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Computer Oscilloscope and Function Generator
Last modified: July 27, 2015
Introduction
What is it? What is it used for? How is it typically made? Pictures
Safety
The input range of the audio jack on a computer is typically from 0mV-500mV.
Inputting a larger signal will permanently damage the computer. Always look
up the safe input voltage levels of a computer before plugging into it.
Materials
Microphone and earphone set (alternatively: audio jack)
(Old) computer
2 resistors
2 variable resistors
4 diodes
Breadboard (solderless or soldered)
(Solder + solderwick)
Wires
Alligator clips
(Heat shrink)
Tools
Scissors
Sandpaper
Soldering iron
Wire strippers
(Heat source for heat shrink)
Procedure
Take an earphone and microphone set, and cut off the earpiece and microphone to
expose the wires. There will typically be 2 wires that go to each: one is the signal
and one is the ground wire. Remember which color wires go to the earpiece and
which go to the microphone.
You may need to sand away the paint on the wires to expose the conducting surface.
The wires are extremely delicate, so be careful as to not accidentally rip the wires
while sanding.
The microphone wires act as an oscilloscope and the earpiece wires act as a function
generator.
Oscilloscope: The input range of the audio jack on a computer is typically from 0mV500mV. Inputting a larger signal will severely damage the computer and will
prevent the microphone from working properly in the future. Thus, you may need to
build an attenuator.
Circuit: (homediyelectronics.com)
Layout'
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Schematics'
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Oscilloscope:'
Layout'
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Functions of the circuit:
 Diodes are used to protect the soundcard of the computer by restricting the
output voltages to 0.5A
 Resistors are used as a voltage divider to attenuate the voltages from the
input
Function generator: Connect the wires to your desired output device. Many sites on
the internet offer good signal generators:
http://onlinetonegenerator.com/
Characterization
Function generator:
 Time step limitation: 10 us (independent of tone type)
 Maximum voltage: V_pp = 3.5 V (dependent on frequency due to computer's
low-pass and high-pass filters)
 Voltage noise: V_pp = 160 mV (without circuit), V_pp = 70 mV (with circuit,
220 Ohm and 3 kOhm give the same result)
 Triangle waves are hairy
Schematics'
Tips and tricks
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Check that the headset is working before you start. That's the best way to
prevent a much longer time than needed for debugging.
Once you figure out which wires go to the microphone and which to the
earphones, label them! Tape is typically a good idea
Improvements
 Make or find computer program (not online) that makes tones
 Can make frequency vs. amplitude for sine, square, and triangle waves from
online tone generator