In the circuit of Figure 1, assume that a brush-type dc motor must drive a load back and forth between two endpoints on a lead screw. Optical sensors determine end of travel, and an SPDT switch selects to which end to send the load. The sensors themselves supply all the necessary directional logic, and a triac powers the motor with the necessary polarity of half-wave pulses from the 13.5 V-ac input. Starting with the load parked at the south end, when you set switch S1 to north, the ac input connects to the LED in the north-side sensor, IC1, through current-limiting resistor R1 and reverse-polarity-protection diode D2. The phototransistor output from IC1 then supplies firing pulses to the gate of triac Q1 during the northbound half-cycle, and the load proceeds toward that detector.
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| Figure 1. | The optocouplers provide both position-control logic and triac gate drive to the motor. | |
Similarly, IC2 drives the motor during the other half-cycle to push the load south, and stops when it reaches the south endpoint. This scheme works even when you change the direction switch while the load is in motion. Power for the gate drives comes from the ac input and the half-wave rectifier comprising D1 and C1. You might need snubber R2 and C2 if the motor's inductance causes spurious firings of Q1 during undesired half-cycles. The motor's stall current is approximately 2 A. You can easily scale the design for larger or smaller motors.
