EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Ian on May 04, 2011, 12:44:59 pm
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Hi
So i am playing around with some IR leds that i got from radio shack. So i supplied about 1.5v from a varible power supply and put a 22ohm resistor and then plugged the IR led in. and then i get out my camera so i can see the iR light. But there is no ir light. I have 2 of them and both of them dont work. here are the specs.
Reverse voltage- 5v
Continuous forward current 150mA
radiant power output 13-15mw
Wavelength at peak emission 950nm
Forward voltage 1.3typ-1.7vmax
PLease help
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If you set the supply voltage to 1.5V and the forward voltage drop of the LED is 1.3V, then with a 22ohm resistor you have 9mA current. If the Vf is 1.5V or more, you'll get no current. Bring up the voltage and increase the current a little bit. You can go up to 150mA, if needed.
By the way, If = (Vsupply - Vf) / R
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...and then i get out my camera so i can see the iR light. But there is no ir light. I have 2 of them and both of them dont work...
can your camera see IR light?
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From the data that sounds like a LD271 IR Diode.
With 1,5V + 22R resistor your LED doesn't get enough current to light up. Have a look at the LD271 datasheet, these things need more If typically about 100mA in continuos mode, often more than 1A in pulse mode.
Turn the voltage up a bit, maybe 3-4V - still using your 22R resistor - and you should see it come to life.
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Hi
so i tried what you said and it worked. My little 1/4watt resistor got nice and hot. i think i might wont to upgrade that lol. Also if i measure the voltage after it comes off the led ( basically farther down the breadboard) it should be no more than 1.7v and no less than 1.3v?
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Spot on. The 1/4 W resistor is awfully close to its limit, 2.3V voltage drop * 0.1A = 0,23W.
So choose a 0,6 or 1 W resistor.
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The voltage across the LED should not be more than 1.7V or it will be out of spec. Before you change your resistor, determine the minimum current you need for your LED. 0.25W resistors should be fine for most low-power LEDs.
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Hi
Now it wont work. I have my PSU putting out about 1.5-1.8v or not to steady. Why wont it work?
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The voltage across the LED should not be more than 1.7V or it will be out of spec.
Watch the current through the LED, not the voltage.
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how
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i tired nocking the led up to 5v and it worked what is the max voltage i wont?
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hi
so i just figured out why its so finkey. i had 10k R on my desk with my 22ohm and that was messing me up. i was wondering i calculated that i would need at least a 10ohm R so i whent to radio shack to get some 1/2watt 10OHM and 22OHM R. Do i not wont to use the 10OHM cause it might be less and blow my led?
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Do you have a multimeter?
Put it in series with the LED for current measurement and start with the lowest voltage your supply can handle.
It might be best to actually start this with a visual LED, rather than IR. It sounds like you haven't done much with LEDs and current limiting at all. It would be good to see the changes visually with the multimeter.
Ideally if you have to meters, set one up for current and the other as voltage across the LED.
Edit: As far as 22 vs 10 ohm, DO THE CALCULATIONS that were given above. Google current limit resistor LED to figure out how to calculate. Or just use a site like this: http://ledcalc.com/ (http://ledcalc.com/)
You really should be able to do it manually and understand WHY it is coming out that way. This is basic electronics that you need to learn to learn later things.
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Hi
I know how to calculate the resistance needed. How can i hook my meter in series? I am asking the min resistance i need is 10ohms so i bought some 10ohm Resistors do i not want to use them cause it might be a little under 10ohms? and that means it would blow my led.
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http://www.electronics-radio.com/articles/test-methods/meters/how-to-measure-current.php (http://www.electronics-radio.com/articles/test-methods/meters/how-to-measure-current.php)
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First you need to determine from what voltage you want to power your LED. Let's say you want to power it from 5V:
Uf = forward voltage / If = forward current / Usup = voltage of power supply / Rled = current limiter resistor
Uf = 1,5V, If = 130mA as given in the datasheet
Usup = 5V
First we calculate how many volts we have to get rid of:
Udrop = Usup - Uf
Udrop = 5V - 1,5V
Udrop = 3,5V
Then you need to calculate the current limiter resistor:
R = U / I
Rled = Udrop / If
Rled = 3,5V / 0,13A
Rled = 26,92R
The closest E value is 27R, so you choose that one.
Then we need to know what power the resistor needs to dissipate:
P = U * I
P = Udrop * If
P = 3,5V * 0,13A
P = 0,455W
So for a supply voltage of 5V you need a 27R 0,5W resistor for this particular LED with the chosen forward current If=130mA.
PS: I wrote U instead of V for voltage because that's the norm in Germany. Don't get confused by it, it's just ingrained in me.
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debugging steps:
1) are you sure your camera can pick up IR light?
2) do you get enough current, hook up your multimeter in series with your circuit. assure that you are drawing enough current
3) If not use your multimeter to verify the LED is in fact working by touching both sides of the LED with your DMM in diode mode and look for a reading on the meter, you should get 1.5-1.7V as mentioned earlier.
4) Check polarity of the LED
5) Do you have any way of detecting the IR light?
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most cameras have a filter designed to block IR which you need to remove if you want the camera to see it.
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most cameras have a filter designed to block IR which you need to remove if you want the camera to see it.
I can see IR out of a remote control with my Palm Pre and other digital camera. I think we are seeing the near IR, but it is usually enough to tell if it is working. A good test is to use a remote with the camera. This should be a known good setup.
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Hey all
so i was just working on taking some voltage measurements to figure out whats happening. here is what i found
Volts from PCU = 6.13v
Volts after diod = 5.78v
Volts after diod with led on = 4.00v
Volts after diode and resistor = 5.79
volts after diode and resistor led on = 1.311
Volts after diode and resistor with led on test on led's leeds = 1.296
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how
LEDs need a constant current source, not voltage.
The tolerance of forward voltage is quite wide and there is no need to monitor it, just limit the current to well within the maximum rating and don't worry about the voltage.
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Another fun simple project would be to build an IR detector circuit with and IR transistor driving a visible LED.
Make sure you buy an IR transistor that is made with a filtered lens so it filters out any non IR ambient light from hitting the IR transitors guts.
IR transistors look like an LED with a black, blue or dark green colored lens. This shaded lens filters out ambient light and only passes IR light.
I assume if you are experimenting with IR LEDs you are going to be sensing it with some kind of IR receiver at some point.
Google "IR remote control tester circuits" and you should find plenty.
Good Luck
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1.3V sounds normal for an IR led
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Another fun simple project would be to build an IR detector circuit with and IR transistor driving a visible LED.
You'll discover that visible LEDs generally peak in sensitivity at the wavelength they emit so if you need a good visible detector pick a green LED.