Photodiodes and LDRs

Thread Starter


Joined Oct 21, 2012
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.


Last edited:


Joined Feb 12, 2009
LDRs are measured in ohms. They will normally be specified by the dark resistance - which will be quite high - probably megohms (millions of ohms) and the resistance when illuminated by a certain intensity of light. This will be much lower - probably only a few thousand ohms.

So in your circuit, the transistor will switch OFF when the LDR is exposed to a bright light. In your circuit, this will not happen suddenly - the transistor will slowly change as the brightness increases.

Photo-diodes can be used in two ways: With no voltage across them, they will generate current when illuminated. So they will be specified as a current (probably microamps) at a certain light level. They are used in an op-amp circuit to maintain zero volts across the diode. This is the best way as leakage current will be zero at zero volts.

Or they can be used with a reverse voltage across them. When dark, they will have a very small leakage current. this current will increase when illuminated. The leakage current will be specified at a certain voltage and temperature. Also the current at a certain illumination.

An infra-red LED will probably work as a photo-diode. Another old trick is to find a transistor in a metal can (the old BC108 used to be good) and carefully saw the top off without damaging the transistor chip inside.