Tachometer circuit

Use a meter to measure the speed of the electric fan, and place the fan blades between the light source (bulb HI.) and the photosensitive transistor . When the light is blocked by the blades,
the light cannot shine on the phototransistor, and the VTI) is in a high-resistance state; after the blades turn a certain angle, the light shines on the VTL from the gap between the blades, and
the phototransistor is turned on and in a low-resistance state. .
    When the fan blades rotate continuously, the phototransistor is intermittently illuminated three times during each revolution (a typical table fan only has three blades). Since a 12V voltage is applied
to both ends of the series circuit of the resistor RJ and the photosensitive transistor , a pulse potential
will be formed on the two poles of the VTL with the intermittent irradiation of light . After     the pulse potential is differentiated by the differential circuit composed of the input resistor of Nan C and VT , a series of sharp pulses are formed, and then rectified by VD1 and VD2 to obtain a pulse signal with twice the frequency, which is output through the capacitor c2 and sent to the microammeter. Head PA, the rotation speed is indicated by the pointer of PA. The higher the rotation speed , the greater the pointer deflection, and vice versa.     If you have a digital frequency meter, there is no need to use a PA. Connect the lead between point A and ground, and directly connect it to the input terminal of the digital frequency meter to display the speed number.