Build your own Mk.2

Parts list:

Microprocessor: 9pin serial Freeduino from NKC Electronics. $16.99. I ordered this version of the Arduino microcontroller board because it has a 9pin serial which will plug directly into my handheld GPS unit. I have ordered from NKC several times for personal use and for Arduino’s to give away at club competitions, and would recommend them to anyone.

GPS unit: a handheld GPS that I stopped using a few years ago. Fortunately, it is one that is able to have its firmware updated. It will do all the “heavy lifting” of GPS calculations to waypoints. The plan is to have the Arduino monitor the output of the GPS to navigate. There are several topics on the Arduino forum dealing with interfacing a GPS.

Sensors: 1ea Parallax Ping ultrasonic distance sensor. $32.99 Radio Shack. I have since found a much cheaper alternative ultrasonic sensor. When I have ordered it and tested it I’ll update this. The Parallax sensor will give long range (a few feet) object detection. The Parallax sensor will be mounted on a servo so I can use one sensor to scan left/center/right of the robot. 3ea Sharp infrared distance sensors. Approx $12.00 each, you can get them at SparkFun. The Sharp sensors are for object detection a bit closer in, and there will be two facing forward and one facing rearward.

Servos: I am using Futaba S3003 servos, $10.00 at any radio control hobby shop. These are the most basic servos Futaba makes, and all I am using it for is to rotate the Parallax sensor back and forth so fancy things like metal gears and digital interfaces are of no use.

Motor Controller: a Pololu motor controller that can provide 14amps continuous and 30amps peak. It seems to handle the inefficient PowerWheels motors just fine.

Motors: Salvaged PowerWheels motors/gearboxes. You know, the ones from a “Barbie Jeep” or similar kids toy. The majority of them run on 12v, are built to stand up to Junior’s use and abuse, and are cheap. Our club members find them regularly in the trash. People seem to pitch them when they develop problems. Usually the batteries, chargers, or switches go bad. The motors are usually fine. Sometimes they are on Craig’s List or similar sites free or very low cost. Note: PowerWheels motors are designed to move the weight of a small kid and the weight of the toy at around 5mph. A relatively low weight robot like this one will do every bit of 10mph using these motors. I am considering some kind of remote kill switch or self-destruct device.

Batteries: 2ea 12v UPS batteries from a local surplus store. $10.00 each. These were pulled from new UPS units that got wet in shipping and were scrapped. You may not need two of them, but my goal is to get 1 hour of runtime per charge. I have yet to determine what the runtime is. I have run one motor at full speed for 15 minutes and the batteries only required a few minutes of charging to get back to full power.

LCD unit: 1ea 2 line LCD unit. $2.00 from a local electronics surplus store. Most LCD units use the the same interface that requires a lot of microcontroller pins to operate. To make things simple, I am using an LCD “backpack” unit that plugs directly into the LCD unit and allows me to control the LCD with serial commands. The advantage is that I only need to use one microcontroller pin to send serial data to the LCD instead of 7 or 8 pins. The serial LCD “backpack” I used is called the K107 and is available online from the Wulfden. I ordered the kit version, and it went together in under an hour. The Wulfden guy even provides an Arduino specific library and test programs to get it up and running. He sells three versions: a general purpose version, an Arduino version, and a PICAXE version, so order the appropriate one. The main difference is the baud rate they operate at. My kit was $9.00, but he also offers a complete kit with LCD unit, and even fully assembled and tested units.

Good ol’ American Pine: the chassis is made from 1/2″ pine boards, cut, shaped, and sanded smooth. Stain to the color of your choice, and put on a few coats of polyurethane for protection. Wood works great…it is cheap, easily shaped, easy to mount things to, and gives your robot a cool retro look.

PVC pipe: the spacers between the layers of Pine are made from sections of PVC pipe. I drilled three holes in the Pine boards to enable a long bolt to pass through the PVC sections. Washers and a few lock nuts squeeze it all together, making a very strong platform. I counter sunk the PVC sections about a 1/4″ into the Pine boards to make it all align and hold together better.

Tires: dolly/handtruck tires from Harbor Freight. $4.99 each. Mine are inflatable as opposed to solid rubber, and have a rating of 300lbs. each. The tricky part is adapting the wheels to the output hub of the PowerWheels motors.

Wheel adapters: Attaching the Harbor Freight tires to the PowerWheel motors took some thought, but we came up with a simple and sturdy solution. (If the original PowerWheels tires are in good shape, you can just use them as is. Mine were in terrible shape and severely cracked, so I had to replace them.) Complete instructions with pictures are coming, here is a brief description. The PowerWheels motors I used have a rectangular hub that matches up to the original plastic wheels. The Harbor Freight tires have a bearing. To make the two join, we used several circular “plates” of MDF (medium density fiberboard, from a hardware store). Each “plate” is a circle cut to the inside diameter of the metal part of the tire, approx. 6″ but yours may be different. The plate that touches the PowerWheels hub was cut with a rectangular hole to fit the rectangular hub of the PowerWheels motor. The plate that touches the Harbor Freight tire needs to have holes drilled to match the bolts that hold the tire together. Mine had 5 bolts. Drilling out the holes for the 5 bolts allows the plate to sit as close as possible to the tire. The remaining plates are just circles that fill in the distance between the tire plate and the PowerWheels plate. All the plates have a hole drilled in the center that matches the diameter of the steel rod axle. Assemble all the plates together and line them up as accurately as you can and glue them all together with 15 minute two part expoxy. Lots of it. When the plate assembly is cured, attach it to the Harbor Freight tire with even more expoxy. That stuff is stronger than you think, and has held my plate assembly to the tire with no problems. There is no expoxy attaching the plate assembly to the PowerWheels….when the axle is installed and a nut attached on each end, the tire/plate assembly is “squeezed” onto the PowerWheels hubs. If all this is unclear, just wait for the photos. I’m going to make a page dedicated to constructing the tire/plate/PowerWheels assembly.

Steel rod: approx. $6.00, local hardware store. You will need a length of steel rod for an axle for the PowerWheels motors. Measure the hole through the hub and get a steel rod that fits just barely snug. The rod needs to be threaded on each end to accept a nut. Mine was not threaded, so I had to learn how to hand thread the rod using a tap and die set. Threading your own steel rod builds character. πŸ™‚

Master power switch: 12v 300amp rated keyed switch. $5.00 each. Harbor Freight. I think they advertise this as a marine battery switch. It is a very heavy duty switch with a removable key. This switch controls all 12v power to the robot.

Lighted 12v automotive type switches: 2 ea. $3.00 ea. Radio Shack. The 12v lighted switches control the 12v feed to the Arduino microcontroller board, and the 12v feed to the relay motor controller board. Having two switches enables me to power up the robot for programming, but leave the motors off. I don’t want this thing streaking across my living room unexpectedly.

Misc. stuff: Nuts, bolts, washers, wire. Terminal strips, and a lot of them. All connections on the Mk.2 are attached to terminal strips. All the pins to the Arduino microcontroller board, all power connections, motor connections, etc. are all attached to terminal strips. This makes it easy to attach/reconfigure stuff, and also adds a lot of durability. You can’t have a sensor wire pop off when a 30 lbs. 10 mph robot is on the loose. LED’s. You have to find a way to incorporate blinking LED’s in any self respecting robot project. I decided to learn about 555 timer circuits and build a circuit to flash 15 super bright 130,000mcd LED’s. The 555 circuit is totally separate from the Arduino microcontroller and simply flashes whenever the main 12v power is on. Kind of like a warning light. The 555 circuit does an awesome job rapidly flashing the LED’s and doesn’t even get slightly warm. Rubber bungie cord, used to hold the two 12v batteries in nice and tight. Low tech and works great. Imagination/creativity: all this is just a guide to show you how I did things, which might be a long way from the best way. If you think of a better or more efficient way to do something, then by all means do it! And then share with me what you did so I can copy what you did and try to figure out a way to take credit for it! πŸ™‚

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