How a Vex can monitor its own battery voltage

Hey folks,

For the traction study that I’m doing, I wanted to log the battery voltage as part of the data, so I came up with the following nifty “sensor” and thought I’d share.

You plug one end into a spare PWM port (or into a Y adapter if you are using all 8 motor ports), and the other end goes into an Analog input port. Reading the analog value gives you a (unit-less) voltage, which you can easily convert to volts.

The raw data from my calibration (using a regulated bench supply) looked like this:
[INDENT]619 = 6.00V
670 = 6.50V
721 = 7.00V
741 = 7.20V
773 = 7.50V
824 = 8.00V
875 = 8.50V
926 = 9.00V[/INDENT]

Which looks something like this:


  • Dean

That’s a very clever Vex sensor that you came up with! What schematic capture program did you use to get the 3D figure?

Actually, it is just a basic page layout & drawing program for the Mac called OmniGraffle.
I use it for lots of stuff and just find it very natural for simple drawings like this.


  • Dean

Does it provide the Vex parts as symbols or did you draw those yourself?

It doesn’t have a library of parts, but it is pretty easy to draw new pieces like these and reuse them. I’ve been slowly creating my own library of pseudo-photographic Vex parts for diagrams like these.


  • Dean

The drawing is really top notch. I was looking for you to do one like it for the potentiometer tip that you gave the other day. I think there would be some value to posting these drawings in the gallery.

Very nice Quazar,

Thank you for the information on monitoring the battery voltage.


Thanks for the kind words! I have a lot of fun putting these together, so expect to see more (I’m shooting for about 1 a month). If you have ideas for any you’d like to see, send me a PM…

Interesting - I was just using a stock Vex potentiometer, so it never occurred to me to diagram it up like I’ve done for these wiring projects. I’ll add this to the list of project ideas to diagram and post next.

I’ll go ahead and post the ones I’ve done into the gallery. [edit: Posted [url=]here] Some of the projects I have in mind will be longer than a half-page, so I’m not sure the gallery will work for all of them.


  • Deam

Diagram seems to have gone missing above and the option to edit has expired.

It is now posted on the gallery here.

  • Dean

i like this idea of measuring the voltage
with this you could wire a speaker and the module will give a 2 second long tone that tells you the batter is going weak
because it only tells you good or dead on the front led