This article uses an infrared detector and a smoke alarm circuit to explain the detailed usage of phototransistors.
1. Infrared Detector
An infrared detector is primarily used to detect whether an infrared remote control transmitter is functioning properly. The infrared detector circuit is shown in the figure. When infrared light from the infrared remote control transmitter strikes phototransistor VT1, its internal resistance decreases, driving VT2 into conduction, causing light-emitting diode VD1 to illuminate in rhythm with the incoming light. Since the brightness of LED VD1 depends on the intensity of the infrared light striking phototransistor VT1, the battery life of the infrared transmitter can be estimated based on the brightness of LED VD1.

2. Smoke Alarm
A smoke alarm consists of a series-connected feedback photosensitive circuit consisting of an infrared light-emitting diode and a phototransistor, a semiconductor switch circuit, and an integrated alarm circuit, as shown in the figure. When the monitored environment is clean and smoke-free, infrared LED VD1 illuminates at a pre-set starting current. When this infrared light is received by phototransistor VT1, its internal resistance decreases, increasing the current in the series circuit of VD1 and VT1. This correspondingly increases the luminous intensity of infrared LED VD1, further reducing the internal resistance of the phototransistor. This cycle forms a strong positive feedback loop, until the current in the series photosensitive circuit reaches its maximum value. The voltage drop across R1, caused by VD2, turns VT2 on, turns VT3 off, and the alarm circuit deactivates. When smoke rapidly increases in the monitored environment, the light transmittance of the air deteriorates. The light flux received by phototransistor VT1 decreases, increasing its internal resistance. The current in the series photosensitive circuit also decreases, and the luminous intensity of LED VD1 also decreases. This cycle forms a negative feedback loop, until the current in the series photosensitive circuit decreases to its starting current value. The voltage across R1 drops to 1.2V, turning VT2 off and VT3 on, activating the alarm circuit and generating an alarm signal. C1 is provided to prevent interference from brief periods of smoke.