Computer Oscilloscope and Function GeneratorLast 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) computer2 resistors2 variable resistors4 diodesBreadboard (solderless or soldered)(Solder + solderwick)Wires Alligator clips (Heat shrink)

Tools

Scissors Sandpaper Soldering ironWire 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 0mV-500mV. 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)

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

Tips and tricks

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