Make: Electronics

by Charles Platt

  Hand saw. This is my old-school preference. My favorite is a Japanese pull-to-cut saw, which makes very clean cuts: the Vaughan Extra-Fine Cross-Cut Bear Saw, 9-1/2 inches, 17 tpi (teeth per inch). If you use one of these, be careful to keep your free hand out of the way, as the saw can easily jump out of the cut. Because it is designed to cut hard materials such as wood, it has no difficulty cutting soft flesh. Gloves are strongly recommended.

  Figure 5-76. The perils of kickback. Plastic easily sticks to the blade of a table saw, which will hurl it at you unexpectedly. Use other tools to cut plastic.

  Curving Cuts

  Curving cuts involve relatively little danger, although eye protection and gloves are still advisable. My preferred tools:

  Band saw with a 3/8- or 1/4-inch blade designed for thin wood or plywood.

  Jigsaw. I have a special liking for the DeWalt XRP using Bosch blades that are designed for hardwood or plastic. This will cut complex curves in ABS as easily as scissors cutting paper.

  No matter what type of saw you use, you’ll have to clean ragged bits of plastic off the cut afterward, and the absolutely necessary item for this purpose is a deburring tool, available from http://www.mcmaster.com and most other online hardware sources. A belt sander or disc sander is ideal for rounding corners, and a metal file can be used to remove bumps from edges that are supposed to be straight.

  Figures 5-77 through 5-80 show various cutting tools. Figure 5-81 shows a deburring tool, and Figure 5-82 shows a disc sander.

  Figure 5-77. A band saw is an ideal tool for cutting complex shapes out of ABS plastic. You can often find them secondhand for under $200.

  Figure 5-80. This DeWalt jigsaw can run at very slow speeds, enabling precise and careful work with plastic.

  Figure 5-78. A handheld circular saw, running along a straight-edge, is much safer than a table saw for cutting plastic, and can produce comparable results.

  Figure 5-81. A deburring tool will clean and bevel the sawn edge of a piece of plastic in just a couple of quick strokes.

  Figure 5-79. This Japanese-style saw cuts when you pull it, rather than when you push it. After some practice, you can use it to make very accurate cuts. Because ABS is so soft, minimal muscle-power is required.

  Figure 5-82. A belt sander or disc sander is the ideal tool for rounding corners when working with ABS plastic.

  Making Plans

  I like to use drawing software to create plans, and I try to print them at actual scale. I tape them to the smooth side of a piece of white or natural-color ABS, then use an awl to prick through the plan into the soft surface beneath. I remove the paper and connect the awl marks by drawing onto the plastic using a pencil or a fine-point water-soluble pen. Its lines can be wiped away later with a damp cloth. Don’t use a permanent marker, as the solvents that you will need to clean it may dissolve the plastic.

  Because ABS will tend to open a fissure when you bend it at any inside corner where you don’t have a smooth radius, you need to drill holes at these locations, as shown in the cart plans in Figure 5-92 on page 275.

  A regular half-inch drill bit is too aggressive; it will tend to jam itself into the plastic within one turn of the drill. Use Forstner bits (shown in Figures 5-83 and 5-84) to cut nice smooth circles.

  Figure 5-83. A Forstner drill bit makes clean, precise holes; a large regular drill bit will chew up ABS plastic and make a mess.

  Figure 5-84. By drilling holes at any location where two bends intersect, you reduce the risk of the plastic fissuring.

  Note that the heat from bending will tend to make any marks on the plastic permanent.

  Bending It

  A big advantage of plastic over wood is that you can make complex shapes by bending them, instead of cutting separate pieces and joining them with nails, screws, or glue. Unfortunately, bending does require an appropriate bender: an electric heating element mounted in a long, thin metal enclosure that you place on your workbench. The bender I use is made by FTM, a company that offers all kinds of neat gadgets for working with plastic. Their cheapest bender, shown in Figure 5-85, is just over $200 with a 2-foot element. You can get a 4-foot model for about $50 more. Check them out at http://thefabricatorssource.com.

  Figure 5-85. Making clean, precise bends in ABS is simply a matter of resting the plastic over a bender that consists of an electric heating element.

  Avoid Burns While Bending

  A plastic bender will inflict serious burns if you happen to rest your hand on it accidentally, and because it has no warning light, you can easily forget that you have left it plugged in. Use gloves!

  To bend plastic, lay it over the hot element of a plastic bender for a brief time (25 to 30 seconds for 1/8-inch ABS, 40 to 45 seconds for 3/16-inch, and up to a minute for 1/4-inch). If you overheat the plastic, you’ll smell it, and when you turn it over you’ll find it looks like brown melted cheese. Naturally you should learn to intervene before the plastic reaches that point.

  ABS is ready to bend when it yields to gentle pressure. Take it off the bender and bend it away from the side that you heated. If you bend it toward the hot side, the softened plastic will bunch up inside the bend, which doesn’t look nice.

  You can work with it for about half a minute, and when you have it the way you want it, spray or sponge water onto it to make it set quickly. Alternatively, if you need more time, you can reheat it. The amount of force necessary to bend the sheet increases in proportion with the length of the bend, so a long bend can be difficult, and I usually insert it into a loose vise, push it a bit, move it along to the next spot, and push it again.

  Because plastic bending is very similar to making shapes in origami, it’s a good idea to model your projects in paper before you commit yourself to ABS.

  If you decide that you don’t want to spend money on a bender, don’t abandon plastic just yet—you can use screws to assemble separate sections with greater ease and convenience than if you were working with wood.

  Making 90-Degree Joints

  Driving screws into the edge of a piece of plywood will almost always separate its layers, but ABS has no layers (or grain, either), and never splits or shatters. This means that you can easily join two pieces at 90° using small screws (#4 size, 5/8-inch long).

  Figures 5-86 through 5-90 show the procedure for joining 1/8-inch (or thicker) ABS to 1/4-inch ABS, which I regard as the minimum thickness when you’re inserting screws into its edge:

  1. Mark a guideline on the thinner piece of plastic, 1/8 inch from its edge. For #4 screws, drill holes using a 7/64-inch bit. If you’re using flat-headed screws, countersink the holes very gently.

  Figure 5-86.

  2. Hold or clamp the pieces in place and poke a pen or pencil through the holes to mark the edge of the 1/4-inch plastic beneath.

  Figure 5-87.

  3. Remove the thin plastic, clamp the 1/4-inch plastic in a vise, and drill guide holes for the screws at each mark, centered within the thickness of the plastic. Because ABS does not compress like wood, the holes must be larger than you may expect; otherwise, the plastic will swell around the screw. A 3/32-inch bit is just right for a #4 screw.

  4. Assemble the parts. Be careful not to overtighten the screws; it’s easy to strip the threads that they cut in the soft plastic.

  Figure 5-88.

  Figure 5-89. Figures 5-86 through 5-89 illustrate four steps to join two pieces of ABS using #4 sheet-metal screws. Cut 7/64-inch holes on a line 1/8 inch from the edge of the first piece, then mark through the holes to the edge of the second piece. Drill 3/32-inch holes that are precisely centered in the edge, then screw the pieces together.

  Figure 5-90. Three #4 screws driven into the edge of ABS, using a 1/16-inch guide hole, a 5/64-inch guide hole, and a 3/32-inch guide hole. respectively Because the first two g
uides holes were too small, the plastic swelled around the screw (but did not break).

  Framing Your Cart

  For reasons that will soon be apparent, I’ve chosen an unusual diamond-shaped configuration of wheels. In the rendering shown in Figure 5-91, the front wheel (at the far end of the cart) applies power, the rear wheel (at the near end of the cart) steers the cart when backing up, and the side wheels prevent it from falling over.

  Figure 5-91. If you have 3D rendering software, it can be a great way to test the feasibility of a construction project before you start cutting materials and trying to fit pieces together. This rendering was a proof-of-concept for the Little Robot Cart.

  Depending on the type of motor that you buy, you’ll have to improvise a way to mount it in the front section of the cart. Don’t be afraid to use kludges such as cable ties, duct tape, or even rubber bands to attach the motor to the frame. We’re making a rough prototype, here, not a thing of beauty (although if you decide you like the cart, you can always rebuild it beautifully later).

  The plan in Figure 5-92 shows the pieces that you will need. Part A is the body of the cart. If you’re going to bend it from ABS, you should drill half-inch holes, with a forstner bit, at the four inside corners, so that these corners have rounded edges. If you simply saw the plastic to make sharp 90° corners, the plastic may develop fissures at the corners when you bend it. If you don’t have a plastic bender and don’t feel inclined to buy one, you can make Part A from three separate rectangles and then screw them together.

  Figure 5-92. These sections of 1/4-inch plastic can be assembled to create the simple cart described in Experiment 32.

  Part B is a wheel, of which you will need four. I cut them using a 3-inch hole saw. The front wheel is screwed to whatever disc or arm you obtained to mate with the shaft of your motor. See Figure 5-93.

  Figure 5-93. A 3-inch wheel is screwed to the disc that mates with the drive shaft of the motor.

  Parts C, D, and E assemble to form a yoke in which the rear wheel is mounted. I used a 2-inch hinge to pivot the yoke. The hinge is mounted on Part F, which is a partition located midway in the frame of the cart. The photographs in Figures 5-94 and 5-95 will help to make this clear. Initially, when you install Part F, use only two screws, one each side, so that you can adjust its angle a little. This will be necessary to optimize the contact of the wheels with the floor.

  Figure 5-94. The assembled body of the cart, before adding control electronics. The wheel at the righthand end will pull the cart from left to right. The hinged trailing wheel will allow the cart to move in a relatively straight line when it moves forward, but will tend to turn it when it backs up.

  Figure 5-95. A closeup of the hinged trailing wheel, which rotates freely and can flip from side to side with minimal friction.

  The side wheels and rear wheel must spin freely, but on the other hand, they shouldn’t wobble. I simply tightened the nuts on the bolts that serve as axles for the wheels, until there was maybe half a millimeter of clearance. I added a drop of Loctite to stop the nuts from getting loose.

  The plans don’t show precisely where to drill holes for the axle bolts, because the location will depend on the size of your wheels. You can figure this out as you go along. Just make sure that the side wheels aren’t mounted too low. We don’t want them to lift the front wheel or the rear wheel off the floor. If the side wheels are a fraction higher off the ground than the front and rear wheels, that’s good.

  If you have tile or wood floors, your cart may acquire better traction if you wrap a thick rubber band around each disc that you use for the drive wheel and the steering wheel.

  The most important aspect of the construction is to place microswitches where they’ll be triggered when the cart runs into something. I placed mine at the front corners, as shown in Figures 5-96 and 5-97. And that brings me to the electronics.

  Figure 5-96.

  Figure 5-97. Two microswitches with metal arms are mounted on each side of the cart, where they will sense any obstacle.

  The Circuit

  The schematic is very, very simple, with only four principal components: two microswitches that sense obstacles in front of the cart, one relay, and one 555 timer. You will also need a small power switch, a battery or battery pack, and a resistor, and capacitors to go with the timer. A trimmer potentiometer will allow you to adjust the “on” time of the 555 timer, which will determine how long the cart takes to back up. See Figure 5-98.

  Figure 5-98. This ultrasimple schematic is all the cart needs to enable it to back up when it hits an obstacle.

  The motor I chose requires 5 volts, so I had to use a voltage regulator with a 9-volt battery. If your motor uses 6 volts, you can wire four AA batteries to it directly. If you have a 12-volt motor, you can use two 9-volt batteries in series, supplying power through a 12-volt voltage regulator.

  Assemble the components, mount them on the cart, and switch it on, and it should move forward slowly in a more-or-less straight line. If it moves backward, reverse your connection to the terminals on the motor.

  When the cart bumps into something, either of the microswitches will connect negative voltage to the input pin of the 555 timer. This triggers the timer, which runs in monostable mode, generating a single pulse lasting about 5 seconds, which closes the relay, which is wired so that it reverses the voltage to the motor.

  When the voltage is reversed to a simple DC motor, it runs backward. So the cart backs up. Because the rear wheel is mounted in a yoke that pivots, the yoke will tend to flip one way or the other, causing the cart to describe an arc as it moves backward. At the end of the timer cycle, the relay relaxes and the cart starts moving forward again. In forward mode, the rear wheel just follows along without applying any steering force, so the cart tends to follow a straight line—until it hits another obstacle, at which point it backs up, and tries another path.

  Fundamentals

  All about limit switches

  The most obvious enhancement for your cart would be a better steering mechanism. You could use another motor to take care of this, with a pair of limit switches. Because limit switches are a basic, important idea in conjunction with motors, I’ll explain them in detail.

  Figure 5-99 shows three successive views of a motor with an arm attached to it, which can press either a lower pushbutton or an upper pushbutton. Both of the pushbuttons are normally closed, but will open when pressed by the motor arm. These buttons are the limit switches. Typically you would use microswitches for this purpose, just like the ones that I suggested as barrier-sensors at the front of the cart.

  In addition, there’s a DPDT relay that is activated by a simple on/off switch at the righthand side. On the cart, the 555 timer takes the place of the on/off switch, by feeding power to the relay.

  Suppose that the motor begins with the arm pointing downward, as shown in the top view in Figure 5-99, and the motor is wired so that when it receives negative voltage at its lower terminal and positive at its upper terminal, it rotates counter-clockwise. This is what happens when the on/off switch closes and sends power to the DPDT relay. Positive voltage from the relay contacts cannot pass through the upper diode, but can pass through the upper limit switch, which is closed. Negative voltage cannot pass through the lower limit switch, because it’s open, but can pass through the lower diode. So, the motor starts to turn counterclockwise. During the midpoint of its arc, it receives power through both of the limit switches.

  Finally, the motor arm reaches the upper switch, and opens it. This prevents positive voltage from reaching the motor through that switch, and the positive voltage is also blocked by the upper diode. So, at this time, the motor stops.

  Now suppose that the on/off switch is opened, as in the top view in Figure 5-100. The relay loses its power, so its contacts relax. The voltage to the motor is now reversed. Negativ
e voltage passes through the upper diode, while positive voltage reaches the motor through the lower limit switch. The motor starts running clockwise, until its arm hits the lower switch, opening it and cutting off power to the motor.

  Limit switches are necessary, because if you continue to apply voltage to a simple DC motor that is unable to turn, the motor sucks more current, gets hot, and may burn out.

  You can easily see how this kind of system could be used to control the cart’s steering. Even though the motor has only two positions, these are sufficient to make the cart turn when going backward, and proceed straight ahead when going forward.

  To reduce power consumption, the DPDT relay could be replaced with a two-coil latching relay. The circuit would then have to be revised so that the relay is flipped to and fro by a pulse to each of its coils.

  Fundamentals

  All about limit switches (continued)

  Figure 5-99. The three diagrams, from top to bottom, show three snapshots of a motor controlled by a DPDT relay and two limit switches. When the on/off switch at bottom-right sends power to the relay, the lower relay contacts cause the motor to run counterclockwise until it stops itself as its arm opens the upper limit switch.