Question about Shield-Bot voltages

hyperritual

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

Mike Green

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

hyperritual

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

Mike Green

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.

hyperritual

Mike, good catch! I shall do likewise. Thanks again.