oscilloscopes, multimeters, & strain...

Community College of Allegheny County Unit 1

OSCILLOSCOPES, MULTIMETERS, & STRAIN GAGES

The Overweight Sub That Cost Billions: After Spain invested $2.7

billion in a program for diesel-electric submarines, in 2012, it

was discovered that the first one — weighing 2,200-tons — was

70-ton overweight and would probably sink if it went out to sea.

The error occurred because a decimal point was put in the wrong

place. "Apparently, somebody in the calculations made a mistake

in the very beginning and nobody paid attention to review the

calculations,"

Revised: Dan Wolf, 1/7/2018


OBJECTIVES:

Measurement Concepts:

• Oscilloscope Measurements

• Digital Meter Measurements

• Analog Meter Measurements

• Signal Generator Operation

• Voltage Measurements: Peak-to-peak (PP), Peak (P), RMS

• Time Measurements: Period and Frequency

• Waveforms: Sine, Triangular, Square, Sawtooth

DELIVERABLES THAT YOU MUST SUBMIT

1. Graph and Data for Tables 1, 2, and 3

2. Table for Experiment 5

3. Practice Problems

On-Line Reading Material:

1. https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope

Read sections:

a) Introduction b) Basics of O-Scopes c) Oscilloscope Lexicon d) Anatomy of an O-Scope e) Using and Oscilloscope

2. https://learn.sparkfun.com/tutorials/voltage-dividers

EQUIPMENT REQUIRED:

1. Signal Generator

2. Oscilloscope

3. Analog Volt Meter

4. Digital Volt Meter

5. Variable voltage power supply

6. 100Ω resister

7. 1000Ω resister

8. 350Ω resisters 1% tolerance (Qty=3)

9. 1000Ω 10-turn potentiometer

10. Strain Gauge type BF350-3AA 350 (mounted on metal bar),

Qty=2

https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope

https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope

https://learn.sparkfun.com/tutorials/voltage-dividers


INTRODUCTION:

The object of this experiment is to learn how to use the

oscilloscope by measuring the periods and amplitudes of various

waveforms (shown in Figure 2). The oscilloscope is an electronic

instrument widely used in making electronic measurements. The most

noteworthy attribute of an (ideal) oscilloscope is that it does

not affect the quantity being measured.

An example of an AC signal is shown in Figure 2. The voltage is

on the vertical (y) axis and the time is on the horizontal (x)

axis. Notice that if we plot a DC (or constant) voltage on this

figure, it would be a horizontal line.

There are two main quantities that characterize any periodic AC

signal. The first is the peak-to-peak voltage (Vpp), which is

defined as the voltage difference between the time-varying signal’s

highest and lowest voltage. Thus, Vpp is defined as:

𝑉𝑝𝑝 = 2 ∗ 𝑉𝑝𝑒𝑎𝑘

The second is the frequency of the time-varying signal (F), defined

by:

𝐹 = 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = 1

𝑇 = 𝑃𝑒𝑟𝑖𝑜𝑑

where F is the frequency in hertz (Hz) and T is the period in

seconds (as shown in Figure 2).

The voltage RMS value is the effective value of a varying (AC)

voltage. It is the equivalent steady DC (constant) value which

gives the same effect. For example, a lamp connected to a

6V RMS AC supply will shine with the same brightness when

connected to a steady 6V DC supply. RMS Voltage is defined as:

𝑉𝑟𝑚𝑠 = 0.707 ∗ 𝑉𝑝𝑒𝑎𝑘 Something Important to Remember: An oscilloscope will show you Vpp

and Vp however an analog or digital multimeter will normally show

you Vrms. This means that the maximum voltage in the circuit will

be higher than the value shown on the multimeter (120Vac in your

house is actually 170Vpeak but shown as 120V on the multimeter).


Figure 1 - Generated Waveforms

SINE WAVE TRIANGLE WAVE SQUARE WAVE

SAWTOOTH SAWTOOTH PULSE

Figure 2 – Sine Wave Fundamentals

Vpeak = Vo

Vpp = 2 * Vpeak

Frequency = 1 / Period

Vrms = .707Vpeak


Experiment #1 Sine Wave:

1. Set a signal generator to a 60Hz sine-wave output with 10V peak-to-peak.

2. Display the voltage on an oscilloscope. a) Measure and record the amplitude, period, and

frequency of the signal.

b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact

60Hz, 10V peak-to-peak signal.

3. Using the signal generator, change the frequency and voltage of the signal without looking at the oscilloscope

or meter. You now have an unknown waveform.

a) Measure and record the amplitude, period, and frequency of the signal.

b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Compare your measurements with the dials on the signal

generator. Are they (reasonably) close to each other?

e) Sketch this waveform and complete Table 1

Experiment #2 Square Wave:

1. Set a signal generator to a 100Hz square-wave output with 5V peak-to-peak.



b) Measure and record the voltage with analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact









Experiment #3 Triangular Wave:

1. Set a signal generator to a 1000Hz triangular-wave output with 6V peak-to-peak.



b) Measure and record the voltage with an analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact








Experiment #4 Voltage Divider:

1. Connect the circuit shown in Figure 3 where R1=100 ohm and R2=1000 ohm and V equal to +5V or +12V (your choice).

2. Measure the voltage across R1 and R2. Note that the ratio of voltage between R1 and R2 should equal the ratio of

resistance values for R1 and R2. Thus:

𝑅1

𝑅2=

100𝑜ℎ𝑚

1000𝑜ℎ𝑚=

𝑉𝑅1

𝑉𝑅2= 1: 10

So: R1 = 100 ohm VR1 = _______

R2 = 1000 ohm VR2 = _______

3. Note that if R1 = R2 then VR1 must equal VR2. Modify the circuit so R1 = R2 then measure VR1 and VR1 and record here:

R1 = _____ ohm VR1 = _______

R2 = _____ ohm VR2 = _______


Experiment #5 Bridge Circuit:

1. Connect the circuit shown in Figure 4 where: V = 10Vdc

Ra = Rb = Rc = 350 ohm (ideally these are 1% tolerance)

Rx = 1000 ohm ten-turn potentiometer

Use both an oscilloscope and voltmeter to measure the

center point. You will notice that the oscilloscope signal

is not easy to read due to low level noise.

2. If you adjust the potentiometer to exactly 350ohms, the voltage measured at the center will be 0 Volts.

3. Carefully turn the potentiometer a small amount and observe that the voltage measured changes with a change of Rx.

4. Replace Rx with one of the strain gages and adjust the potentiometer until you get zero volts at the center point.

5. Apply pressure on the strain gage and observe that the voltage at the center point changes. Record your

observations below.

Voltage

Measured

(One Strain

Gauge)

Voltage

Measured

(Two Strain

Gauges)

Balanced circuit with

no pressure applied.

Light Pressure applied

Heavy Pressure Applied

6. The output voltage of the bridge, VO, will be equal to:

𝑉𝑜 = [𝑅𝑥

𝑅𝑥 + 𝑅𝑐−

𝑅𝑏

𝑅𝑎 + 𝑅𝑏] ∗ 𝑉𝑖𝑛

From this equation, it is apparent that when Ra/Rb = Rc/Rx,

the voltage output VO will be zero. Under these conditions,

the bridge is said to be balanced. Any change in resistance

in any arm of the bridge will result in a nonzero output

voltage.


7. Replace the potentiometer with the second strain gage and replace the Rb resister with the potentiometer. Now adjust

the potentiometer until you get zero volts at the center

point. Re-test and complete the last column of the Table.

This circuit is a half-bridge strain gauge circuit and

should have twice the sensitivity as the quarter-bridge

strain gauge circuit.

Figure 3

Figure 4


Table 1

SINE WAVE

Signal Generator Measured

Frequency Voltage Vp and Vpp and

Vrms Period Frequency

Meter

Voltage

60Hz 10Vpp


Table 2

SQUARE WAVE


Frequency Voltage Vp and Vpp Period Frequency Meter

Voltage

100Hz 5Vpp


Table 3

TRIANGULAR WAVE


Frequency Voltage Vp and Vpp Period Frequency Meter

Voltage

1000Hz 6Vpp


PRACTICE PROBLEMS:

These do not have to be turned in but you should take a look at them and make sure you

understand the concepts. Ask the instructor to explain anything that you are not comfortable

with.

1. With regards to Figure#3, if V = 10V, R1 = 5000 ohms, R2 = 2000 ohms, how much current will flow in R1? How much current will flow in R2?

2. With regards to Figure#3, if V = 10V, R1 = 8000 ohms and the current in R1 is 1mA, what is the value of R2?

3. With regards to Figure#4, if Vin = 10 Volts, Ra = Rb = Rc = 350 ohms and Rx = 352 ohms, what voltage will be at Vo?

4. With regards to Figure#4, if Vin = 5 Volts, Ra = Rb = Rc = 350 ohms and Rx = 352 ohms, what voltage will be at Vo?

5. Looking at questions 3# and #4, we can see that reducing Vin affects the value at Vo. So, if want to see a larger value at Vo, we can increase Vin. But if we increase Vin,

more current wil flow through the Ra/Rb and the Rc/Rx nodes. This is ok as long as the

amount of current flow doesn’t exceed the capacity of the resisters and strain gauge.

How much current will flow through Rc in question #3? If Rc is a quarter watt resister,

will the current be acceptable? Note that the power equation is: P=I2R.

6. There is one inherent problem with interfacing switches to embedded microprocessors –


switch debounce. Anytime a mechanical switch closes (or opens), the contacts do not

make clean contact immediately, they “bounce” a quantity of times before making final

closure – see Figure 5. An embedded microprocessor is fast enough to detect each of

these bounces as a valid switch closure. With this in mind, each of the bounces could

be incorrectly acted upon. The solution is to implement either debounce hardware (see

Figure 6) or a “debounce delay” function in software.

Debounce delay is when the uP detects the first switch closure, waits for a period and

then rechecks the switch. If the switch is still closed the switch closure is assumed

to be complete and the new state is accepted.

The length of the debounce delay period is dependant on the construction and condition

of the switch. The amount of switch debounce is primarily determined by the switch type

and its’ age. In general, switch bounce will occur for a period between 5 and 60mS.

Note: Solid state switches are switches with no moving parts and are very popular.

They are more expensive than mechanical switches and require additional support

circuitry but they do not suffer from contact wear and do not experience the bounce

problem. This means the switch debounce task is not required so the software is smaller

and faster. The decision whether to use mechanical or solid-state switches really gets

down to a tradeoff of hardware versus software costs. If the product quantities are low

(i.e. space shuttle), the extra hardware cost for solid-state switches will be less than

the software costs (development, size, and risks). If the product volume will be in

large quantities (i.e. clock radios), the one-time software costs will be less than the

cost of the more-expensive solid-state switches. This decision is important and should

always be considered by both software and hardware engineers.

Use the internet to identify a software algorithm, source code (any language), or

detailed explanation for switch debounce. Print it out and turm it in as part of this

lab.


Figure #5 – Switch Debounce

Switch is

Released

Switch is

Pressed

0 Volt

+5 Volt

Switch Bounce

Non-Debounced

Switch

Debounced Switch

Coin Acknowleged

Coin

Initially

Detected


Figure 6 – Electronic Debounce Circuits

+V

R1

R2

C1

RC Debounce

Circuit

+V

To

CPU

Digital Debounce

Circuit

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