Build a cheap oscilloscope and sensor
probes for physics experiments in school
Wichit Sirichote, kswichit@kmitl.ac.th
Physics Department, Faculty of Science
King Mongkut's Institute of Technology Ladkrabang, Bangkok
THAILAND
Rev. 1.0, April 19, 2014
Programme
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Day 1
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Morning: basic oscilloscope, introducing a cheap oscilloscope, basic
operations
–
Afternoon: assemble the oscilloscope and voltage probe, learn how
to use trigger mode and hands on experiments
Day 2
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Morning: basic of sensor probes, using sensor probes with the
oscilloscope
–
Afternoon: assemble the probes, and test the probe for many labs
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Discussions and summary
What is oscilloscope?
An oscilloscope is a laboratory instrument commonly used
to display and analyze the waveform of electronic signals.
In effect, the device draws a graph of the instantaneous
signal voltage as a function of time.
http://whatis.techtarget.com/definition/oscilloscope
3
Cathode Ray Tube (CRT) scope
http://www.williamson-labs.com/scope-main.htm
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Digital Storage Oscilloscope, DSO
http://en.wikipedia.org/wiki/Digital_storage_oscilloscope
5
Vibrations and Waves
http://ocw.mit.edu/courses/physics/8-03-physics-iii-vibrations-and-waves-fall-2004/
6
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Waves
A wave is a disturbance that travels through a medium
from one location to another. A wave is the motion of a
disturbance.
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Mechanical wave: sound, Seismic waves
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Electrical wave: voltage, current
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Magnetic wave: magnetic field
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Electromagnetic wave: Light
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Human heart wave: ECG or EKG
http://ocw.mit.edu/courses/physics/8-03-physics-iii-vibrations-and-waves-fall-2004/
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Using oscilloscope to display waves
Sound probe
Voltage signal
The sound probe built with condenser microphone
converts sound wave to electrical signal. The scope can
display the waveform of sound pressure.
9
JYE Tech DIY oscilloscope
DIY(Do It Yourself)
Schematic and
firmware are
available for
educational use
DSO 062 1Channel, 1MHz, 20MSample/s
http://www.jyetech.com/
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DSO 062 : front view
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DSO 062 block diagram
ADC
Input
signal
MEGA64
Amplifier
AVR
Microcontroller
128x64 pixels
Graphic LCD
ADC: high speed analog-to-digital converter
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Hardware schematic DSO 062 oscilloscope
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Specifications
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Maximum equivalent-time sampling rate 20MSa/s
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Maximum real-time sampling rate - 2MSa/s
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Resolution - 8 bits
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Record length - 256 bytes
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Analog bandwidth - 1MHz
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Vertical sensitivity - 100mV/Div - 5V/Div
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Vertical position adjustable with indicator
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Input impedance - 1M ohm
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Max input voltage - 50Vpk
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DC/AC/GND input coupling
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Horizontal - 0.5us/Div - 10m(minute)/Div
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Auto/Normal/Single trig modes
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Rising/Falling edge trigger
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External trig available
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Trigger level adjustable with indicator
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Hold/run feature
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Save/recall up to 6 captures
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FFT function available - size of 256 points and 512 points
selectable, sampling rate of 1Ksps to 2Msps selectable
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Cheap oscilloscope: renovate the
DSO 062
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Bigger display 128x64 Graphic LCD
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Add RCA jack for +5V sensor probe and trigger input
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Big size push buttons
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Add sensor probes for school lab
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1500THB (scope) + 500THB (sensors)
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AVR MEGA64 Microcontroller & ADC & Keypad & LCD
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Amplifier & Power supply
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Board layout
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AVR MEGA64 Microcontroller & ADC
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Preamplifier TL084 & NE5532
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Negative power supply & reference
voltage
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Break
Handmade acrylic box
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Blue Scope with DIY sensor probes
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Set in the box: scope & sensors
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Voltage probe
alligators
Shrink tube
RCA jack
Black wire
Red wire
50cm stereo shield cable
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Power supply: AC adapter 9V
AC adapter +9V 300mA
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HOLD/RUN state
Vertical position
Coupling
Y-scale
X-scale
Trig-mode
Trig-source
Window position
Trigger-edge
Screen layout and indicators
Trigger-level
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T(period)=4 div=4x5ms=20ms frequency=1/ T, 1/20ms = 50Hz
X scale has10 Divisions, 1 division= 5 points, 1 point= 0.229
Amplitude=4div=4x5V=20Vpk-pk
Y scale has 6 Divisions, 1 division= 5 points, 1 point= 0.2
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RCA input terminal
Multi-Function
Terminal, MFT.
1. 500Hz test signal
Channel 1 input
2. Ext.Trigger input
(White color)
3. Trigger output
+5V power supply for
(Yellow color)
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sensors (Red color)
Input signal slide switches
Y scale per
division
Input signal
coupling types
AC or DC
Select frequency
meter
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LCD display contrast adjustment
POT
LCD contrast
adjustment
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Keypad layout
Main function
Sub function
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Keypad Functions
Select triggering modes, with key + and 1. AUTO
2. NORMAL
3. SINGLE
AUTO This trigger mode allows the oscilloscope to acquire a
waveform even when it does not detect a trigger condition.
NORMAL The Normal mode allows the oscilloscope to acquire a
waveform only when it is triggered. If no trigger occurs, the
oscilloscope will not acquire a new waveform.
SINGLE The Single mode allows the oscilloscope to acquire one
waveform, and the trigger condition is detected. The display will
be in HOLD state.
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FFT, Fast Fourier Transform
Press key MODE and hold for 2-3 seconds,
the FFT display will be displayed.
FFT
scope
50Hz sinewave
Time domain display
50Hz sinewave
Frequency domain display
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Original waveform
Fundamental Frequency
FFT
2nd harmonic
rd
3 harmonic
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Sec/Div: select time scale for
second/division. Use key + or – to adjust
from 0.5 microsecond to 10minutes.
Toggle between RUN mode or HOLD state.
Vertical Position: adjust waveform position
in vertical up or down with key + or -.
Save waveform to memory in HOLD state.
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Horizontal Position: adjust waveform
position in horizontal with key + or -.
Select FFT size 256 or 512 points.
Select triggering edge, positive going or
negative going.
Display waveform that saved in memory.
Adjust trigger level with key + or -.
Send BMP file of LCD screen to serial port, 38400 8n1
with XMODEM protocol.
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Press level key also selects the function of
Multi-Function Terminal by indicating the
single letters “i”, “e” and “o”.
Indicator
i
e
o
Trigger Source
Test Signal
Trig Output
Internal
External
Internal
Enabled
Disabled
Disabled
Disabled
Disabled
Enabled
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Basic operation
Connect the signal input to TEST signal.
Press LEVEL key to select internal triggering source. (shown i )
Question1: What is the frequency of this signal?
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TEST signal is square waveform. It can be measured by
the frequency meter directly. Slide the 3rd switch to Freq.
Position. The readout will show 500Hz.
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Aliasing
Apparent lowfrequency waveform
due to aliasing
Aliasing occurs when the oscilloscope does not sample the signal fast
enough to construct an accurate waveform record. When aliasing
happens, you see a waveform with a frequency lower than the actual
waveform being input or a waveform that is not stable even though the
oscilloscope triggered.
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One way to check for aliasing is to slowly change the horizon scale with
the SEC/DIV then press key -. If the shape of the waveform changes
drastically, you may have aliasing.
To display a signal accurately and avoid aliasing, you must sample the
signal more than twice as fast as the highest frequency component.
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Afternoon hands on exercise
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Assemble the scope
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Play with basic scope operations
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Build a voltage probe
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Learn Triggering modes: AUTO, NORMAL, SINGLE
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Measure period, amplitude, and frequency
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Learn FFT mode
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Adjusting display position
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waveform saving, displaying waveform
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Discussions
45