Measuring light with a photodiode

It is often useful to measure light with a photodiode.  It is quite easy.


  • Determining when a light source turns on / off or oscillates
  • Establishing that the expected timing of system or light sources is correct

Required hardware

One needs a photodiode, such as:
  • VTB1113, part 980-2625 from Allied Electronics
  • SLD-68HL1A, part 699-0008 from Allied Electronics, which has the specs 
               "Photodiode, Planar; TO-46; 450 muA (Typ.); 0.40 V (Typ.);
                100 nA (Max.); -40 de"
standard oscilloscope probe (also known as test probe or scope probe).

How to do it

One can simply connect a standard oscilloscope probe across the 2 leads of the photodiode and tape the scope probe to the side of the monitor, so that the photodiode is near to the light source.  If the light source is a screen, it is helpful to tape the scope probe to the edge of the monitor (not the glass!) so that the photodiode position does not vary during any recording.  If you want to test light in a particular region of the screen, it is useful to put some cardboard on the screen and cut a small hole so that only light in the region of interest shines through to the photodiode. Then, just plug the BCN end of the scope probe into a data acquisition device or oscilloscope.

Photodiode connected to an oscilliscope probe