DC Motor Control
This circuit was made to control the on-off and left-right functionality of a dc motor. Based on the op-amp to deliver the control current to the push-pull transistor pair, this particular circuit can give control to a microcontroller safely while at the same time providing current to the motor directly from a power source, using simple and cheap materials.
While it is by no means an ideal control circuit, it was designed to provide this basic functionality reliably at a low cost and, most importantly, at small size. This circuit was required 5 times in the ROV me and my team built for our graduation project, thus it had to be small and simple in order to fit into the hull along with all the other electronics required. Additionally, it is quite fool-proof, as there is no combination of its input that produces unpredictable or destructive behaviour.
The additional resistor at the op-amp's output and the diodes were suggested by a member of edaboard.com (thanks!) to provide additional protection - they do not directly influence the functionality of the circuit.
Import data for Circuit Simulator
Import data for Circuit Simulator (improved a bit)
The NAND gates are used because they are cheaper and easier to find, but essentially they function as a single AND gate. While the ENABLE input is high, the op-amps non-inverting input is at 2,5V, thus its output becomes approximately -12V or +12V depending completely on the state of the LEFT/RIGHT input. If ENABLE is low, the LEFT/RIGHT input doesn't affect the output, which becomes approximately 0, shutting the transistors down. Depending on the output of the op-amp, either transistor will turn on and connect the motor to the corresponding power supply, making it turn in the direction the voltage makes it turn.