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Author Topic: ir led is too weak  (Read 985 times)
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hi,
i am trying to emulate a remote control.

i got the key code, and i can transmit it successfully from a distance of up to 10 inchs.
farther than that, the receiver doesnt detect the signal.
whereas with the real remote, it can detect it from 10 feet.

i tried using a transistor, connected the led directly to 5v, using some resistors (33 ohm, 100 ohm, 170 ohm)
but same outcome.

ideas ?
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Tasmania - Australia
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green-man, can I suggest you might have better success with this question in another section of the forum, not in Home Automation and Networked Objects. This question should be asked in 'General Electronics' or 'Sensors' I believe.

Maybe it should be moved.

Rockwallaby.
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A high power IR LED needs more current (100 mA?) than an Arduino pin can supply (40 mA). The transistor should have worked if it was wired correctly.  Please provide more details about your LED, transistor, and how they were wired.
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I don't think you connected the grounds, Dave.
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Topics merged.
OP, do not cross-post.
IT WASTES TIME.
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You need to provide a lot more detailed information before anyone has any chance of helping you better.

The order you tried each option and exactly what you did and how you wired it. Diagrams and photo's are worth a thousand words.

One thing that leaps out is you seem to have just 'tried' different things and it appears you don't have an appreciation of the underlying theory behind what you were 'trying'.

You need to know the forward voltage drop of your IR LED and it's typical and maximum forward current. You can then determine what current limiting resistors you need for a specific supply voltage, what type and rating of transistor to use, and the transistor base current and resistor required.

You also mentioned connecting the LED 'directly'. If that is the case then there is a posibility you could have damaged or destroyed the LED, the Arduino output pin, or both.
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thanks,

u are right ! this is a hobby, i dont have a formal education. so i am trying. even though i understand some basic theory, putting it all together is difficult.

npn transistor:
collector +12 v
base --- 2k ohm ---- arduino control
emitter ---- 33 ohm ---- led ---- ground

i dont know the specific type of the led nor the transistor - how can i find it out ?

how to calculate the resistance to the base ?

how to calculate the resistance to the led ?

by wiring a multimeter to the led can i measure the voltage drop and current ?

thanks
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...i dont know the specific type of the led nor the transistor - how can i find it out ?

If you don't know what devices you are using then you need to do some more investigation to find out. The transistors should have some kind of markings on them that you can search on. For the IR LED it may be a case of searching for similar devices until you can find one that looks like what you have.

Where did you get your components from?

You need to find the data sheets for the devices you are using. All the required information is contained in the manufacturers data sheets.

Post photo's here. Someone may even be able to tell you what you have from a visual. There are guys here with years and years of experience and knowledge.

EDIT: For example, I've just looked at a 5mm clear IR LED on Sparkfun that quotes 1.5VDC forward voltage and 50mA current. You stated 12VDC supply and 33R resistor. That is over 300mA if you fully saturate the transistor. Assuming you are using an NPN; your transistor has a Base resistor of 2k, from a 5V Arduino output, meaning 2.5mA into the Base. You'd usually want 2-5 times the saturation current, so IF your transistor is capable of switching 300mA it would need a Hfe of around 120-150 @ 300mA to even have any chance of being saturated. Realistically it would need to be in the 300-700 range, or you'd need a lower Base resistor and a correspondingly more Base current. If using a cheap transistor, it may well only be rated 100mA  and Hfe of 30 @ 100mA anyway, so you could have damaged the transistor and/or LED due to overcurrent. Without knowing exactly what you are using then then it's mostly 'what if', assumption and educated guesswork.
« Last Edit: February 03, 2013, 12:06:53 pm by tack » Logged

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npn transistor:
collector +12 v
base --- 2k ohm ---- arduino control
emitter ---- 33 ohm ---- led ---- ground

You have your circuit a bit backward.  The LED and resistor go between +12 and Collector.  The Emitter connects directly to Ground.  Your Base circuit is OK.
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I don't think you connected the grounds, Dave.
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33 ohms in a 12V circuit sounds a bit low to me.
What are the specs of the LED?
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33 ohms is almost certainly too low for a 12v circuit.
For example if the infrared LED is rated for 100mA at 2v forward:

12v - 2v = 10v.
10v / 0.1A = 100ohms.

You should use atleast 100 ohms.

Any reason you don't just use 5v? Why dissipate more power than necessary.
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You are missing two other things as well
1) you need to make sure you are modulating the IR at the same frequency as your remote
2) you need to make sure that your IR LED is the same wavelength of IR as the remote.
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For optimum distance you should make sure the wavelength matches but you should still get up to 2 meters distance with a red LED (or white works well also) if it's a good bright one. Being able to see the LED light up is obviously a huge advantage when you're testing.
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In my testing, when you get the modulating frequency closer to correct the range increases dramatically.
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Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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