Hardware Hacking Projects for Geeks

9

Introduction

After you have completed building this circuit, con-

nect a 9V battery and test it out. When you press the

button, the LED should turn on. If it doesn’t work,

don’t despair. The next section covers how to use a

multi-meter to troubleshoot your circuits.

How to use a multi-meter

A multi-meter is often called a Volt-Ohm meter

(VOM), a digital volt meter (DVM), or simply a meter.

A multi-meter measures voltage, resistance, and some-

times current. Advanced models can also measure the

value of capacitors, time, and temperature.

The multi-meter typically has two plastic sticks with

metal tips (called probes) that are attached by flexible

wires connected to two sockets in the body of the

meter. The probe wires are removable. In the center of the instrument is

a dial that selects the electrical characteristic you will be measuring. The

meter shown in Figure I-6 measures resistance, DC voltage, AC voltage,

current, diode forward voltage, and battery voltage.

The multi-meter lets you see what is going on inside an electronic circuit.

Unless you always build every circuit perfectly, you will be using a multi-

meter to find out what is wrong.

We will now use a multi-meter to measure voltage and current in the circuit

you just built.

How to measure voltage in a circuit

The multi-meter’s dial will usually have two areas marked with the letter V.

One will have two straight lines (a solid one above a dashed one), and the

other will have a single wavy line.

Overview

When the dial is rotated to the area with the V with the two straight lines, it

is set to measure a static or slowly changing voltage such as a battery. There

are several different numbers in this area as well, which tell the meter what

range of voltages you want to measure. Select the numeric value closest to

what you think the signal might be. Touch the tip of the black probe to a

point in the circuit that you know to be “ground” (look on your schematic

diagram or select the minus terminal of the batteries in the circuit), and

touch the red probe to where you want to measure the voltage. You do not

always have to touch the black probe to ground in the circuit. The meter

will measure the voltage between any two points in the circuit. If the meter

reads all zeros or single 1, change the range and try again. Some meters will

change the range for you.

Solder Flux

Solder flux is a liquid that makes it

easier for two surfaces to be soldered

together. It is a solvent (slightly acidic)

that helps to clean away oil and resi-

due from the surface of the materials

being soldered. Several types are

typically available: rosin-based, no-

clean, and water-soluble. Rosin-based

is one of the oldest types, and may

leave an orange residue. This residue

should be removed from the circuit

board before testing the circuit. Use a

flux remover (another solvent) to dis-

solve the rosin-based flux. No-clean

type flux leaves a clear residue that

does not need to be removed. Water-

soluble fluxes can be cleaned off with

water.

I recommend using a no-clean flux, as

it requires no action when soldering

is complete.

Figure I-6: Multi-meter

The Basics

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