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Infrared Emitter and Detector Question

  1. Nov 6, 2007 #1
    Hi nerdy geeky folks,

    I have two questions:

    1. can a regular cadmium-sulfide detect infrared in a complete absence of light (but a beam of infrared)? if not, what can be used to detect infrared (for a beam break sensor)?

    2. I just want to make sure that I am building the circuit right. So, I am building an infrared emitter whose forward voltage is 1.3-1.7V, reverse voltage is 5V, and continuous forward current is 150mA (according to he specification provided by Radioshack). I am using a 9-V battery as a power source. Since 9V-1.3V=7.7V and 7.7V/150mA=51 Ohm, I should use a 50 Ohm resistor, right?

    This will help me tremendously, and I appreciate it!

  2. jcsd
  3. Nov 6, 2007 #2


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    Staff: Mentor

    The resistor value sounds right. You'd get much better battery life by using 3 C cells and making a 150mA current source with transistors, but you can do it with the 9V battery and dropping resistor.

    As for the detector, I haven't used CdS detectors, but you should be able to compare the datasheet spectra for the emitter and the candidate CdS detector to see how they match up. I've used PIN photodiodes for detectors, and they match up well with the IR spectrum. I'm usually concerned with the speed of the photodector response, so that's why I use reverse-biased PIN photodetectors (in a current-to-voltage opamp circuit configuration).
  4. Nov 6, 2007 #3


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    Science Advisor
    Gold Member

    I'm fairly sure, without looking it up, that CdS detectors have a very broad range of frequencies it can see well. So will have to have it dark of other light to get a good change in resistance. And like Bekerman implied, CdS detector response times are kinda slow.
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