Question about Shield-Bot voltages

hyperritualhyperritual Posts: 10
edited 2019-03-07 - 01:47:04 in Robotics
Hello!

Inspired by the Python-friendly cyber:bot (which I just received -- thank you, Parallax), I have been wondering about using the Parallax Shield-Bot with Adafruit's METRO M0 Express, which can be programmed in CircuitPython (Adafruit's beginner-oriented flavor of MicroPython). The M0 has (almost) the same form factor as the Arduino Uno, but it operates at 3.3V logic levels rather than 5V.

At first, I assumed the voltage difference would be a big problem, but then I examined the schematic for the Shield-Bot and now I think using the M0 with the Shield-Bot might be quite feasible. It appears that power is regulated by the Uno/M0 board, and the shield merely "brings it up" for the servos and breadboard. Looks like Vin from the Uno/M0 feeds the Shield's VR1 and VR2 regulators, and they just output to the X1 headers to power the breadboard. In other words, so long as I am careful not to (directly or indirectly) connect a 5V or Vin power pin to one of the M0's I/O pins, then there should be no problem -- right? The continuous-rotation servos allow 3.3V as well as 5V control signals (IIRC, the Propeller boards use 3.3).

I am posting this here in case anyone has already attempted what I have proposed, or in case any of you sees something that I have missed that would cause the M0 to fail when used in conjunction with the Shield-Bot.

Thank you.

Sincerely,
Joshua

Comments

  • You have to be really careful about 5V signals with the Metro M0's 3.3V I/O pins. Other than that, you should be OK and your observations look correct. It's sort of like using a Propeller BoeBot since that also has 3.3V logic levels on a board with a lot of 5V signals and connection points. Remember that some 5V devices that you might use on the little breadboard may not be "happy" with 3.3V logic levels
  • Cheers, Mike. I appreciate the confirmation. If I get something interesting going with the M0 and Shield-Bot, I will share it here on the forum.

    Joshua
  • Look at the schematic for the Propeller Activity Board WX. There's a 3.9K resistor between the servo and the Propeller I/O pin. This is to protect the I/O pin from noise or leakage from the servo electronics even though the servo control line is an input to the servo's electronics. The M0's I/O pins may need the same sort of protection. I spliced a 3.9K resistor into the white control wire about an inch from the plug/socket(?) on the end of the servo cable when I was using a 3.3V logic controller without the built-in resistors.
  • Mike, good catch! I shall do likewise. Thanks again.
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