I decided to create I–V curve of leds. But I have only one multimeter, so I created simple I-V meter with Arduino Uno.

From Wiki: A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or potential difference across it.

Step 1: List of Materials

For this project, you will need:

Arduino Uno with USB cable

breadboard and duponts cable

leds (I used 5 mm red and blue leds)

drop resistor (shunt resistor) - I decided for 200 ohm (for 5V is maximum current 25 mA)

resistors or potenciometer, I use mix of resistors - 100k, 50k, 20k, 10k, 5k, 2.2k, 1k, 500k

Step 2: Circuit

Circuit consist from testing led, shunt resistor (R_drop) for measure current. To change voltage drop and current I use various resistors (R_x).

Basic principle is:

get total current I in circuit
get voltage drop on testing led Ul

Total current I

To get total current, I measure voltage drop Ur on shunt resistor. I use analog pins for that. I measure voltage:

U1 between GND and A0
U2 between GND and A2

Different of this voltages is equal voltage drop on shunt resistor: Ur = U2-U1.

Total current I is: I = Ur/R_drop = Ur/250

Voltage drop Ul

To get voltage drop on led, I substract U2 from total voltage U (which should be 5V): Ul = U - U2

Step 3: Code

float U = 4980; // voltage between GND and arduino VCC in mV = total voltage

float U1=0; // 1 probe

float U2=0; // 2 probe

float Ur=0; // voltage drop on shunt resistor

float Ul=0; // voltage drop on led

float I =0; // total current in circuit

float R_drop=200; // resistance of shut resistor

void setup()

{

Serial.begin(9600);

pinMode(A0, INPUT);

pinMode(A1, INPUT);

}

void loop()

{

U1 = float(analogRead(A0))/1023*U; // get voltage between GND and A0 in milliVolts

U2 = float(analogRead(A1))/1023*U; // get voltage between GND and A1 in milliVolts

Ur=U2-U1; // drop voltage on shunt resistor

I=Ur/R_drop*1000; // total current in microAmps

Ul=U-U2; // voltage drop on led

Serial.print("1 ");

Serial.print(U1);

Serial.print(" 2 ");

Serial.print(U2);

Serial.print(" //// ");

Serial.print(" voltage drop on shunt resistor: ");

Serial.print(Ur);

Serial.print(" voltage drop on led: ");

Serial.print(Ul);

Serial.print(" total current: ");

Serial.println(I);

// pause

delay(500);

}

Step 4: Testing

I testing 2 leds, red and blue. As you can see, blue led has knee voltage bigger, and that´s why blue led need blue led start to blow around 3 Volts.

Step 5: Testing Resistor

I do I - V curve for resistor. As you can see, graph is linear. Graphs show, that Ohm´s law work only for resistors, not for leds. I calculate resistance, R = U/I. Measurements are not precise at low currents value, because analog - digital converter in Arduino has resolution:

5V / 1024 = 4.8 mV and current -> 19.2 microAmps.

I think measurement errors are: