Building Robot Electronics—the Basics—Part III

Using Headers and Connectors

Most robots are constructed from subsystems that may not be located on the same circuit board. So you need to connect these subsystems together using some kind of wiring system. For very simple connections, you can directly solder wires between boards and other components. But as the electronic systems of your bot get more complex, such direct connections make it harder to experiment.

The solution: Use connectors whenever possible. In this approach, you connect the various subsystems of your robot together using wires that are terminated with a connector of some type or another. The connectors attach to mating pins on each circuit board.

MAKING YOUR OWN MALE CONNECTORS

You don’t need fancy cables and cable connectors for your robots. In fact, these can add significant weight to your bot. Instead, use ordinary 20- to 26- gauge wire, terminated with single or double-row plastic headers.

Figure 30-11 Make your own male header connectors by soldering wires to the short end of the header pins. For a professional look use heat shrink tubing over the solder joint.

You can make these yourself using breakaway header pins. You buy them in lengths of 10 or more pins, then break off as many as you need. Solder wires to the underside of the pins. Insert the top side into sockets on your circuit boards. Figure 30- 11 shows how. The optional heat shrink tubing is applied by cutting a small piece of the tube and slipping it over the soldered wire. Shrink the tube using a heat gun or hair dryer set on high.

PREPARING FEMALE CONNECTORS

If you’re connecting to a circuit board that already uses male pins, you’ll need to use a female connector to hook things up. You can make these by purchasing a connector set that’s designed for the pins you’re plugging into. These vary by the number of pins and the pin spacing, so be sure to get the right set. Most pin headers use 0.100? spacing. There are smaller and larger spacings, but these are obvious just by looking at them.

Referring to Figure 30- 12 as a guide, to make a connector prepare the end of a wire by removing about 1/4? to 3/8? of insulation. When using stranded conductor wire, twist the strands together. Insert the end of the wire into a crimp- on connector piece. Secure the wire in the connector by crimping it; a crimping tool made for the job works best. You then insert the connector piece into the plastic shell. The connector snaps into place in the shell.

Figure 30-12 Make your own female connectors using a connector shell and crimp on sockets.

BEST CONNECTIONS

Use plastic ties to bundle the wires together. This keeps them all tidy. Or, instead of using individual lengths of insulated wire, use ribbon cable, which is made of many pieces of wire bonded together as a unit.

When making interconnecting cables, cut the wires to length so there is a modest amount of slack between subsystems. You don’t want the wire lengths so short that the components are put under stress when you connect them together. But don’t go overboard; you also don’t want, or need, gobs of excess wire.

Using Clip-on Jumpers

Another kind of cable is used when experimenting with and testing your robot electronic circuits. These are clip- on jumpers. The jumper is made with flexible insulated wire, where both ends have some kind of spring- loaded clip. You attach the clips to wires, components, or another part of the circuit to make temporary connections.

You should get at least one set of clip- on jumpers for routine testing and experimenting. Jumpers are available with three basic kinds of clip- on ends:

• Small alligator clamp, useful for smaller components and wires. Don’t use these to attach

to an individual pin on an integrated circuit (IC). They’re too large and will cause a short.

• Large alligator clamp, good for motors and other, bigger components (the wiring is heavier, too).

• Push- in hooks, ideal for use when connecting to the individual integrated circuit pins.

All three kinds are shown in Figure 30- 13.

Good Design Principles

While building circuits for your robots, observe the good design principles described in the following sections, even if the schematic diagrams you are working from don’t include them.

Figure 30-13 Assortment of common jumper cables: hook, small alligator, and large alligator. They come in various colors and lengths. Always use the jumpers that have fully insulated (plastic- covered) clamps. They help prevent short circuits.

Some of these concepts assume you already know what a resistor and capacitor are. If you 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 kΩ; 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 perform. The value of the capacitor isn’t supercritical. I like to use 1 µFto 10 µF (1 to 10 microfarad) tantalum electrolytic capacitors positioned between the positive and ground terminals of the noisy component. Some designers like to use a decoupling capacitor on every integrated circuit, while others place them beside every third or fourth IC on the board.

Figure 30-16 A ground loop is when there is more than one path for the ground connection. Ground loops can cause erratic behavior in circuits. Always connect the ground leads of components to one central point.

It’s also a good idea to put decoupling capacitors between the positive and ground connections of any circuit at the point of entry of the power supply wires. Many engineering texts suggest the use of 1 µF to 100  µF tantalum capacitors for this job. Remember that tantalum capacitors are polarized— they have a + and a - side. Be sure to properly orient the component in the circuit, or the capacitor (and maybe some other parts) will be ruined.

KEEP LEAD LENGTHS SHORT

Long wire leads on components can introduce electrical noise in other parts of a circuit. The long leads also act as a virtual antenna, picking up stray signals from the circuit, from overhead lighting, and even from your own body. When designing and building circuits, try to keep lead lengths as short as you can for everything. When soldering, this means soldering the components close to the board and clipping off any excess lead length.

AVOID GROUND LOOPS

A ground loop is when the ground wire of a circuit comes back and meets itself. The positive and ground connections of your circuits should always have “dead ends” to them. Ground loops can cause erratic behavior and excessive noise in the circuit. See Figure 30- 16 for a visual depiction of a nasty ground loop that almost guarantees problems.

RoHS Demystified

Ever wondered about that “RoHS” thing that you see with many other electronic parts these days? RoHS standards for Restriction of Hazardous Substances directive, a worldwide effort to reduce the amount of toxic materials in commercially produced electronic devices. These materials— which include lead, mercury, and cadmium— present significant health hazards as tons of discarded electronic circuit boards are dumped into landfills. Yecch!, as Alfred E. Neuman would say.

In following various international laws, many sellers of electronic components and completed circuits list whether a product is RoHS compliant. In the case of electronic components, RoHS compliance typically indicates that the use of lead is either reduced or eliminated. Not being in compliance does not mean the product is inferior, just that its manufacture does not yet meet the very strict RoHS standards.

Excerpted from Robot Builder’s Bonanza, 4th Edition by Gordon McComb (McGraw-Hill; 2011) with permission from McGraw-Hill.