Phototransistor Circuit: Make an Automatic Light On/Off Switch The more light that hits the photodiode, the more electrons flow into the base, and the stronger the current becomes. This turns the transistor on so that current can flow from the collector to the emitter. When light hits the photodiode, it produces a flow of electrons into the base of the transistor. Here’s a summary of the phototransistor working principle: How Phototransistors WorkĪ phototransistor has a base, collector, and emitter like a regular transistor, but instead of a pin to connect to the base, it has an internal photodiode that converts light into current that acts as the base current.

PNP phototransistors are less common and are used for applications such as smoke detectors or IR sensors. NPN phototransistors are the most common and are used for low-power applications, such as proximity sensors or optical encoders. There are two different types of phototransistors, NPN and PNP. A simple circuit to convert light into voltage

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