PIC-based speed controller (1)

Let me start with an outside view of the speed controller and explain what it does. It converts a 15V DC power supply into a 12V constant voltage, pulse width modulated power output for a DC motor. There are three pre-defined speed settings in addition to "off". And there is a watchdog timer, so if the motor is left running for 15 minutes, the controller automatically turns it off.

The user can cycle the speed setting between off - 1 - 2 - 3 - 2 - 1 - off - 1 - etc by pressing the black button. The three LEDs light up according to what speed is set.

After pressing the button, the speed gradually changes to the next setting; this transition takes three seconds. During the three seconds, the relevant LED flashes repeatedly before going steady when the speed has reached the new level.

The red button is emergency stop. It actually resets the PIC and hence immediately sets the speed to "off".


I got most of the components from Rapid Online. This included some very neat sprung terminals for connecting the power input and motor output.

The circuit is built on stripboard. More details on the circuit design and PIC program in my next post.