Servo - August 2013

your computer (we’ll get to why you might want to do this in the next installment). By simply disconnecting the batteries for the motors, your ArdBot II will think it’s driving around, but, in fact, won’t move.

Another important reason to use separate supplies is changing out those unrechargeable 1.5 volt AA cells with rechargeable ones. In the long run, it’ll save you a ton of money — robots use up batteries like kids eat candy. The rechargeables are also better on the environment.

The nominal voltage output of nickel cadmium or nickel-metal hydride cells is 1.2 volts; four together provide 4. 8 volts. That’s enough to motivate the robot’s two servo motors, but not enough to power the Arduino’s regulator.

Making Your Own ArdBot II

With the design basics out of the way, let’s turn to building the ArdBot II. Construction is simple, requiring only one square foot of material. You need four pieces total: one for the bottom base, with small cut-outs for the wheels; one for the top base; and two for the servo mounts.

A cutting and drilling diagram for the main top and bottom decks is provided in Figure 3; Figure 4 shows a cutting and drilling guide for the servo mounts.

You can use 1/4” plywood or my preference: 6 mm expanded rigid PVC plastic. The plastic is lighter weight, and if purchased locally from a plastics distributor, is often cheaper than wood. Either way, plywood and PVC can be readily cut using ordinary wood tools. I suggest a coping saw for working by hand, but recommend a motorized table scroll saw for the fastest and most accurate cutting.

12

Servos and servo mount hardware

4-40 x 1/2” pan head machine screw

4

4-40 x 1/2” flat head machine screw

12

4-40 hex nut

4

3/4” x 3/4” plastic angle bracket*

Arduino mounting standoff hardware

3/4” to 1” (approx.) length nylon standoffs, 4-40 thread

4

4-40 x 1/4” pan head nylon machine screw

4

2

4-40 x 1/2” flat head machine screw Skids hardware

6-32 cap (acorn) nut

2

6-32 × 3/4” pan head machine screw

4

6-32 hex nut Deck risers (standoffs)

Standoffs with 4-40 or 6-32 threads, at least 2” or longer

8

4-40 or 6-32 pan head machine screws * For the 3/4” x 3/4” plastic angle brackets, you can substitute the nearest smallest metal L brackets available. The plastic brackets weigh a little less.

Table 1

Table 2

You can use any type of drill — all holes are 1/8” unless otherwise indicated. Some hole locations are critical, so follow the old adage: Measure twice, cut (or drill) once.

When marking off the holes for the Arduino in the top deck, use your Uno board for the hole locations. I include all four mounting holes, although on the latest Unos you’re likely to only use three of them since the one in the upper left corner is very close to the top row of headers. The hole is still there in the board in case you want to mount it with a shield stacked on top.

If you’d prefer not to do the cutting and drilling, see the Sources box for a kit of parts, including precut body pieces and assembly hardware.

To complete the ArdBot II, you’ll need the hardware listed in Table 1 to put all the pieces together. Except for the skids and riser standoffs, I’ve specified 4-40 size hardware for everything. While 4-40 machine screws and nuts are a little harder to find than their more common 6- 32 siblings, their smaller size makes them better candidates for a small robot. They also weigh less, and the less weight a robot has, the longer its batteries will last.

If your local home improvement store doesn’t have the 4-40 hardware, you can always get it online; see the Sources box for a small selection of Web retailers that carry miniature fasteners.

Tip: When searching the local digs for miniature nuts and screws, try the smaller hardware stores first. They’re more likely than the “big box” retailers to have what you’re looking for.

In addition to the assembly hardware, at a minimum you need the additional components in Table 2 to build your ArdBot II. Some additional parts will be needed to complete other functionality of the robot. We’ll cover these

2 Standard size servos, continuous rotation, Futaba compatible splined hubs

2 2-1/2” or 2-5/8” wheels with Futaba compatible hubs

1 Arduino Uno microcontroller board

1 Mini solderless breadboard (170 tie points, double-sided foam tape on back)

1 4 x AA battery holder

4 AA batteries (alkaline or rechargeable)

1 Nine volt battery snap and wires (lead length should be at least 6”)

1 Nine volt battery

1 2.1 mm barrel solderable connector

1 Miniature piezo speaker

2 Snap action leaf switches (standard size)

1 12” length three-wire (signal, power, ground) extension Misc Pre-made female-female jumper wires* (some will be cut to length)

Misc Set of 10 or more double-length break-away header pins Misc 12” length of 0.25” OD (outside diameter) aquarium tubing**

Pre-made female-female jumper wires and double-length break-away headers are available from Pololu. I recommend the 50 pack of 3” length female-female jumper wires (rainbow assortment), part number 1706. This is far more than you’ll use for the ArdBot II, but the extra will always come in handy.