Community College of Allegheny County Unit 1 Page 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
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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
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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).
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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
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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.
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 analog meter. c) Measure and record the voltage with a digital meter. d) Adjust the signal generator until you have an exact
100Hz, 5V 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 2
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Experiment #3 Triangular Wave:
1. Set a signal generator to a 1000Hz triangular-wave output with 6V 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, 6V 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 3
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 = _______
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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.
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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
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Table 1
SINE WAVE
Signal Generator Measured
Frequency Voltage Vp and Vpp and
Vrms Period Frequency
Meter
Voltage
60Hz 10Vpp
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Table 2
SQUARE WAVE
Signal Generator Measured
Frequency Voltage Vp and Vpp Period Frequency Meter
Voltage
100Hz 5Vpp
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Table 3
TRIANGULAR WAVE
Signal Generator Measured
Frequency Voltage Vp and Vpp Period Frequency Meter
Voltage
1000Hz 6Vpp
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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 –
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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.
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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
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Figure 6 – Electronic Debounce Circuits
+V
R1
R2
C1
RC Debounce
Circuit
+V
To
CPU
Digital Debounce
Circuit