Servo - January 2007

Beginner’s Robotics on $50 a Month

FIGURE 2. Motor Noise Capacitors.

FIGURE 3. Caps and Wires on Motor.

the assumption that it will be easier for the new builder to use these components. However, they add over $50 to the cost of the robot. That is more than our entire budget!

For the CIRC Bot, you will be using a $2.99 AVR Mega48 microcontroller. The Mega48 is not only less than a tenth of the cost of a BASIC Stamp, it is also more powerful.

The robot will be programmed using BASCOM-AVR from MCS Electronics. BASCOM is a version of the Basic programming language. You will need to download a free version of the

BASCOM-AVR compiler complete with manual from www.mcselec.com/ index.php?option=com_content&ta sk=view&id= 14&Itemid= 41. BASCOM runs on Windows 95 or newer PCs. We will be programming using the parallel (printer) port, so you will need to have a PC available with a parallel port. If you do not already have one, you can often pick up an older PC at a computer repair shop or a second-hand store for less than $50. Just make sure that the computer’s operating system is Windows 95, 98, 2000, NT, or XP.

Ordering the Parts

Description Qty

Solderless Breadboard 830 contacts 1

Wire 3’ each of red, green, and black 1

.1 µF Axial Ceramic Capacitor 11

7805T 5V Positive Regulator, 1 Amp 1

100K ohm 1/4 Watt 10

360 ohm 1/4 Watt 11

4 AA Battery Holder w/leads 1

Heat Shrink Tubing 1

40 Pin Header 1

D-Sub Mini Connector/25 Pin Male 1

DB- 25 Hood 1

6 Conductor 26GA Multicolor Wire 10

1” Squares 1/16” Thick Foam Squares ( 15) 1

SN754410 Quad Half-Bridge 1

ATMega48 — Atmel Mega48 Microcontroller 1

Tamiya Truck Tires (1 Pr) 1

Tamiya Double Gearbox 1

Tamiya Universal Plate Set 1

Tamiya Ball Caster (1) 1

CDS Photoresistor 2

2,200 µF 16V Radial Electrolytic Capacitor 1

TABLE 1. Bill of Materials.

The list of components necessary to build the CIRC Bot is shown in Table 1. The entire component list can be purchased as a kit from Wright Hobbies.

Subassemblies

Building the Gearbox

The gearbox kit comes with instructions; however, they are a little limited. Wright Hobbies has made a video of the gearbox assembly procedure. You can find it online at www.wrighthob bies.net/guides/. You will be assembling the gearbox in its highest gear ratio configuration (344.2:1).

Adding Caps to the Motors

This is probably the hardest part of the build, but if you practiced your soldering

using the LED project described in last month’s article, you will be able to handle it. Three of the 0.1 µF capacitors need to be soldered to each motor. One cap will be connected between the two motor leads. The other two will connect each lead to the motor case. In an ideal situation, the caps between the leads and the case would both connect to the case in the same location. However, with the configuration of the Gearbox we are using, this will not be possible, so we will have to solder to two different locations on each motor.

Bend the leads on three capacitors as shown in Figure 2. Hook the leads from the capacitor with both of its leads bent into the two motor lugs. The leads bent into a “J” shape on the other two capacitors also hook into the motor leads. These capacitors should be positioned so that their straight leads touch the metal part of the motor case. Before soldering the capacitors into place, lightly sand the parts of the motor case (which are under the capacitor leads) with a very rough sand paper or emery board. The goal is to make the surface rougher so that solder will bond to it easier. Clean the roughed areas with alcohol. Apply rosin paste to the roughed areas and the leads so that the solder flows into all of the crevices and makes a good bond between the motor and both of the leads. The motor is a fairly large heatsink so you will need to take some time with your iron. Do not solder the connection yet.

You need to add hookup wires. Cut two five-inch pieces of wire: one red and one green. Strip about a quarter inch of insulation off of each end. With the wires side by side, twist them together leaving only about one inch at either end untwisted. Take a stripped end of the red wire, twist it around the capacitor lead going through one of the terminal lugs, and take a stripped end of the green wire and twist it around the capacitor lead going through the other terminal lug.