Mbed turns off 3pi after a few seconds

Hello, I’m currently having problems with m3pi…

  • 3pi robot works everything fine by itself (demo tested)
  • 3pi robot works everything fine with mbed expansion board attached (NXP LPC1768 not connected)
  • 3pi robot turns off automatically with expansion board connected and NXP LPC1768 connected on the expansion board
    • turns off few seconds after power on
    • Serial-slave program is loaded in the 3pi (issue happens with any program though)
    • mbed m3pi test software is loaded in the NXP LPC1768 (issue happens with any program tough)
    • batteries are brand new
    • NXP LPC1768 not connected to USB
    • I remember seeing the m3pi working fine once (problem started after I detached expansion board and re-connected )
      • I was able to control the 3pi from the mbed controller in that occasion (moving motors, screen, etc)
    • NXP LPC1768 works fine on an ARM mbed Application Board …

please help


It sounds like you might be having connection issues. Could you verify that you are placing the NXP LPC1768 in the correct orientation on the mbed expansion board? Could you also verify that the mbed expansion is connected to the 3pi correctly? Could you post pictures of the m3pi completely assembled with the NXP LPC1768 (pictures from multiple angles will help determine if everything is connected correctly)? How long was the m3pi working with the mbed controller before it started to turn itself off?

- Jeremy

It worked for like 2 minutes or so…I’m not sure exactly because I disconnected everything and then when I wanted to test again a couple of hours later… it would not work
in the file DSC_0032.JPG you can see the mbed working fine (its running a program that turns a led on) but after a couple of seconds, everything goes turns off
I guess one can assess that is properly connected because the leds on the expansion board + leds on the mbed are on right ?

additional note:
the 3pi robot also goes off right away if you hit its reset button (when expansion board+NXP LPC1768 are both connected to the 3pi board)

You mentioned that you were using new batteries in your original post. However, it sounds like you are having a power related issue. Could you measure the voltage across the charge port when everything is off and again when everything is on and see if there is a significant drop in voltage?

- Jeremy

batteries on the 3pi by itself reach 5v when turned off, drops to 4.6 when on…
when the expansion board and mbed are connected: drops from 5v to 2.8v and then goes totally off
am I better off using Duracell Quantum batteries and/or rechargeable ones ?
I’m afraid that it worked a while long time ago because the batteries were brand new and they got drained a lot very quick after a couple of hours (2 or 3 maybe)
turned off…is this a normal behavior ?

It sounds like those batteries are severely discharged, and you will definitely want to replace them. In general, we recommend using rechargable batteries for applications that use DC motors, as alkaline batteries tend to not last. You might consider using our rechargeable AAA NiMH batteries.

By the way, running the m3pi for 2 - 3 hours could use a lot of energy depending on what it was doing. If the motors were running the whole time, I would expect the batteries to be depleted. For line-following robots, people often recharge/replace batteries between runs during competitions.

- Jeremy

I have bought the iMAX-B6AC Balance Charger and Discharger from Pololu and 4 AAA NiMH batteries…
I have connected it to the 3pi trough its charge port…
can you please indicate what is the procedure to charge the batteries?..I do not know what to select
in the menu for amperage and volts…
also, can I charge the 4 batteries at the same time? how long would they take to fully charge?

please and thank you

Yes, you can charge all 4 batteries at the same time. The time it will take to charge them will depend on your charge rate and how discharged they are. We generally recommend charging those batteries at 1/2 C (450 mA). You should set your iMAX charger to the correct battery type (NiMH) and charge them at 0.4 to 0.5 A. You can refer to the iMAX-B6AC instruction manual for how to set various charging parameters like the battery type. You might also search the web for additional tips on charging NiMH batteries.

- Jeremy

I finally charged the batteries and now the 3pi+mbed combo are working fine.
My problem has been fixed, thank you very much.

Thanks for letting us know. I am glad you got it working!

- Jeremy

one last question:
how long is the m3pi supposed to last after a full recharge ?
(before having to recharge again)
(how long moving motors, and how long without moving motors(just turned on) and no additional hardware connected)

It is difficult to say how long a charge will last. If the motors are moving, the batteries might last for about an hour depending on the load. According to this page from mbed’s website, the mbed NXP LPC1768 draws about 140 mA when it is not doing anything, so the m3pi might last a couple of hours if the motors are idle.

- Jeremy