A Simple, permanent-magnet dc motor is an essential element in a variety of products, such as toys, servo mechanisms, valve actuators, robots, and automotive electronics. In many of these applications, the motor must rotate in a given direction until the mechanism reaches the end of travel, at which point the motor must automatically stop. Although you can use microswitches to stop the motor at the end of travel, their size, weight, and cost can be prohibitive, particularly in low-cost, portable items. The circuit in Figure 1 implements a low-cost, micropower, latching motor controller that uses current sensing rather than switches to stop the motor. The design is optimized for a supply voltage of 3 to 9V, making it well-suited to battery-powered applications. To understand how the circuit works, assume that cross-coupled flip-flops IC_(1A) and IC_(1B) are both in a reset state, such that the D input of each one is high. Because both Q outputs are low, the H-bridge transistors, Q_1 to Q_4, are all off, and the motor is idle.
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