Photoelectric sensors

The detection of light and light intensity is used in many IoT sensor devices, such as security systems, smart switches, and smart street lighting. As the name implies, a photoresistor varies in resistance depending on light intensity, while photodiodes convert light into an electrical current.

Photoresistors are manufactured using a high-resistance semiconductor. The resistance decreases as more light is absorbed. In the dark, a photoresistor can have quite a high resistance (in the megaohm range). Photons absorbed by the semiconductor allow electrons to jump to the conduction band and conduct electricity. Photoresistors are wavelength sensitive, depending on their type and manufacturer; however, photodiodes are true semiconductors with a p-n junction. The device responds to light by creating an electron-hole pair. The holes move towards an anode, the electrons migrate to a cathode, and a current is produced. Traditional solar cells operate in this photovoltaic mode, producing electricity. Alternatively, a reverse bias can be used on the cathode to improve the latency and response time if needed: