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Pardon my noobness. I'm trying to set up a simple IR LED - Phototransistor pair but am having difficulties. My setup is just as in the following link, except my resistor values are 1K.

http://www.nerdkits.com/videos/theremin_with_ir_distance_sensor/

I'm using an Arduino to read the value of the voltage at the emitter but it does not change no matter what I do. I know I'm using the Arduino correctly because I can change the analog pin around and get expected values. I'm guessing I have some fundamental misunderstanding. Is it perhaps where you place the IR LED in relation to the phototransistor? They are right besides each other and I moved them around a bunch with no change. I will post a picture when I get home.

Also, what does varying the amount of current through the IR LED change? The wavelength is always the same, right?

Another question, this is the datasheet for the phototransistor: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_373001_-1

A red LED will work to, since this is somewhat sensitive to red light, right?

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    \$\begingroup\$ DV for not accepting a good answer, leading this question to keep resurfacing, zombie-like, when its pinged by the "Community" user. \$\endgroup\$
    – The Photon
    Nov 16, 2012 at 1:21

2 Answers 2

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Are you using the 1K resistor in series with the IR LED? Because based on the datasheet from that page that won't be enough current to really drive it. That will give you about 3.5mA and you need more like 100 mA. So try using a 100 Ohm resistor there instead.

Another cool trick is if you have a camera phone, old webcam or digital camera a lot of times if you point them at your IR LED you'll be able to see it turn on, on screen. Try it yourself by pointing your remote control at your phone and hitting some buttons. Also that might be a good way to see if your detector is outputting anything too. Point it at your IR detector and hold down the volume button and see if you get anything on the Audrino (or even better a scope if you have one).

The wavelength stays more or less the same, but more current = more light or in your case any light at all.

Red might work but I suspect it might be that your LED isn't on.

Good luck!

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  • \$\begingroup\$ Thanks for the help. Though I confirmed that the IR LED is on by viewing through my cell phone camera and still no change. Just for kicks I switched out to a 470 resistor (the lowest I have) and nothing was different. \$\endgroup\$
    – ballaw
    Oct 17, 2012 at 1:39
  • \$\begingroup\$ You have the photo transistor in correctly (not upside down)? \$\endgroup\$ Oct 17, 2012 at 1:48
  • \$\begingroup\$ Also at 470 that's about 5 times less current then they show in their design. Do you have 4 or 5 470 ohm resistors you could put in parallel? \$\endgroup\$ Oct 17, 2012 at 1:50
  • \$\begingroup\$ Oh and try them facing each other (dome tip to dome tip) if that doesn't work. \$\endgroup\$ Oct 17, 2012 at 1:54
  • \$\begingroup\$ Well, I tried again and was able to get it working with the 470 Ohm resistor. The problem was the current was so small it went unnoticed. I changed to 4 470 resistors in parallel and that worked too with little change. Then something weird happaned. I went off for a bit and when I came back it stopped working. The weird part is that the IR LED was still turning on (I confirmed with my cell phone) but the current dropped so far that it was no long activating the phototransistor. Is this the sign of a bad IR LED? \$\endgroup\$
    – ballaw
    Oct 18, 2012 at 21:30
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Direct DC IR LED emitter to a phototransistor path only works for a short distance (1m) You can increase that with the square root of drive current due to distance inverse square law. If you want a high gain receiver for long distance then you need to modulate in the 30~50KHz range with a matched IR Rx part just like most IR remote controls.

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