Tricks about LEDs

I recently learned two tricks about LEDs

Trick 1 - Ifr you put current through an LED you get light. What happens if you put light into an LED? Yup, you get current so an LED is also a photodetector. It's pobably not very efficient but if you are looking for a cheap detector an LED might work. You really want to measure voltage since the current won't be very large. My limited experiments show that green LEDs deliver the most voltage (about twice what a red LED will do). the actual voltage depends on the light intensity as well as the specific LED so play around.

Trick 2 - I was working on a broken shaft encoder which used an infrared LED and photodiodes. We wondered if the IR LED was working or not but couldn't tell because our eyes don't see IR. Another guy at the lab wandered past and noted that our phone cameras COULD see IR and sure enough they do detect it and make it visible! Again, some cameras filter IR more than others.