This is my first project on EasyEDA. It is a simple circuit with a Led and 390-ohm Resistor and a 9V battery. The resistor R is calculated with the formula: R=(Vcc-Vd)/I=(9-1.7)/0.020=365 Ohm The commercial resistor with a value nearest to 365-ohm is 390-ohm. Here some insights on how I designed this circuit: 1. I assumed a Led absorb 20mA; 2. the junction voltage on led terminals is around 1.7V; 3. you always should connect a resistor in series to the led to avoid to burn it out; 4. the resistor value can be calculated with the formula above. On the web, you can find that the junction voltage on the terminal depends on the led color according to this table: * Red led: 1.8V * Yellow led: 1.9V * Green led: 2V * Blue led: 3.5 See here for details: https://it.wikipedia.org/wiki/LED However, when I tested the circuit with real components I found out that practice is a bit different from the theory: 1. the led absorb almost 16mA; 2. there are no particular differences among the different led colors (I tested a red, green, yellow led); 3. the junction voltage was around 2.12V; 4. My 9V battery was 9.76V and not 9V. It is normal that real value are a bit different from the declared one. In general, real tests are always a bit different from the theory and you should always consider this.