A brushless dc motor needs several voltage levels to control its speed: 0V to stop the motor, 5V to run it at maximum speed, and some voltages between these extremes to run it slower. When you use such a motor in a system under microcontroller supervision, the microcontroller should generate all these voltages. But a microcontroller is a digital device, and it usually has no analog output. Several methods are available to overcome this deficiency. For example, you could use a DAC, a digitally programmable potentiometer, or some analog switches connected to resistor dividers. However, when you need only a few intermediate voltage levels, it would be more attractive to find a method that uses microcontroller software. This Design Idea exploits the fact that you can program a microcontroller's I/O pins as either input or output. When you program a pin as output, you set its voltage level to high (5V) or low (0V). When you program a pin as input, it has no effect on output voltage. Figure 1 shows a circuit example.

The circuit needs no external components, except for a few resistors. You can set the output-voltage levels during the main program execution or by an external interrupt. For testing purposes, using an external interrupt is preferable (see Listing 1). A pushbutton-mode switch triggers the external interrupt, the service routine of which consequently sets all the predetermined voltages. This design uses 0, 1.25, 2.5, 3.33, and 5V levels. Resistors R₁ to R₄ determine the intermediate levels. The circuit uses the inexpensive, 8-bit MC-68HRC908JK1 flash microcontroller from Motorola (www.motorola.com), but this method applies to almost any kind of microcontroller.