This speed controller is designed to drive a DC gearmotor with a maximum current drain of 600mA at 12VDC. It is a "chopper" circuit of my own design. Speeds from 50 to 1800 RPM were obtained in my application with a significant load on the motor. My specific application is described as an example. Naturally some mechanical changes may be required to fit your application.

Circuit Description

The reflective sensor S1 detects a series of 4 equally spaced white stripes placed on the motor pulley. There should be a black background.

IC U1 is a frequency converter containing a charge pump, op amp, and an output transistor. Pot VR1 is used to set a threshold voltage between the minimum and maximum swings of the input voltage. A fixed resistor divider could be used here once the sensor is placed. A 5 Vpp signal should be easily obtained.

The converter output is amplified by op amp U2. VR2 is used to set the motor speed.

Device U3A is a 40KHz pulse generator. U3B is a pulse-width-modulator. As the motor speed decreases, the U2 op-amp output increases to widen the output pulse, providing more power to the motor and maintaining speed.

The output from U3B drives the transistor which in turn drives the motor. A heat sink is required on the transistor for heavy motor loading.

Notes

Part values and numbers are shown on the schematic. The caps are ceramic. All resistors are 1/4 watt 5%. The reflective sensor has an NPN phototransistor output. It is made by QT Optoelectronics. Any similar type should work.

Use decoupling caps on the power-supply pins of the IC's to ground.

The stripe spacing on the pulley is critical or speed "jitter" may occur. Do this step carefully. Improved low speed response can be obtained by increasing the number of white stripes to 8. However resistor changes around U2 may be required.

This circuit is somewhat temperature sensitive. I noted an increase of about 4% in RPM when the ambient board temperature was elevated from room temperature to 115 deg F. The problem area is U1 and the two caps associated with it. I would expect no problems if the board is not enclosed.