Adventures with micro:bit robotics: DAVE 2

DAVE 2 is the second Digital Autonomous Vehicle for Education. It is built around the Kitronic Motor Driver board for the micro:bit. This has an edge connector that the micro:bit slots into. It provides the 3 volt power supply that the micro:bit requires from a 4.5 volt or 6 volt battery. It has power transistors that boost the power from some of the micro:bit's pins so that you can drive a DC motor in either direction with your code. It has terminals for connecting sensors and switches that your code can read and respond to. All the terminals have screw connectors so no soldering is needed here.

The motor, gearbox, wheels and caster come from the model maker Tamiya. These are the 70097 Twin-Motor Gearbox Kit, the 70101 Truck Tire Set, and the 70144 Ball Caster Kit. These come as kits of parts that need to be assembled. This can be fiddly and many of the parts are small but the result is a robust unit that is perfect for robotics. The ball caster is the same height as the motor and wheels so the chassis sits level.

The chassis is made from 3mm plywood. The two decks are held together with M3 x 25mm Female/Female Threaded Brass Hex Spacers, M3 x 12mm screws and M3 washers.

A 4 x AA battery holder provides 6 volts to the Kitronic board and a separate 9 volt PP3 battery powers the optional external sensor circuits. A separate power supply is needed because the motors running on the 6 volt supply create a lot of interference and electrical noise. The micro:bit is isolated from this by the 3 volt regulator on the Kitronic board but running sensitive circuits on the 6 volt supply is impossible. The simplest solution is to use a different power supply for sensor circuits. The two power sources must be connected together at the 0 volt rail. The 6 volt and 9 volt rails are both controlled by a double pole switch.

Lets have some pictures....

These are some of the parts I started with.

After assembling the gearbox and fitting the wheels I measured the space I would need to cut out to make room for the wheels. I folded a piece of paper in half and marked out one half of the body with one wheel cutout. When unfolded, the template was symmetrical. I checked it for fit against the wheels and then marked out the two decks on the plywood. Only the lower deck needs wheel cutouts.I clamped the plywood to a workbench and cut out the decks with a hacksaw.

I found that sawing the long straight sides was problematic. It was impossible to keep the cut straight, the saw would wander off course and leave a wobbly edge. I rough cut the decks from the large piece of plywood leaving about a centimetre of excess around the outside. I also cut an extra bit of plywood to protect the decks from being damaged by the clamp. I laid the edge of the hacksaw blade along the line I wanted to cut and made several slow careful sawing movements across the surface. This created a groove all the way along the surface of the plywood which guided the saw and kept it perfectly straight as I cut all the way through. Unfortunately I was concentrating so much on making nice straight cuts that I forgot to take a picture of this. Sorry!