Servo - September 2011

FIGURE 7. Close-up of the power terminal. The L298 board provides its own regulated five volts for the circuitry on the breadboard.

cells. I prefer NiMH batteries because they’re a little more environmentally friendly. Though they initially cost more than alkaline cells, when you factor in recharging them hundreds of times, they’re a very good deal. (Be sure to use a recharger made for NiMH batteries.)

When using NiMH or NiCd batteries, the six-cell pack delivers a nominal 7. 2 volts. When using alkaline batteries, the pack delivers nine volts. Either voltage is acceptable. However, be sure to recharge your NiMH or NiCd batteries when the pack voltage falls under seven volts.

of the wired power terminal. I’ve used a small cable tie to keep everything together.

Use a small flat-bladed screwdriver to tighten the terminals so the wires remain snug. You DO NOT want any of these wires to accidentally come loose, or else they could cause a short circuit and damage to the L298 module.

Attach the battery holder to the Beginner Bot base using two 1” Velcro™ squares as shown in Figure 8. This arrangement allows you to pull the holder off the base to replace or recharge the batteries.

For the prototype Beginner Bot, I used a set of six AA size nickel-metal hydride (NiMH) cells. You can use AAA size if you wish, and substitute nickel-cadmium (NiCd) or alkaline

Add the Solderless Breadboard Deck

Augmenting the L298 motor module is a small collection of circuitry to provide the Beginner Bot with sensitivity to light. Rather than solder together this circuitry, you can use a small solderless breadboard. A mini breadboard with 170 contact points will do.

The control circuit uses two cadmium sulfide (CdS) photocells, an inexpensive and readily available 74HC14

integrated circuit, a couple of resistors, and some wire. The solderless breadboard connects with the L298 module via the two power wires you provided earlier, plus a pair of 10” length three-conductor male-to-female R/C servo extensions.

At the heart of the control circuit is a CdS photocell, also called a light sensitive resistor. It’s connected as shown in Figure 9 with a 22 kΩ resistor to form a voltage divider. The output of a photocell is a varying resistance — the darker it is, the higher the resistance. With the resistor added and a connection point in between, the output becomes a voltage that varies between zero and five volts, depending on the brightness of the light.