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The circuit in this Design Idea controls the speed of a 5V permanent-magnet dc motor through the PC’s parallel port (Figure 1). You use the C++ computer program to run the motor at three speeds. The circuit uses PWM (pulse-width modulation) to change the average value of the voltage to the dc motor. You connect the motor to the PC’s parallel port with an interface circuit. The design comprises IC₁, a 74LS244 buffer; IC₂, a ULN2003 driver; relay switches S₁, S₂, and S₃; IC₃, a 555 astable multivibrator circuit; and Q₁, a 2N2222 driving transistor. The 555 timer operates as a variable-pulse-width generator. You change the pulse width by using relays to insert or split resistors in the 555 circuit.

The computer program controls these resistors. When S₁ is on and both S₂ and S₃ are off, the timer output is set to logic one, thereby driving the motor with its maximum speed. When S₁ and S₂ are on, the 555 timer generates a pulse signal with a 50% duty cycle. In this case, the charging resistor, R_A1, is equal to the discharging resistor, R_B. In the third case, S₁ and S₃ are on, and the charging resistor is R_A2,where R_A2=0.1×R_B, reducing the on time of the pulse signal and, consequently, the speed of the motor to the lower limit. Table 1 summarizes the on/off-operation conditions of the relays and the corresponding dc-motor speeds.