Digital Multimeter Basics

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This is referred to as a ‘Digital Multimeter’ (DMM) because it does more than one type of measurement: volts, ohms and amps. A really nice DMM may also have other features, like frequency, capacitance and temperature (with an accessory probe).

Voltage Measurements

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Voltage measurements are the easiest and safest DMM measurements. Note the red probe plugs into “V” and the black probe into “COM” (for common ground). The dial selector should be at “V” with a flat line over it for DC volts, the squiggly line for AC volts. Since the meter is high-impedance, you may go directly across a power source, even and electrical outlet without damaging the meter. On DC measurements the display will tell you when you have the polarity wrong, as in the car battery photo and the -13.02V.

Current Measurement

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I used to manage 52 technicians; and nearly every one of them did not know how to perform a current measurement as evidence by all of the blown DMM fuses I had to replace. I finally resorted to taping over the “A” holes to avoid damage. You can damage a meter by not doing this correctly. A current measurement is made by opening the circuit under test and putting the meter in series with the current using the current and common ground leads. You never go across a component, as with a voltage measurement; because the current measurement is practically a dead short. Also, always begin with the highest current range and work your way down.

Recap:

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Ohm Measurements

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Resistance is NEVER measured on a live circuit. The DMM uses its internal battery to extrapolate the resistance. The best ohmic measurements are taken out of circuit because in-circuit you can not be sure that other components might be providing paths across your resistor under test and making it read lower than it is. I have often had to lift one leg of a resistor just to be sure of my measurement. Before taking any resistance measurement touch your probe tips together and note how close to zero the meter reads (and mentally subtract that from your readings). If it is over 10 ohms repair or replace your meter leads.

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diodetesting

Use one of the MakerSpace DMMs and check out the diode on your breadboard. Then use the same technique on your NPN transistor measuring from its base to either the emitter or collector.

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Oscilloscope Measurements

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An oscilloscope with all of its knobs and buttons might seem to be an intimidating piece of test equipment; however, it is not hard to learn to use and it greatly aids troubleshooting by displaying invisible waveforms.

Voltage Amplitude and Time

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The first thing to wrap one’s mind around is the display method of voltage amplitude versus time. The trace begins at the left and moves to the right in a period defined by its timebase. Most scopes will adjust from microseconds to milliseconds to actual seconds.

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The horizontal line in the middle of the graticule is the ground line. The amplitude of the peaks and troughs of pulses above and below it are based on the volts-per-division setting. In the picture above it must be 2V/Div since 2 and a half divisions represents 5VP.

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Triggering

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I have used scopes with 1, 2 and 4 channels; the picture above shows a 2-channel scope. The trigger controls the voltage level at which the waveform is captured. The oscilloscope can be set to capture a single waveform event; though most often one sees repetitive waveforms displayed. If the trigger is set too high or too low you  might not see any waveform at all. Most scopes have a ‘free run’ or ‘roll’ mode that displays anything (but not with a fixed beginning or end) so you can estimate how to adjust the trigger for a stable waveform display.

Frequency Versus Period

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Frequency is simply the inverse of period. When setting the timebase it is in terms of period.

Waveforms

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The sine wave is a natural analog wave. Digital circuits most often generate square or rectangular waves or pulses. Old cathode-ray television sets used the saw tooth wave to produce a raster on the screen line-by-line.

Over/Undershoot and Ringing

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High-frequency digital signals can suffer from parasitic inductance and capacitance that distorts the intended signal. The term ‘parasitic’ in this case means that real world attributes of electronics differ from their theoretical models; a circuit trace contains some resistance, inductance and capacitance; a capacitor also has some inductance, and and inductor has some capacitance and resistance. Thus, a squarewave under close inspection is rarely perfectly square. There can be overshoot, ringing and noise present; and if the amplitude of these is too high they spurious signals can be confused for logic 1s and 0s resulting in a malfunctioning circuit.

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OLYMPUS DIGITAL CAMERA
OLYMPUS DIGITAL CAMERA

Not all oscilloscopes are big, expensive beasts. At our MakerSpace we have a DSO Nano scope that is smaller than a cellphone that retails for about $100 USD. We also have a USB BitScope that costs a little more but it also contains an arbitrary waveform generator and a logic analyzer. We also have some conventional scopes. In this exercise we will build a signal generator using our Arduino and will take turns observing the signal using these scopes.

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Arduino Variable Audio Generator

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If you want to use a regular 8- ohm paper cone speaker rather then the piezo element; add this circuit:

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This little signal generator is far from perfect (but it sure is a lot of fun!) The tome will warble a little bit because the single-core processor takes turns doing other things; like the serial comm and running the loop. If you need a pure tone, dispense with the potentiometer and hard code just one frequency like this:

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going-further

Arduino Mini-Bus Pirate Blog

Cheep Component Tester

Arduino Data-Logger

Arduino Frequency Counter

 

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