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2pins IR LED

GreenGiant

Feb 9, 2012
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so the diode is reverse biased, so nothing will pass through it, and therefor the output will read nothing, when the diode "sees" the IR the diode essentially lets the voltage through

Think of it like a transistor, you have 2 arms that wont pass anything but when the third is activated the voltage will pass between the first 2, typically these circuits use a transistor of some sort to create the switch
 

vick5821

Jan 22, 2012
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but diode need certain voltage to be forward biased right ?
 

Harald Kapp

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the IR the diode essentially lets the voltage through

More precisely: Not the voltage but the current.
When light of the correct wavelength (in this case infrared) falls on the photodiode a small photocurrent is generated within the photodiode. This photocurrent flows from the cathode to the anode and on through the load resistor. Across the load resistor the photocurrent produces a proportional voltage drop. See this explanation.

Since the photocurrent is generally very small, you need a high resistance toi achieve a good voltage signal. Another drawback of this simple circuit is that the voltage acros the photodiode changes with the photocurrent. Mostly you will add a high input impedance amplifier to decouple the photodiode from the rest of the circuit and to amplify the photodiode's signal.

Here are a few application examples.
 
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Harald Kapp

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but diode need certain voltage to be forward biased right ?
Right, but you don't forward bias a photodiode. You use it in reverse bias.
 
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