 The following is a version of a project that first appeared in my book the Absolute Beginner's Guide to Building Robots. You can find out more about the book, look at details of two other projects, and order it, on the book's companion website. This Walker was also pictured in Make Vol. 6 and on the Make website. Here's how to make it yourself!Note: Most of the images are larger in size than they appear inline here. View them in a new window if you want to see a bigger view. |
The Coat Hanger Walker
By Gareth Branwyn
Photos by Jay Townsend
Illustrations by Mark Frauenfelder
In this project, we'll build a little critter out of surprisingly simple, minimal parts. We'll even make use of one of those coat hangers that seem to breed in the bedroom closet. This project is an ingenious little hardware hack dreamt up by a Canadian BEAM enthusiast named Jérôme Demers. It wonderfully illustrates a number of principles of bottom-up, BEAM-friendly robotics. Here are a few:
FIGURE 1: What our finished coat hanger walker will look like. Cute, ain’t it?
The persnickety among you will be quick to point out that by many definitions of a robot, this ain’t no stinkin’ robot. It doesn’t gather, process, and respond to feedback of any kind. It’s really a walking machine rather than a proper robot. We still think it's a cool walker, because of the reasons bulleted above, and because it teaches many of the fundamentals in construction and electronics that can be used in other bot building projects. You also end up with a nifty little “monitor pet” that'll merrily motor across your desk and impress all your geek friends (especially when you tell them: “I made it out of a coat hanger and some junked electronics!”
We’re not going to lie to you. Although this critter has relatively few parts, the Bicore control circuit can be a little harrowing to solder (you’re up for a challenge, right?). The dual inline package (DIP) IC socket that the control circuit plugs into is very small and the pins are closely populated. Soldering these pins will take some patient effort. That’s the bad news. The good news is that DIP IC sockets cost about 10 cents a pop, so you can mess up quite a few in practice and it’s no big deal. Hopefully, you’ll have logged plenty of soldering practice time by now and be ready for some big boy/big girl soldering. If not, go ahead and read through this entire project, order any parts you need, and then do some soldering practice sessions while you wait for the components to arrive.
Gathering the Parts
This project requires some materials and parts you might already have lying around the house and some components you’ll likely need to purchase at your local electronics store, an online retailer, or some other source. Our pals over at Solarbotics have kindly put together a Parts Bundle of most everything you need for this project. It does not include the 40-tooth gear. We got ours from Jameco. The Parts List
The Coat Hanger Walker requires the following parts:
- (1) hobby servo motor with “Servo Horn”
(Solarbotics Part #GM4)
- (1) 1.5 inch (4cm) plastic gear (around 40 teeth are good)
- (1) 2 feet of 8 to 10–gauge wire; can use coat hanger wire or 10-guage copper wire (Solarbotics Part #Wleg)
- (1) short piece of 1cm diameter plastic tubing (can use “jacket” from preceding leg wire)
- (1) terminal block (Radio Shack Part #274-678)
- (2) AAA battery holders (each holds two AAA batteries)
- (1) length of 1cm diameter heat-shrink tubing (can use “jacket” from preceding leg wire)
- (2) .22 µF monolithic capacitors
- (1) 100K to 10MΩ resistor (we recommend 3.2MΩ)
- (1) 74HCT240 integrated circuit (IC)
- (1) 20-pin DIP IC socket
- (1) on/off toggle switch (smallest you can find)
- (2) leg mounting pads (Solarbotics Part #LMP2); optional, but recommended
- pieces of scrap plastic (from product packaging) or .030" plastic sheeting
- several feet of stripped hook-up wire (or other 22-guage wire)
FIGURE 2: All the parts you’ll need to build your first robo-critter.
The Tools and Supplies List
You’ll also need the tools and supplies listed here:
and supplies
recommended)
Freeforming the Bicore Control Circuit
The first thing we’ll want to do in building our robot is to assemble its brain. The Coat Hanger Walker makes use of the ingenious BEAM Bicore circuit. It’s prefixed bi because it has two states, or nodes, and core because, well, it’s the central part of the robot’s nervous net. Our Bicore uses the 74HCT240 integrated circuit. This chip is an inverting octal buffer. That’s just a fancy way of saying that it is a chip with eight logic gates that invert the signals going into them. Whatever goes in each gate gets inverted, so a low signal becomes a high signal and a high signal becomes a low one. By combining the three gates on one side and three gates on the other (by soldering their pins together), we end up with two “teams” inverting gates that “buffer” the signal and make it more powerful. Bicore! The signal passing back and forth between the two nodes sends high and low (or “on” and “off”) pulses to our servo motor. The result is back and forth movement of the motor shaft, which is transferred to our gears to create a reciprocating walking motion. By the way, if you’re wondering, the remaining two gates are used as sort of the controller for the two three-gate teams.
Breadboarding the Bicore
Before we actually heat up our soldering iron and start dripping molten metal all over components, we want to breadboard our circuit to make sure that all the components are working properly and that we have a sound design for our Bicore circuit (If you don’t know how to breadboard, read the “Thumbnail Guide to Breadboards” later in this project doc before continuing).
You’ll want to hook up the wires (from your breadboard jumper kit), the resistor (whichever value you decide to start with), and the two .22µF capacitors to the following tie points on the breadboard. For these numbers, assume that the pin in the upper-left corner of the IC is pin 1 (the pin to the left of the little dimple). Then it’s pins 2–10 on the left side, straight across (from 10) to 11, and then up to pin 20. Connect jumper wires to the follow tie points:
The last thing you’ll need to do is to connect the positive (+) power wire from pin 20 to a positive tie point on the power bus, and ground (-) wires from pin 1 to a negative tie point and pin 10 to a negative tie point. Make sure both of these pins (1 and 10) go to negative.
Figure 3 shows what the breadboard should look like when you’ve hooked up all of the pins on the IC. (In this photo, the motor is not yet connected.)
FIGURE 3: The basic breadboard set-up. Note that motors are not shown and the top and bottom breadboard busses are not connected.
Put hook-up wire, the resistor, and the two .22µF capacitors in the tie holes as shown. Note: Make sure the top and bottom power bus strips on your breadboard are connected together (not shown here, but see Figure 13).
At this point, your breadboarded Bicore circuit is all hooked up and ready for juice (battery power) and something to drive (your motor). Before we hook up these final components, you might need to do a little work on your motor to get it ready for reciprocating (back and forth) motion (see Hacking a Hobby Servo Motor below). Make sure the battery pack that you’re going to use to power the breadboard is not connected to the breadboard power bus before connecting positive and ground (negative) wires to the bus.
Hacking a Hobby Servo Motor
FIGURE 4: (see right) The steps to removing the control circuit from a hobby servo motor.
If you got your servo motor from Solarbotics (Part #GM4), it already has the control electronics removed. If not, you’ll need to remove them yourself. Servo motors have built-in control circuitry. We don’t want this control on our servo—we want to control its movement with our Bicore chip. Removing the control PCB is simply a matter of opening the case, removing the board, and resoldering the positive and negative wires directly to the motor (see Figure 4).
If you did get your motor from Solarbotics, there are no electronics to remove, but the motor may have been configured for continuous rotation. We want reciprocating back and forth motion. Usually, servo motors have a final gear (as the drive gear is called) with a mechanical pin on it that prevents full rotation. The Solarbotics servo has a final gear with no stop pin, but the “servo horn kit” that comes with the motor includes the original final gear with the stop on it. To re-install this gear, all you have to do is
Before putting the case back together, make sure the gears are well seated and meshed. Also, make sure the plastic stop on the gear is facing toward the wired end of the servo case. When you have a servo motor with a mechanical stop final gear in it and no control electronics, you’re ready to hook it up to your breadboarded circuit to see if it works. Plug the red (+) wire into pin 9 and the black (-) wire into pin 12. Connect 6 volts (V) of power to your breadboard’s positive and negative terminals. If you’re using the two AAA battery holders for the robot, connect them in series as seen in Figure 5.
If, when you power up the motor, you see the motor shaft twisting back and forth, congratulations! You’ve just built your first robot control circuit. If nothing happens, go back and check each connection on your breadboard to make sure that they’re wired correctly. Look at each hook-up closely, as it’s easy to sometimes put a wire or component lead in the wrong tie point on the board. If it still isn’t working, try different resistor values. On our robot, we got good action on resistors in the 3.2 to 4.2 MΩ range. If your circuit still isn’t working, it’s time to get out your digital multi-meter and check all of your components (battery packs, resistors, capacitors, and switch) to make sure that everything’s working properly. Consult the manual that came with your DMM to find out how to properly test each component type. To test the motor, touch its wires directly to the battery pack’s wires (positive to positive and negative to negative, of course). If you do all this, your circuit should be working properly. There aren’t that many parts that can fail here.
Creating a Breadboard Power Supply
FIGURE 5: The proper way of hooking up your battery packs (in series) to deliver 6V of power to your circuit. Add a switch to the circuit, if you’d like.
Soldering Up the Circuit
Now it’s time to heat up your soldering iron, take a few deep breaths, and begin soldering the pins of your IC socket together, and then the discreet components onto it. Here are the steps involved:
1. Get your 20-pin DIP socket and turn it over. Using your needlenose pliers, bend over pins 1 and 19 so that they are as close to touching each other as possible. Don’t hold metal parts directly. Use needlenose pliers or other tools. Next, bend pins 2, 5, 7, 9, and 10 outward. Bend pins 6 and 8 inward. Try to keep all the pins as straight and on the same level as possible. Now bend outward pins 12, 14, 16, 17, and 20. Then bend inward pins 11, 13, 15, and 18. When you’re finished, the chip should look similar to the one shown in Figure 6.
FIGURE 6: All of the pins on the IC socket bent and ready for soldering.
- Using a small piece of component lead (clipped from one of your resistors, capacitors, techno-junk, and so forth) bridge the gap between pins 1 and 19. Solder the lead to one pin and then the other. If it makes it easier for you, you can solder the lead to the first pin when it’s upright, and then bend it over, solder it to the other pin, and then clip off the excess.
- Now find pins 3 and 4 and solder them together (see Figure 7).
- Solder together pins 6 and 8 and pin 4 (which was already soldered to pin 3 in step 3) with a piece of component lead. In other words, you should have one long wire connecting pins 6 and 8 to pins 3/4 (see Figure 8).
- Solder together pins 11, 13, 15, and 18 with a piece of component lead (also shown in Figure 8).
FIGURE 7: (below) Pins 1 and 19 (the enable pins) and pins 3 and 4 connected to each other as shown.
FIGURE 8: (above) Pins 6, 8, and 3/4 soldered together on one side, and pins 11, 13, 15, and 18 on the other.
- Get a piece of hook-up wire and cut it so that it reaches from pin 10 to the join you made between the enable pins 1 and 19. Strip off just enough of the wire jacket to solder it to these pins and try to get the wire as straight (with as little slack) as possible (see Figure 9).
- Solder together pins 5, 7, and 9 with a piece of component lead (see Figure 10).
- Solder together pins 12, 14, and 16 with a piece of component lead (also shown in Figure 10).
FIGURE 9: Pin 10 (the negative, ground pin) connected via insulated wire to the enable pins 1/19.
FIGURE 10: Pins 5, 7, and 9 connected together, and then pins 12, 14, and 16.
- Get the two .22µF monolithic capacitors. Trim the leads down (if you haven’t already cannibalized them for lead clippings) so that they’re of a manageable length. Solder one of them to pins 2 and 3 and the other one to pins 17 and 18. Solder them with enough“slack” lead so that they can be bent to the sides, and out of the way when you flip the socket over and insert the IC into it (see Figure 11).
- Now we need to add our final component: the resistor. Hopefully, you experimented around during the breadboarding stage and found a resistor value that gives your walker the right amount of back and forth action. Here, you don’t want to clip the resistor’s leads because you’re going to want to solder it around the top edge of the socket, from pin 2 to pin 17. We need to go around the socket because we don’t want the resistor to get in the way of plugging the IC into the socket, or when mounting the socket/IC assembly to the top of the walker.
With plenty of lead on the resistor, we can bend and twist it during the final assembly phase to make sure it’s out of the way of other components (as shown in Figure 11).
FIGURE 11: Our two capacitors soldered in place (one across pins 2 and 3 and one across pins 17 and 18) and our resistor installed (from pins 2 to 17).
11. Flip the IC socket over and carefully plug the 74HCT240 chip into the soldered-up IC socket (see Figure 12). If it has trouble going in, carefully inspect your socket assembly to make sure that you didn’t melt any of the plastic package, and therefore disturb the sockets where the IC pins plug in. If they are misaligned, we hate to break it to you, but you might have to get another socket and do the whole thing over again. We told you to be careful!
FIGURE 12: The completed soldered up IC socket with the 74HCT240 plugged in.
That’s it! If all went well, you should have aworking Bicore control circuit. All you have to do now is connect the power and the motor to the appropriate pins. We’ll hold off on doing that though until we’ve built the rest of our robo-critter.
Go ahead and take a break, indulge in your junk food of choice, play a round of Enter the Matrix, run a lap around the house, or otherwise cool out for a bit. I don’t know about you, but these solder fumes are makin' me feel kinda funny.
Thumbnail Guide to Breadboards
Every wirehead worth his or her propeller beanie knows about breadboards. And no, we’re not talking about a cooking utensil from Martha Stewart’s kitchen; we’re talking about an essential piece of equipment for every electronics hobbyist and professional.
A breadboard is a simple, inexpensive device (available at any electronics store) for temporarily hooking up and testing an electronics circuit before you solder it together. By using a breadboard, you’re checking to make sure that the circuit is designed properly and that all of your components are working as specified.
A breadboard is usually comprised of a metal base plate with a white nylon block mounted on it that’s covered with a grid of holes (known as tie points or wire receiving sockets). There are usually three binding posts also on the board, which are screw-down connectors for bringing power to the board.
The nylon grid on a breadboard is divided into two major sections split down the center. This center channel (called the “trench”) is just wide enough to accommodate the two rows of connecting pins found on standard DIP ICs. Radiating from the center of the board are vertical rows of tie points (usually five on each side). All of these “5-position groups” of tie points are connected together, so a wire in any one of them electrically connects any wire or component lead in any of the others in that row. Along the top and bottom of the breadboard are a series of power “distribution buses” (or simply “buses”). They (usually) run horizontally and there are often four groups of them (two on the top and two on the bottom). To power the circuit you’ve set up on your board, you simply connect the leads from a battery to the binding posts (positive to a red post, negative to a black), connect a short wire from each post to the appropriate channel (+ or -) on the power bus, and then a wire from the bus to the appropriate socket in the row of tie points connected to the parts of your circuit that need power. Confused? Check out Figure 13. You’ll get the hang of it pretty quickly.
To make life easier down on the breadboard, electronics stores sell jumper wire kits with different lengths of 22-guage hook-up wire cut to appropriate lengths that correspond to the tie-points on the board. You’ll want to get one of these kits. You’ll also need to power your board. For this, all you need to do is hook up the battery pack of the correct voltage needed to power the circuit you’re building (for instance, in the Bicore circuit for this project, we need 6V of power). To make things even more convenient, the tops of the binding posts also accept banana plugs (also available at electronics shops). You can solder sets of these to various battery packs (6V, AAA, 9V, and so forth) and have these available for your breadboard power source. Then all you need to do is plug the banana plugs into the tops of the binding posts to power your circuit. (Breadboards? Bananas? Is anyone else getting hungry?) If you want to get really fancy, you can hook up a power switch to the board so that you can turn the power on and off without having to disconnect the battery.
FIGURE 13: Anatomy of a breadboard.
When you first get your breadboard, you’ll want to hook it up with a standard power configuration that distributes power throughout the buses on the top and the bottom of the board. As we said earlier, there are usually four power bus groups on the board. To connect them, you need to put a jumper across the two positive and negative groups on the top and the two positive and negative groups at the bottom. You then need to connect the top bus groups to the bottom bus groups (with a positive-to-positive wire and a negative-to-negative wire). Use the standard red for positive and black for negative, if you have them; otherwise, just be consistent (all positive wires one color, all negative wires another). Refer back to Figure 13 if this is getting confusing.
Besides the breadboard itself, the power supply, and the jumper kit, you’ll need two other tools for effective breadboard work. A pair of needlenose pliers are essential for getting those pesky jumper wires onto and off of the board, especially in tight spaces. You’ll also want an IC extractor (or a chip puller). This is a funny-looking pair of tweezers that is used to safely remove IC chips from a breadboard (or an IC socket) without damaging their delicate pins. (And trust us, without a puller, this is very easy to do.)
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