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Basic Electronics » IR current question

April 16, 2011
by missle3944
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Hi all, I just bought 2 pairs of IR detector and emmitor at radioshack to try out the theremin project. The package says that the detector runs on 35mA of current but the one u guys use is 50mA.I am just using a 100 ohm resistor in parrelel to it. The reason why I'm asking this is because it doesnt reallly seem to want to work. I am using the printf through hyperterm and I am getting reading on the pitch but the volume just jumps around alot. also the step always stays on 1 and the volume always says zero. I also noticed that on the emitor I can see a faint light on it. Why is that?

-missle3944

April 17, 2011
by BobaMosfet
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missle3944-

You need to specify more specific information in your posts. You don't tell us what emitter/detector you have, so we can't go look at the specs to help you. There are 3 (at a minimum) things that are involved- voltage, current, and resistance. If these are not properly in balance with one another, you can fry your emitter.

Secondly, you need to understand the difference between parallel and series. You put a resistor in series with your emitter, otherwise it won't do you any good.

Since you haven't specified voltage, I'm just going to assume it's 5VDC, so you can see the equation for determining resistor size. Resistor size is about controlling current, not voltage (voltage will take care of itself).

If your emitter can only handle up to 35mA, then (assuming 5VDC input), and further assuming that the emitter requires (just a guess, because you didn't say), 2.3VDC, the equation looks like this to determine resistor size:

R = E/I

R = (5-2.3)/.035
R = 2.7/.035
R = 77.14 Ohms.

Now, since resistors don't come in this specific side, you pick one that errs on the side of caution that's close: For example, in this case, you would choose a larger resistor (to resist more current, rather than less), and the next available size larger than 77.14 Ohms in a metal-film resistor (cheap and common) is 82 ohms.

To find out how much that limits the current to, work the resistor the other way:

I = E/R

I = (5-2.3)/82
I = 2.7/82
I = 32mA

Which is perfectly fine and will make sure the emitter is not fried.

Since you're using 100 Ohms, try using the above formula, with the proper numbers, to obtain what your resistor would limit it to.

As for 'faint light', some infrared emitters are close enough to the visible spectrum, that you can see a faint light from the emitter- usually a very cool/faint blue-white. That's okay, and is just because of the frequency. If you don't want that, you have to move up in frequency (but then you'll need NVGs or something else that can see into the IR spectrum) just to see if the lamp is on.

BM

April 17, 2011
by BobaMosfet
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Couple of minor corrections to the above (sorry)--

I meant 'size' not 'side' in a couple of places, and importantly: I meant CARBON FILM resistors, not METAL FILM. Most of the time, you will have CARBON film resistors in your inventory.

BM

April 17, 2011
by Rick_S
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Just a side note, if you want to check to see if an IR led is on or not, often digital camera's or web-cams will see IR light. An easy test if your phone camera or web cam will or not is to take your tv's remote, point it at the camera and press a button. I know for a fact the camera in my motorola droid see's it. It makes for a quick easy way to see if the LED is lighting or not.

Rick

April 17, 2011
by mongo
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That's how I check them too. Quick and dirty.

April 17, 2011
by missle3944
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Hey guys, I was able to see the IR light with my camera. But I still get bad readings on the volume ADC. Any help?

April 18, 2011
by BobaMosfet
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Rick, thanks for that- I had no idea cell cams could see IR like that- sure enough!

:P

BM

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