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Topic: LED as Sensor ?in reverse or forward bias?? (Read 1 time) previous topic - next topic


Jun 20, 2013, 09:01 am Last Edit: Jun 20, 2013, 09:09 am by devendra Reason: 1
I read some post related to this and what I got is LED can generate small photo current when you throw light towards it.
I also tried with LM358(Op-Amp),I connected in reverse bias and got same result.

But my question is when I connected LED Anode to BC547 Base and cathode to GND(as shown in attached file),
I mean in forward bias it worked!and it was able to switch LED on (not fully but ther is "some" light) which is connected to collector?

i mean to use LED as sensor it should work in reverse bias?they why it worked with BC547 in forward bias?please tell me I am very curious to know about this.. :smiley-mr-green:


The LED can make a positive current when connected in forward bias. If it is reversed, it will provide a negative current. The forward bias, as shown in the schematic, is the correct bias to turn on the npn transistor using photocurrent.


MITSUBISHI has very nice paper about Very Low-Cost Sensing and Communication Using Bidirectional LEDs



Very neat paper, thanks for sharing. I just might integrate a simple LED communication interface on a project I'm working on.

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