7 years, 6 months ago.

Power Supply Puzzle for the whole team

Hello mbed community,

We are team of students, trying to build a line following robot with objection sensing and lifting sequence, and we are stuck at selecting correct voltage supply for the robot.

What we are using:

1) mbed arm plc 1768

2) Three IR proximity sensors, 2 for line following, 1 for object detection, Each rated at 3v.

3) Two 3Vdc motors for driving wheels controlled by L293D, using 0.8 Amps each.

4) One 5V Stepper Motor 28BYJ-48 controlled by uln2003.

What kind of voltage source do you guys recommend?

1 Answer

7 years, 6 months ago.

Li-Po (lithium polymer) batteries are best suited for 'model' type projects. They can supply high current and steady voltage, small and light weight, cheaply available from the usual places (Ebay).

2 cells would be required, that will give you 7.4v. Mbed (LPC1768) can be powered directly from these batteries (-ve or 0V to pin 1 and 7.4v to VIN, pin2).

In your case do not use any regulated supplies from the Mbed module itself, it will not be happy!!

Use a separate voltage regulator for the 3v and 5v sensors and motor drive circuits. Again these can be fed from the Li-Po cell. The regulators can be linear (78 series type, ie 7805 for 5v) or you could use switch mode modules again from Ebay.

There are considerations on the choice of regulator. Linear regulators will require heat sink's to dissipate their heat but will not introduce RFI (radio interference). Whereas the switch mode type will be more efficient do not generate the heat like linear type but will introduce RFI that can have an effect on signals from the proximity sensors. The only way to find out is by trial and error. But in all cases keep the power and signal leads as short as possible and definitely use screened cables for the sensor connections, otherwise you may experience data errors and think you have a problem with your code.

If you use Li-Po batteries you will need an appropriate balancing charger, again easily available. One warning, Li-Po batteries can be dangerous if the terminals are shorted. These cells can supply extremely high current. So use an inline fuse from the battery just in case.

Battery capacity. Difficult to advise exactly as this depends on the space available, average current consumption and required running time. However in your case I would start with a 1000mAh - 2000mAh, see here:


Suitable charger.


Please ask if you have any further questions. Good luck and don't forget to publish your project here on Mbed.