Sandwich is a line-following robot described in the book Robot Building for Beginners. The circuit is basic enough to implement on a single-sided board. Furthermore, Solarbotics created a low cost kit that leaves the board exposed rather than encased. This combination is perfect for showing off colored PCBs.
You may also notice the use of a transparent chip carrier in the center of the board, as described on the previous page of this article.
Open Face Sandwich robot with hidden elements designed to focus attention on the red PCB.
The motors, battery, and body frame are located underneath the footprint of the PCB. Removing the circuit board shows the following:
Robot frame from above.
There are four nylon washers to raise the circuit board to make room for the soldered tips of the electronic components.
The two motors on the sides are Solarbotics GM7 “Baby GM3” gearmotors. Screw holes have been drilled all the way through, as demonstrated on the Atari Joystick robot.
Pictured below, the 9V battery is held in place beneath the circuit board by a Keystone Electronics #79 flat battery holder (Mouser 534-079). What I really like about this holder is that it has more than one screw hole (it has two pairs of two), so it doesn’t awkwardly rotate as you try to attach a battery.
Underneath robot frame showing battery holder and rear slide.
The body consists of three pieces of 3/16-inch thick ABS plastic and one piece of Delrin plastic for the slide. I machined this on my mill.
Robot frame parts made from ABS.
Everything attaches together with 2-56 size screws.
The wheels are homemade.
Solarbotics motor with lathe hub Lego wheel and drilled through screw holes.
I started with a one-inch diameter ABS rod. Using a parting tool on a lathe, grooves were cut and sections were narrowed by eye to create five hubs (two pairs plus a spare). The grooves were finished with a triangular cutting tool to provide a more tapered slot rather than square, to better fit the tire.
Machining hubs from inch ABS rod on lathe.
The tires came off of some standard LEGO wheels. They were chosen to provide a cool, compact look.
Unfortunately, the wheels and their positions are critical design flaws that prevent the robot from operating correctly, particularly compared with the standard Sandwich robot. The tires are too narrow and slippery, causing the robot to slide when it wants to stop or pivot. Putting the wheels so near the line-following sensors causes the robot to rotate more than it drives forward, resulting in severe oversteer.
Let’s look at a rear-wheel drive version and a few remaining details of the robot.