Servo - June 2009

until the battery voltage drops to around 5. 3 volts. The 22 µF capacitor next to the voltage regulator acts as a momentary battery. It helps the voltage regulator maintain a more constant voltage by discharging when the robot controller requires more power and charging when the voltage level climbs. The two two-pin headers are connection points for the robot’s motors. The three-pin header is the programming jack for the PICAXE- 14. One of the pins is the ground connection and marked with the letter G.

Close to the edge of the PCB (printed circuit board) are two input and output receptacles. These are where external circuits are plugged into the robot controller. The receptacles are laid out in three columns. The column closest to the PCB’s edge is the ground column. Any pin plugged into this column is connected to ground, or 0 volts. The middle column is + 5 volts, so any pin plugged into the middle column connects to + 5 volts. The inner column is the individual connections to the I/O pins of the PICAXE- 14. The pins they connect to are listed in the top silk diagram included here. You’re probably familiar with this

arrangement of pins since it’s used with servos.

Using the CheapBot- 14

After testing, the next step is to mount your CheapBot- 14 to a robot body. In my opinion, the best material for a robot body is 1/8 inch thick Syntra, or foamed PVC. The material is rigid for small robots and easy to work with. There are four holes near the corners of the PCB to mount the controller to the robot body. Use #2-56 hardware and stand-offs, or replace the stand-offs with a layer of Foamies (foamed neoprene). Be careful not to over-tighten the mounting bolts when using Foamies. Since the material compresses quite easily, the PCB gets bent when the bolts are over-tightened. It’s best to use nylon insert locking nuts when bolting the robot controller to a layer of Foamies.

To keep the robot’s footprint small, stack a second Syntra layer above the first using four bolts, nuts, and spacers. On the top layer, mount the battery holders, toggle

Step by Step Soldering List

• Three jumper wires (use cut resistor leads).

• 330 ohm resistor (orange, orange, brown, gold)

• 10,000 ohm resistor (brown, black, red, gold)

• 22,000 ohm resistor (red, red, orange, gold)

• Two-pin right angle header

• 14 pin DIP socket Verify Polarity

• 10 microfarad capacitor Verify Polarity

• 22 microfarad capacitor Verify Polarity

• Three-pin straight header

• LM2940

• TA8080K H-bridge

• 5 by 3 receptacle

• 2 by 3 receptacle

Verify Polarity

Cabled Components

The last six components — the battery holders, the power switches, and the indicator LEDs — are soldered to the PCB through wires. This allows them to be placed somewhere convenient on the robot. The battery packs are ready to solder to the PCB, however, the switch and LED cables must be prepared as follows:

• Strip one end of all four wires back by 1/2 inches.

• Twist the ends tightly and tin.

• Cut the leads of the LED to half their length.

• Tin the leads of the LEDs and the toggle switches.

• Place a tinned wire against a tinned lead.

• Heat with a soldering iron until the solder on the wire and lead fuse.

• Repeat for the other three leads and wires.

• Slide heat shrink over the soldered connections and apply heat to shrink.

• From the bottom of the PCB, insert the bare end of a wire through a large pad (strain relief pad), bend it over, and insert it into the neighboring smaller pad shown in the photo.

• Solder the wire ends and clip any excess length.

• Repeat five more times to solder the two battery backs, two LED indictors, and two toggle switches to the PCB.

A strain relief wire ready for soldering.

If you would prefer a complete CheapBot- 14 kit instead of rolling your own, check out my website under the Catalog link. I’m selling the complete controller kit for $26.50 plus $6 shipping. Directions — written for first-time roboticists — are available as a free download along with sample code. You can also find a line-follower kit at my website. Over time, I’ll add ideas for other sensors you can use with this robot controller. I now have a Twitter account and occasionally send tweets. If you'd like to stay up to date with some of my activities, then follow NearSys.