This project was published as an article in the February issues of SERVO magazine.
Welcome SERVO magazine readers. I hope you enjoy the project. I would love to hear how you used your little ankle biter.
Here it is! The dirt cheap cassette bot. It is fairly simple to make and comes together quickly if you already have a controller. If not we can make that too. Read on.
Unscrew the case screws on the bottom of the cassette. You want the tape reels for wheels and the case for the body. The metal strip will serve as a bump sensor. Once you are done ripping out all the tape from the reels, you should have the parts shown below. Pull the metal strip off the case and set it aside.
You'll need some motors. I use hobby servo motors that have been modified to rotate continuously. Servos fit nicely in the case and are easy to control with a very simple control circuit. Cut some holes in the case to fit the servos. The mounting tabs should overlap the frame. Then drill a couple holes in the plastic to accept the mounting bolts. Using small bolts, bolt the servos to the case. I only bolted the servo to the bottom half of the case. This makes it easier to open the case later on.
Drill a small hole in the center of the reels. Make it the same size as the hole in the servo horn. Take the screw out of the servo horn and use it to attach the reel to the servo. At some point you'll want to hot glue the ping pong ball to the underside of the case where the holes are. It fits nicely and makes a good caster for a light bot like this.
The bump sensor is very simple but effective. I drilled a couple of holes in the front of the case using the holes in the spring strip as a guide. Then I bolted the strip to the front of the case. A wire under the nut is used to connect the strip to ground on the controller.
For the other half of the bump switch, I used duct tape. This is aluminum tape for real duct work, not duck tape. This is wonderful stuff and if you don't have a roll, you need to go to your local hardware store and buy one now. Once you have it you'll wonder what you ever did without it. Just remember the glue on the sticky side is a pretty good insulator so make the electrical connections on the top side. I cut a couple of strips and stuck them under the bump sensor spring and ran the excess under the body of the bot. Then I just used another piece of tape to stick the stripped end of a wire to the duct tape.
I then used a 10k pull up resistor from +5V to pin b6 and the left bump sensor wire also connects to pin b6. The right sensor wire is connected in a similar way to pin b7. The full schematic is shown below. This would be pretty easy to breadboard or you could solder it up on a perfboard. RadioShack has a board that would work well here.
The battery is from a 9.6V rechargeable pack that I cut in half lengthwise to make a flat four cell (2X2) 4.8V pack. I have had good success running 5V micro controllers from a 4.8V NiCd pack without any voltage regulation. It is crude, but it works.
The controller is a Pic 16F88 loaded with the Tiny bootloader and programmed using a max232a TTL to RS232 converter. I worked up a little printed circuit board for servo control that eliminates the need to breadboard the control circuit.
Programming was quick and dirty. I use JAL (Just Another Language),and Tiny bootloader as my development platform. I like JAL compiler because it's free, and it's easy to read. The self programming capability of the 16f88 coupled with a bootloader make it very easy to program. The code for the bot is available in the downloads section. Also check out the worksheets in the downloads section for some easy getting started with JAL code.
This little bot is capable of quite a lot of modification. There is plenty more space inside the case. The clear inside disk of the wheels is screaming for some sort of optical odometry, the clear windows on top need some sort of display, and of course more sensors.