As shown in the figure, when the phototransistor receives the light emitted by the LED, the phototransistor is turned on, and its emitter level is high, resulting in BG90 14 being turned on, and its collector, that is, the output voltage terminal, is low. On the contrary, when the optical path between the phototransistor and the LED is disconnected, the phototransistor is turned off. At this time, the output voltage is high. I wonder what kind of photoelectric speed measuring device you want to make? In addition, the voltage output can be connected to a single chip microcomputer, and the speed can be measured by timing interruption. Specifically, you can check this information, there are many online.