Phototransistor Circuit Diagram. A phototransistor is depicted as a bjt with the base terminal removed and the arrows imply that the base is sensitive to light. A phototransistor is mostly configured with its base pin unconnected as shown in the following two circuits.
A phototransistor is mostly configured with its base pin unconnected as shown in the following two circuits. It is more or less proportional to the amount of incident light when vout voc. Therefore the voltage at vout will move high and low based on the amount of light.
The photo detectors on this page use lm339 quad or lm393 dual voltage comparator integrated circuits to detect the change in voltage across the sensor.
Common emitter phototransistor circuit the common emitter phototransistor circuit configuration is possibly the most widely used like its more conventional straight transistor circuit. Therefore the voltage at vout will move high and low based on the amount of light. In the left side figure the connection effectively causes the phototransistor to be in the reverse bias situation such that it now works like a photodiode. One example is a darlington pair phototransistor circuit which provides double amplification of the output current.
