LM2907 Motor Speed Control – Part 1

We can purchase DC motors with or without brushes with ac tachometer outputs already provided by the manufacturer. A very low cost speed control can be constructed with these motors in combination with the LM2907. The most simple version is illustrated in figure below where the tachometer drives the non inverting input of the comparator up towards the preset reference level.

The output is turned off and the power is removed from the motor when that level is reached. The voltage from the charge pump output falls and power is restored as the motor slows down. Thus, by operating the motor in a switching mode, the speed is maintained. Hysteresis can be provided to control the rate of switching. [Circuit’s schematic diagram source: National Semiconductor Application Note]