Digital Circuit Production 03-Digital Random Turntable

1. Circuit simulation and principle explanation

      The circuit consists of a pulse generator and a shift register. When the S1 switch is pressed, the transistor Q1 is turned on, and the NE555 forms a multivibrator. The 3-pin output pulse sequence causes the Q0~Q9 pins of the CD4017 to output high levels one after another. When S1 is disconnected, that is, when the button is released, due to the existence of the large capacitance of C1, the triode will not disconnect immediately, but will slowly turn off as the capacitor discharges. The phenomenon is that the rotation speed of the LED slowly slows down until a certain light is turned on. Stop at , realize the function of random turntable.

      In this circuit, you can change the time it takes for the LED to stop after releasing the button by changing the capacitance size of C1. The larger the capacitance value, the longer the waiting time. Changing the value of R1 can change the speed of LED flow. The larger the resistance, the slower the diode rotates.

2. Precautions

(1) Pay attention to the positive and negative directions of the capacitor and LED when welding, and do not install them upside down;
(2) If the circuit cannot function after welding, check whether there is a short circuit against the schematic diagram and PCB, especially the transistor pins are relatively close and easy to connect. together to cause a short circuit;