Another form of providing fast feedback for BDC motors is BEMF voltage measurement. The BEMF voltage is proportional to the speed. Figure 11 shows where the BEMF voltage is measured in the bidirectional drive circuit. A voltage divider is used to reduce the BEMF voltage to the 0-5V range so that it can be read by the analog-to-digital converter. The BEMF voltage is measured between PWM pulses when one end of the motor is floating and the other end is grounded. In this case, the motor acts as a generator and produces a BEMF voltage proportional to the speed.
Due to differences in efficiency and materials, all BDC motors will behave slightly differently. Experiments are the best way to determine the BEMF voltage for a given motor speed. The reflective band on the motor shaft helps the digital tachometer to measure the motor's speed (in RPM). When reading the digital tachometer and measuring the BEMF voltage, the relationship between the motor speed and the BEMF voltage will be obtained.