Three-speed fan with light-controlled speed regulation circuit

 The circuit is shown in Figure 3-5. It can be manually controlled (button SB) and light controlled (photosensitive transistor VTj). When the flashlight is
pointed at VTi, the fan will start running and automatically adjust in three levels: slow, medium and fast.
    The clock pulse generator is composed of CMOS six inverters Icl (gates F2-F4) and resistor and capacitor circuits. The fan speed (gear) switch circuit is composed of the integrated circuit
Icz which is a decimal counting/timing decoder, transistors VTz~VT4, bidirectional thyristors Vl-V3, etc.