diferent signals with the same emitter, and have more possible functions that the reciever can do, like a real remote control. I dont know if i'll ever get that done.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?