Jon's Place

Friday, February 7, 2014

Of H-Bridges and 3.3 Volts

So, in another twist of this ongoing saga, it turns out the h-bridge I was using for uCee is not designed to work properly with 3.3 volt logic. I had it hooked up, but the motors weren't turning. I took it all apart, hooked up my logic analyzer to the logic pins, and sure enough I was getting a valid signal. I took my multimeter and was going to test the battery voltage to make sure everything was okay, but when I touched the battery probe to the ground pin on my MicroPython board, the motor started running.

That was weird. So I reset it, and tried again. Same results. I tried it with the tip of a screwdriver, touching one of the +5 volt pins - same results.

I went and found the (now discontinued) product page on, and sure enough, the minimum required voltage is around 4.2 volts.

This is only the second robot I've built with 3.3 volt logic (NanoSeeker was the first), and the ARM chip I'm using (STM32F405) has 5-volt tolerant logic pins, so I haven't had to think about it too much so far. Clearly, I didn't think enough about it. It worked fine with the Teensy 3.1, but it clearly doesn't with my new board.

Anyways, I found this cool Canadian company called Diigiit Robotics, that had one of Pololu's new much cheaper h-bridges in stock, so I ordered it. This h-bridge will work with logic down to 2 volts, so I should be safe.

I'll have to print a mounting plate to hold this board in place - the board is much smaller than my current h-bridge, and has no mounting holes. I'll design a plate that is in the shape of the old h-bridge, with the middle cut out and some clips to hold the board in place.


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