The Light‐Emitting Diode as a Light Detector

Alight‐emitting diode (LED) and operational amplifier can be used as an affordable method to provide a digital output indicating detection of an intense light source such as a laser beam or high‐output LED.1 When coupled with a microcontroller, the combination can be used as a multiple photogate and timer for under $50. A similar circuit is used as a data logging system for a mirror galvanometer, providing a record of changes in the Earth's magnetic field as small as a few nanotesla (nT).