This is a circuit for DC motor circuit. This circuit is used PWM speed control using 555 timer IC. This is the figure of the circuit;
The 555 Ic is wired as an astable and the frequency is constant and independent of the duty cycle, as the total resistance (R charge + R discharge, notice the diode) is constant and equal to 22Kohm (givin a frequency of about 1Khz, notice the hum). When the potentiometer is all up, the Rcharge resistance is 1,0 Kohm (the diode prevents the capacitor to charge through the second potentiometer section and the other 1,0 Kohm resistor) , and Rdischarge is 21 Kohm, giving a 5% on duty cycle and a 1Khz frequency.
When the potentiomenter is all down, the Rcharge resistance is 21,0 Kohm (the diode prevents the capacitor to charge through the second potentiometer section and the other 1,0 Kohm resistor) , and Rdischarge is 1 Kohm, giving a 95% on duty cycle and a 1Khz frequency. When the potentiometer is at 50% , the Rcharge resistance is 11,0 Kohm (the diode prevents the capacitor to charge through the second potentiometer section and the other 1,0 Kohm resistor) , and Rdischarge is 11 Kohm, giving a 50% on duty cycle and a 1Khz frequency. The 555 provide good current capability to drive the mosfet fast and to drive a bipolar transistor.
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