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I am trying to control multiple stepper motors sequentially (only one at a time) with one controller, one driver a multiplexer and a dedicated "relay" circuit (shown in red). The idea is instead of having each motor have a dedicated driver, one driver is connected in parallel to all motors but with a "relay" circuit which lets the controller decide (via a mux) which relay to switch on and therefore which motor will rotate. For simplicity, the following diagram consists of only 2 motors:

enter image description here

I need inputs on the "relay" circuit (in red) since I built one and it is producing inconsistent results. Sometimes it works well sometimes it's constantly "on" with no way to influence through the "relay" circuit.

Any suggestions on tagging this question better are also appreciated!

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  • \$\begingroup\$ What is the question? \$\endgroup\$ – JYelton Mar 1 at 18:08
  • \$\begingroup\$ You would need, essentially, a power analog switch for each "multiplexer" channel -- this means two FETs instead of one, and gate drivers powered by the motor power rail. By the time you're done, you'll have all the power circuitry you need to build your own motor driver. I suspect you'll find that it's less expensive to use one driver per motor, and multiplex the logic signals. \$\endgroup\$ – TimWescott Mar 1 at 18:16
  • \$\begingroup\$ have you got enough gate drive? are you really using 2N7000? have you considered a counter like CD4013 instead of the mux? \$\endgroup\$ – Jasen Mar 1 at 21:52
  • \$\begingroup\$ The question is whether there exists a better solution than the 4 2n7000 (as shown above) and if anyone can spot a potential flaw or bug in the design. TimWescott: A driver for each mux channel is bulky and not as compact as I was looking for and arguable when it comes to comparing prices (number of motors per project, number of projects). Jasen: The mux here is not the problem, rather the need for some sort of small enabling (on\off) circuit trigered via the mux. \$\endgroup\$ – Roee Mar 2 at 13:47
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I think the problem is that the pull down resistor was too high. This made the gates of the transistors susceptible to the noise induced by the alternating currents\voltages of the motor nodes (12v, 0v, 12v, 0v, ...).

I think that I will try working with a shift register instead of the multiplexer. I hope that its OL voltage can pull down the noise on the gates.

Insights will be appreciated...

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