switches, and indicator LEDs. This places the switches and
batteries on top where they can be accessed more easily.
Use #4-40 flat head bolts to attach the battery holders. The
LEDs and toggle switches are bolted through holes drilled
in the top Syntra layer. A good design places the proper
LED indicator next to the toggle switch that turns it on.
The motors and drag wheel are mounted on the
bottom of the robot. The best motors for the CheapBot
are gearhead motors in a clear servo case (SKU# GM4)
from HVW Technologies www.hvwtech.com/products_
view.asp?ProductID= 58. (The motors are servos with
gearhead trains and motors, but no controlling electronics.).
They have plenty of torque and mount just like servos.
Cost is $11.95 each (you’ll need two).
I found the easiest way to mount the two motors to
the robot is with two parallel basswood strips. Use two 1/2
inch basswood strips cut to 5-7/8 inches long and bolted to
the bottom of the robot. Make the spacing between them
equal to the width of the servo case (which is 40. 6 mm, or
nearly 1.6 inches). Now, the servo motors can be mounted
between the rails with the screws included in their package.
The third wheel is a dragger and it’s a cheap plastic
pull knob. The pull knob’s original mounting bolt must be
replaced since it is too short for the robot. Hardware stores
sell a replacement sheet metal screw two inches long of
the proper diameter. You’ll need a spacer to fill the gap
between the base of the pull knob and the bottom of the
robot base. A styrene plastic tube from a hobby store works
well for this and for the spacers between the two layers of
the robot body. The servo motors connect to Cheapbot- 14
through the right angle headers on the back. Crimp a pin
on the end of each servo motor wire and push the two of
them into a plastic housing. The servo motors can now be
plugged into the controller. This still leaves the wheels to be
completed. Here are two options. First, HVW Technology
sells a 2-5/8” diameter wheel designed for servo motors.
Test Procedures
Before inserting the PICAXE- 14 and flipping the switch,
verify the circuit has been properly assembled using the
following checklist:
• Inspect the bottom of the PCB for solder that has
overflowed its pads to other pads and traces.
• Turn on the switch and verify there is no continuity between
the positive and negative terminals in the battery packs.
• Insert AAA cells into the battery packs and flip the switches;
the LEDs should light.
• Measure voltage across the IC sockets pins 1 (+5V) and
14 (Ground).
• Shut off power and insert the PICAXE- 14 (align the notch
of the chip with the notch in the socket).
• Connect a programming cable to the programming header
and start the PICAXE program Editor.
• Set the Editor to PICAXE- 14 and proper Comm Port.
• Type and download the following one line program:
DEBUG
• The Debug terminal will pop up and you should get a single
reply with all variables being zero.
46 SERVO 06.2009
(The wheels are SKU# SW and cost $4 a pair.) They come
in several colors and will add some pizzazz to your bot. The
other option is to use plastic peanut butter lids. To give the
robot some traction, wrap a #84 rubber band around the
rim of the peanut butter lid wheels. Now, load batteries
into the robot and it’s ready to drive. The commands to
drive the robot look like this:
LOW 2 LOW 4
HIGH 3 HIGH 5
To make the robot turn or reverse, just change the
pattern of highs and lows. Just remember that pins 2 and 3
control one motor and pins 4 and 5 control the other.
To stop the robot, use this code:
LOW 2 LOW 4
LOW 3 LOW 5
You can apply braking to the motors by setting all four
PICAXE pins high.
Wrap-Up
I designed the CheapBot- 14 robot as an inexpensive
way for 4-H and scout troops to start a robotics project. So
far, the 4-H in Lawrence, KS is having a great time with it.
I’ll put updates on my website about their adventures and
the research I hope to accomplish with this robot controller
combo. Check it out at nearsys.com.
That’s it, until I describe the CheapBot- 18 and the
Moon Rover I’ve designed around it. So, stay tuned! SV
Parts List
CheapBot- 14 PCB *
Two 4-AAA battery packs
PICAXE-14M
14-pin IC socket
Two Toshiba TA8080K H-bridges
Two LEDs (use two different colors)
Two 330 ohm resistors (can be up to 1K ohm in value)
10K ohm resistor
22K ohm resistor
Three-pin straight header
Two two-pin right angle headers
Two 10 microfarad capacitors
22 microfarad capacitor
LM2940 low-dropout voltage regulator
5 by 3 receptacle
2 by 3 receptacle
Two subminiature toggle switches
#24 AWG stranded wire (two feet will be sufficient)
Thin heat shrink tubing (to cover leads on toggle switches)
Shoot your own board from the copper pattern available on
the SERVO website.
The best place to order these H-bridges is Surplus Gizmos
( www.surplusgizmos.com)
Purchase a 3 by 38 receptacle and cut pieces two and five
pins long. Finding this size of receptacle can be a hit or miss
affair, so if you can't locate one use a combination of one and
two pin wide receptacles.
