Motor controller

The layout for the motor controller PCB was, again, fairly straightforward as there was already had a board layout ready, also the board composes of little more than a single H-Bridge, a voltage regulator and some side connectors. On the other hand, finding components which worked together was a little more challenging.

Ever since starting to designing the modular robots, I was constantly thinking of shield boards that could be developed to allow for more complex behaviour and an Arduino/Atmel shield seemed to be an obvious option for a programmable control structure. The main Atmel chips are typically powered off 3.3v or 5v power supplies, also, 5v is a standard supply voltage for logic chips. However, no battery supplies 5v, therefore a voltage regulator would have to be used, the most common 5v voltage regulator appeared to be the LM7805, but it requires a minimum input voltage of 7.5v to function properly and my intended battery array is not that high (I was thinking of using AA batteries, as they are widely available, cheap, and have a large capacity. So 3 or 4 in series would be good; 4.5v/6v respectively) This lead me to consider a 3.3v voltage regulator such as the LM1117, I had a look and found it requires a minimum input voltage of 4.5v, which is perfect.

Another component which required some research was the H-bridge, I originally intended to use the SN754410 which is sold by Sparkfun (or more accurately, resold by Hobbytronics) so it could be sourced easily. Unfortunately, the SN754410 requires a regulated logic supple voltage of 5v, which, as was just mentioned, I am unable to provide with the current battery amount. I then found a few more H-bridges, but they all provided me with the same problem, I finally stumbled upon the L293D, which operates down to 4.5v also. A common issue with motor controllers of all types that they can become hot when operating, but I found a few suitable DIP heatsinks on UK Farnell, such as this and this. The maximum current output of the L293D is 600ma with a 1.2A peak, both of which are way above the current use of the motor/s I have been looking at, more on that at a later date though.

Although there are two revisions of the board, the only difference between the two is the presence of mounting holes in the second, the width and height between the centre of the holes is 1400 and 1500 mils respectively, the mils measurement is a standard PCB measurement, but almost useless anywhere else, so those dimensions in metric are (annoyingly) 35.56mm and 38.1mm and the diameter of the holes is 3mm.

The schematic, you can see that it is basically two circuits; the H-bridge and the voltage regulator

The PCB

The top side of the PCB as seen in the OSH Park preview/checker

The latest motor controller PCB design files (v0.2) can be downloaded from Dropbox.