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Photodiodes and LDRs

McDroogie

Dec 25, 2012
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Dec 25, 2012
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I want to create a very simple dark sensor. Browsing through many schematics I find all I might need is some resistors, LEDs and transistors and a key component called a photodiode. So my questions are:

Can I use an infared detecting led in replace of a photo diode?

Secondly, would a phototransistor or a Light Dependent Resistor perform the same function as a photodiode?

And finally are photodiodes or LDRs measured in ohms?

The attached schematic is what I would like to do.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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That key component is actually an LDR (light dependant resistor)

A photodiode has a very tiny output signal that would need more amplification that a single transistor can provide. A phototransistor can give a larger signal, but still small compared to the signal an LDR can give (an LDR has disadvantages too, so don't think they must be "better").

Your circuit essentially used light to turn off the transistor. To keep the transistor turned off until the light level gets quite low, R2 has to be quite large. The problem then becomes that it may not be able to provide sufficient base current to turn the transistor on in the dark (it all depends on your definition of "dark".

Also, the LED will turn on slowly as the light level falls, it won't just snap on.

Your image is so small that I can't read the component values. Can you post a larger one?
 
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