Arduino Oscilloscope (with Pictures)

A very basic and easy to make arduino PC oscilloscope.

Features:

50K samples/second
(actually it can go up to 110K but the signal will become noisy)
Auto trigger
Frequency counter
Reasonably accurate voltage readings (depending on the accuracy of the resistors used for the voltage dividers)
Optional: selectable voltage range: 5V, 6.6V, 10V, 20V

You'll need:

An Arduino Leonardo or Arduino Micro
2 crocodile clamps
a 0.1µF capacitor (optional)
a 5.1V zener diode (optional)
a pc with Processing

For the voltage dividers (optional, if you want to measure than 5V or want selectable range):

2 two-pole dual throw switches
two 3K resistors
two 1.5K resistors
one 1K resistor
a small perfboard or breadboard

If you only need to measure op to 5V, you can skip the voltage dividers and connect the probes directly to GND and A1. You'll have to modify the code a bit:

In the arduino code, replace:

ADMUX =  B00000000;         // select external reference and port 5 (A0)

with:

ADMUX =  B01000000;         // select internal reference (Vcc - 5V) and port 5 (A0)

In the processing code, replace:

// read switch position & set voltage range
 boolean switch1=((buffer[writeIndex*2]&(byte)8)==8);                                                 
 boolean switch2=((buffer[writeIndex*2]&(byte)4)==4);
 if (!switch1&&!switch2) voltageRange=20;
 if (!switch1&&switch2) voltageRange=10;
 if (switch1&&!switch2) voltageRange=6.64;
 if (switch1&&switch2) voltageRange=5;

with:

voltageRange=5;

Step 1: Adding Voltage Dividers

The circuit show above consists of:

On the left: a 1:4 voltage divider between the probe and A1

This will bring the voltage down to 1/4 of the input voltage. The analog pins can handle 5V, so this will allow for voltages up to 20V.

Note that there are 2 input channels in the picture of the breadboard. Adding an extra channel slowed down the sampling rate dramatically (because continuous mode can't be enabled on the ADC), so I decided to leave it out in the final code.

On the right: a switched voltage divider between 5V and the Analog Reference (AREF) pin
You can use the switches to set the measuring range: 5V, 6.64V, 10V of 20V

How this works:

If configured to 'external reference', the ADC compares the voltage of the analog inputs with AREF, instead of 5V.
Here's an example: suppose the probe is measuring 5V. The voltage on the A1 will be 5V/4 = 1.25V

If both switches are off, the voltage on the AREF pin is 5V.
The ADC will read 1.25/5 = 25%
If switch 1 is off and switch 2 is on, the voltage on AREF is 2.5V
The ADC will read 1.25/2.5 = 50%
If switch 1 is on and switch 2 is off, the voltage on AREF is 1.66V
The ADC will read 1.25/1.66 = 75%
If both switches are on, the voltage on AREF is 1.25V
The ADC will read 1.25/1.25 = 100%

The second pole of each switch is connected to a digital input. We can read this pin to automatically adjust the voltage scale.

A capacitor between the probe and ground
Might not be necessary, but for some reason some pc's measure a lot of noise without it. The capacitor will solve that, but may slightly affect the signal when measuring high frequencies.

A zener between A0 and ground
To protect the arduino a little from overvoltage or reverse voltage (thanks, tttapa, for the tip!)

Be careful:

If the analog reference is set to internal (default) while you are supplying voltage to the AREF pin, the arduino could get damaged. I did that, and it didn't damage mine, but better be safe and upload the proper code before connecting AREF.
the analog inputs can't handle negative voltages.
Don't exceed 5V on the arduino pins. It's probably a good idea to test the circuit with a voltage below 5V, so
you don't damage the arduino in case the voltage divider on A1 was wired incorrectly.

The arduino code was based on this excellent article:

http://meettechniek.info/embedded/arduino-analog.html

Good luck!
Bram