Clips and Test Leads
The components on Sandwich’s circuit board are permanently soldered together. You’ll learn about that later. But what do you do if you want to make a quick connection for a simple experiment? No, don’t use electrical tape. It’s too loose and sticky!
This chapter covers a simple method for making a few temporary connections. It also describes a multimeter test that you can perform to check if an electrical connection has been made.
The Gators Are Hungry Tonight
Alligator clips have spring-closing “mouths” that grip parts (see Figure 8-1). Squeezing the center of the clip causes the mouth to open and release whatever it’s holding. When you let go of the clip, the mouth closes and can hold onto things.
Figure 8-1. Alligator clip with insulated shroud
Alligator clips are friendly. They have a lighthearted name and you can pinch their mouths open and shut like they are eating or saying hello.
A pair of clips attached by a wire is called a jumper lead or clip test jumper (see Figure 8-2). The wire itself is usually flexible copper surrounded by an insulating colored plastic casing. Now you’ve got something that can grip a part on both ends and connect them with a wire.
Figure 8-2. A jumper lead is alligator clips with a connecting wire
Obtaining Hook Clips
Clips are available in many sizes and colors (see Figure 8-3). Beyond the “alligator” style, there’s the mini IC hook, which is smaller and tends to hold onto wires a little better (see Figure 8-4). For these reasons, you’ll probably use hooks more often than alligator clips.
Figure 8-3. Top to bottom: large alligator, medium alligator, mini IC (hook), micro IC (hook)
Figure 8-4. Close-up of hook
Sometimes the end of a hook bends flat during use. It becomes more like a letter ‘L’ than a letter ‘J.’ It’s not supposed to be that way. Bend it back. When the hook retracts, it should fit into the hole in the end notch.
Purchase at least five medium alligator and five mini IC hook jumper leads (see Table 8-1). They are available in a variety of colors. Try to obtain red, black, and at least one other color.
Table 8-1. Alligator and Hook Jumper Leads
Testing Jumpers
You’re now going to learn how your particular multimeter displays an electrical connection or lack of connection. Then, you’ll make a test connection with an alligator jumper lead.
Setting Up a Multimeter for Continuity Testing
Figure 8-5. Multimeter dial set to continuity
(alternate) If your multimeter doesn’t have a continuity feature, turn the dial to the lowest ohm range (see Figure 8-6). This measures resistance. A short wire like the jumper lead should have almost no resistance to electricity flowing through it.
Figure 8-6. Multimeter dial set to lowest ohm range
Testing an Open Connection
4. Do not touch the metal tips of the probes to anything at this point (see Figure 8-7). If the meter isn’t already turned on, press the power-on button.
Figure 8-7. Multimeter probe tips not touching
A meter with a continuity feature should now display open and should not be beeping. Some meters display “0L mV” instead of the word “open” (see Figure 8-8). Check your meter manual.
Figure 8-8. Multimeter displays open (no connection) and very high resistance
If you don’t have a continuity feature, the meter should display 0L or ∞ (infinity) or some very large number in the megohm (M) range.
Since the probe tips aren’t touching each other, this connection is “open.”
Testing a Shorted Connection
5. Touch the probe tips together (see Figure 8-9).
Figure 8-9. Touching the probe tips together to make an electrical connection
Figure 8-10. Multimeter displays short (connection) and almost zero resistance
A meter with continuity should now display short (see Figure 8-10) and should beep annoyingly. Some meters display “0 mV” instead of the word “short.” Check your meter manual.
If you don’t have a continuity feature, the meter should display zero Ω (ohms) or a very small number.
Ever heard of a “short-circuit?” Well, this one is pretty short. The electricity from the meter goes out of one probe and then directly into the other!
Experiment with the ohm ranges and the continuity setting of your meter. You want to get a good sense for what the meter displays when there is a connection and what it displays when there isn’t a connection.
Testing an Alligator Connection (Shorted)
6. Instead of touching the multimeter probe tips together directly, connect them with an alligator jumper lead (see Figure 8-11). Using a single jumper, attach one alligator clip to the tip of the black probe and the other alligator clip to the tip of the red probe.
Figure 8-11. Making a connection with alligator jumper leads
You should get the same value displayed on your meter whether you touch the probe tips together directly or whether you connect them with a jumper. The alligator clips and wire are just as good at allowing electricity to flow through them as touching the probes together directly.
During use, alligator clips may become loose or grimy and fail to make a solid connection. The wire between the clips may rip and detach. In those cases, if the jumper lead no longer conducts a continuous connection, you can test for it on your multimeter. A broken connection will read “open” or some ohm value greater than touching the probe tips together.
Think of the wire in the alligator jumper lead as electrical pipe. Electricity flows through the copper wire (or any metal) like water flowing through a pipe. Unlike water pipes, if you disconnect one end of the wire, the electricity doesn’t spill all over.
Discovering Unintended Connections
The continuity mode (and also the ohm mode) of a multimeter has a very beneficial use in robotics. It can detect if an unintentional electrical connection exists between various robot body parts and circuits. Although you won’t perform these steps now, here are some example steps you might take to test a robot.
If an electrical connection exists between the part connected to the black test probe and the part being touched by the red test probe, the meter will beep (or display “short” or a low ohm value or whatever). It doesn’t matter how long or how complicated the connection is. If there’s an electrical connection between the parts, the multimeter can detect it.
One of my line-following robots, Sweet, uses a metal candy container for a body. After drilling holes for the sensors and screws (see Figure 8-12), I didn’t consider that cutting away the paint coating had exposed the metal in the container to the robot’s circuits. When I proudly powered on the robot, the circuits went crazy. Fortunately, I only lost a $15 chip that day.
Figure 8-12. Exposed metal on drill holes causes unintended electrical connections
I should have probed the robot a bit with the continuity mode of my multimeter before powering up. By quickly checking the motors, each circuit board, major screws, battery connections, and body, I would have noticed that direct electrical connections had mistakenly occurred between metal parts of the circuit board and metal parts of the robot’s container.
Plumbing with Jumpers
In the last chapter, you learned that a 9 V battery works like an electricity pump. In this chapter, you learned that alligator clip jumper wire (or any other piece of metal) acts as electrical pipe. All you need now is something worth hooking up to the pipe.