On: How to use LEDs to detect light

[jacquesm]

Old diodes were painted black for that reason. If you scraped off the paint you had a pretty good photodetector. Old transistors ditto (OC71 for instance), the housings were made of glass so already transparent.

The Germanium ones were more susceptible to this than the Silicon ones.

[aequitas → jacquesm]

I remember old books with circuit 'recipies' where you had to sand off the top of a metal casing transistor to turn it into a detector.