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Background: I am not a professional teacher, but I have been helping out some kids (including my own) with math and science. I thought it might be fun for the kids to decide on a project of their choosing, and work on it together to make math and science a little more "Real World". They chose to build an e-cycle (we compromised on a trike for safety reasons). I don't have much budget-wise for the project, so I challenged them to also turn this into an ecology project, and use used/recycled items. For the motor, one of the kids got a mechanic to donate a couple of slightly-used, but matching starter motors from a heavy-duty truck. Since starter motors are short duty-cycle motors, I figured the pulsed controller would be a good way to go, but also want to alternate between the two motors to allow more cool-down, and keep them from burning out too quickly.

Question: I have found designs for PWM controllers, but am lost at alternating the signal between the two motors. At present, my only solution is a mechanical one, where a disk would be connected to the drive wheel, with contacts routing power to one motor around one half, and the other motor around the other. This is far from an ideal solution, but is so far the only one we have come up with.

Is there a circuit that can be used in conjunction with the PWM controller to alternate which motor is getting the pulses? Or are we best off keeping with the mechanical solution?

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If you have a motor driver with an enable input, you can feed each driver enable with complementary values. This will turn one driver off and put its motor in freewheeling mode (due to both outputs of the driver being Hi-Z) while the other motor is actively driven.

schematic

simulate this circuit – Schematic created using CircuitLab

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