Page 409 - Robot Builder's Bonanza

378 BUILDING ROBOT ELECTRONICS— THE BASICS

Some of these concepts assume you already know what a resistor and capacitor are. If you
FYI don’t, then no worries; these topics are covered in Chapter 31. This is just a review of ways to
improve the functionality of your robot circuits.

USE PULL- UP/PULL- DOWN RESISTORS
When something is unplugged in your robot, the input voltage might waver back and forth.
This can influence the proper functioning of your robot. Use pull- up or pull- down resistors on
any circuit inputs where this could be a problem. A common value is 10 k ; 10,000 ohms).
In this way, the input always has a “default” state, even if nothing is connected to it.
A pull- up resistor is connected between the input and the + (positive) power supply of the
circuit; a pull- down resistor is connected between the input and (negative or ground), as
shown in Figure 30- 14.

TIE UNUSED INPUTS LOW

Unless the instructions for a component say otherwise, tie unused inputs to ground to keep
them from “floating”—floating means an indeterminate voltage state. A floating input can
cause the circuit to go into oscillation, rendering it practically unusable.

USE DECOUPLING CAPACITORS
Some electronic components, especially fast- acting logic chips and the venerable LM555 timer
IC, generate a lot of electrical noise that can spread through the power supply connections.
You can reduce or eliminate this noise by using decoupling (also called bypass) capacitors, like
that in Figure 30- 15.
These aren’t specific types of capacitors; rather, “decoupling” refers to the job they per-
form. The value of the capacitor isn’t supercritical. I like to use 1 Fto 10 F (1 to 10 micro-
farad) tantalum electrolytic capacitors positioned between the positive and ground terminals

1 1 55
A A
B B
C C
D D
E E
Integrated
Place decoupling circuit
capacitor F F
close to IC's G G
+ power pin
+V H H
I I
J J
Pull-up 1 1 5 5
Output
resistor
Pull-down Figure 30- 15 Concept of the
resistor Output
decoupling capacitor: It’s a capacitor
placed near the power pins of an
Ground
0 volts, - terminal integrated circuit or other component,
on battery which acts to “decouple” (separate out)
Figure 30- 14 Concept of the pull- down and pull- up any noise that may be in the power
resistor. A common value of the resistor is 10 k . supply.

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