Brain Design

Microcontroller

An Atmel ATmega328 microcontroller

Last time I said I was going to use 2 microcontrollers – a mega328 and a tiny24. If I feel like I am going to run out of code space on the tiny24, I can quickly swap it out for tiny44 or 84. The surface mount vs. through hole debate has morphed into “why not both”. The schematic will be the same for both, just the PCB layout will be different.

Serial Port

Having a USB port would be nice, but it is not a main requirement. Neither is having an RS-232 level serial port. I can get by with a TTL level serial port. I have USB to serial port adapters that provide RS-232 and TTL voltage levels. I will think about these some more.

Driving Motors

Sparkfun Motor Shield

One of the big decisions to make is about a motor driver. The design of my first robot brain had headers to plug-in a motor driver daughter board. Sparkfun makes a motor shield for the Arduino that plugs in as a daughter board. The issue that comes up is how much prototyping space do I want. If I leave off the motor driver circuit, then I leave myself a larger area for prototyping. Then again I could include the motor driver and make a prototype expansion board. Pololu makes the 3pi robot and they offer an expansion board that mounts on top to give you a good-sized prototyping area. The 3pi has a cool look to it when the expansion board is mounted. It is a good idea to give yourself some area to experiment with. I like to add a few LEDs for debugging purposes or other circuitry for more functions.

A Pololu 3pi with expansion board mounted

So if I put the driver circuit on the main board, which motor driver should I use? I can use the old standby chips like the L293D/SN754410, the L298, a pair of LMD18200Ts, or a FAN8200 if I could find them. These are bipolar transistor parts that have a voltage drop and a bit of current draw while conducting. Looking into MOSFET drivers appeals to me for the lower voltage drop and less current wasted while driving motors. I could use something like a TC4427 or more exotic parts from Allegro and Freescale. They make motor driver chips with extra features. I just have to decide which components I want to use. Either way I have decided that like the Pololu 3pi robot, I will have a separate expansion board for prototyping. This way I can plug it in only when I need to and the robot’s main circuit board does not have to change.

So I still have a few more things to think about. Next time I will go through the pros and cons of having the motor driver circuit on the main controller board. I will also post a few more pictures and details on my current mini sumo robots. At some point talking about the software tools will come up. Schematics, PCB layout, and parts lists are going to be required at some point. Check back for the next exciting article, or at the very least better than average.

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