Figure 5-73. An AM radio transmitter can be made from just two components: a transformer (left) and a crystal oscillator (right), available from http://www.scitoys.com.
Experiment 32: A Little Robot Cart
Robotics is another application of electronics that deserves a book in itself—or several books. So, once again, I’m going to give you an introduction followed by some points that you can follow if you want to go further. As always, I will start with the simplest possible device, which in the world of robotics is a cart that finds its way around your living room.
You will need:
SPST or SPDT microswitches requiring minimal pressure to activate them. A force between 0.02 and 0.1 newtons would be ideal. Quantity: 2. See Figure 5-74.
Figure 5-74. A microswitch has a small button (at the front, righthand side in this picture) that is often actuated by a pivoted metal lever. The switch can respond to a very light pressure, but can handle relatively high currents.
DC gear-motor, rated for any voltage between 5 and 12, drawing a maximum of 100mA in its free-running state, output shaft turning between 30 and 60 RPM. Quantity: 1. A motor is shown in Figure 5-75.
Figure 5-75. For the Little Robot Cart, I found this 5-volt motor, which is supplied with a disc that fits its output shaft. The combination cost less than $10.
Disc or arm that fits securely onto your motor shaft. Quantity: 1.
555 timer. Quantity: 1.
DPDT nonlatching relay rated for the same voltage as your motor. Quantity: 1.
1/4-inch plywood or plastic, one piece about 2 feet square.
#4 sheet-metal screws, 5/8 inch or 3/4 inch long. Quantity: 2 dozen.
#6 bolts, 3/4 inch long, with nylon-insert lock nuts. Quantity: 2 dozen.
1/4-inch bolts, 1 inch long, with nuts, to mount the wheels. Quantity: 4.
I’m not specifying one particular motor, because if I did, it might not be available by the time you read this. Motors aren’t like logic chips, which have retained their basic function throughout various improvements over a period of several decades. Motors come and go, and many that you may run across will be surplus parts that will never been seen again. Search online for “gear-motor” or “gearhead motor” and find one as close as possible to the specification that I have provided. The mechanical power output of the motor shouldn’t be important, because we won’t be requiring it to do much work.
The important consideration when you buy your motor is that you should also obtain something that fits onto its output shaft. Typically, this will be a disk or arm that can be screwed into place. To this you can then add a larger wheel of your own, which you can cut with a hole saw or make from the screw-on lid of a jar, or anything else circular that you may find in the house.
A larger wheel will make your cart move faster than a smaller wheel, but will reduce its torque, thus limiting its power to overcome obstacles.
This brings me to my next topic: fabrication. Although this is an electronics book, motors are electromechanical devices, and you have to be able to install them in some kind of a machine to get any interesting results. You can use plywood to complete the two little robotics projects here (ideally, the kind of thin, high-quality plywood sold by hobby stores) but I recommend something that looks better and is easier to work with: ABS plastic. Before you start on the robotic cart, you may want to check the section “Fundamentals: All about ABS.”
Fundamentals
All about ABS
Unless you think the steampunk movement isn’t going back far enough, you probably don’t want your autonomous robot cart to resemble a relic from before the 1800s. Therefore, wood may not be the best construction material. Metal can look nice, but is not easy to work with. For quick results that have a twentieth-century look (maybe even a 21st-century look), plastic is the obvious choice, and I feel that ABS is the best type of plastic to use, because it provides such quick, easy results. ABS stands for “acrylonitrile butadiene styrene.” Lego® blocks are made of ABS. Car-stereo installers and model-railroad buffs use it. You can use it, too. You can saw it, drill it, sand it, whittle it, and drive screws into it, and it won’t warp, split, or splinter. It’s washable, doesn’t need to be painted, and will last almost forever.
Delrin is another type of plastic, but tends to cost more and is a little tougher to drill and cut. It’s a matter of personal preference. ABS machines fairly well, but when you drill it, for example, it can “catch” on the bit and the piece will spin with the bit due to the way that plastic chips off with the bit. Delrin is self-lubricating and has better melting properties under the heat of machining, so it drills and cuts much more cleanly and easily than ABS.
Where to find ABS
Pieces of ABS a couple of feet square are available from online sources such as http://hobbylinc.com or estreetplastics (an eBay store), but you’ll save money if you can truck on down to your nearest plastic supply house and buy it like plywood, in sheets measuring 4 by 8 feet. To discover whether you have a nearby plastic supply house, search for “plastic supply” in your yellow pages or Google Local.
Piedmont Regal Plastics has many supply centers around the nation, but you’ll have to collect it yourself, and they may not be willing to cut small pieces. You can check online at http://www.piedmontplastics.com for their locations.
Stock colors of ABS include black, white, and “natural,” which is beige. Sheets usually are
