Electronics

• LEDs
  • Resistance
  • Wiring schematics
  • Pinout
• Potentiometers
• References

LEDs

The forward current of a LED depends on its color. The regular cheap 3-10 mm LED diodes range from \( 1.8-2.2 V \) (red, orange, yellow, etc) to \( 3.2-3.8 V \) (white, green, warm white, etc). They have typical a forward current of \( 20 mA \), and a maximum forward current of \( 30 mA \)¹ (for reference, a GPIO pin of an ESP32 provides up to ca max \( 40 mA \)).

Resistance

Since these LEDs all generally have consistent forward currents and forward voltages, we can just use Ohms law and work out a reference table for common supply voltages.

The resistor values are assume the typical forward current of 20 mA.

The voltage across the circuit is equal to the sum of the voltage across the components in the circuit, the resistor and the LED.

So for an LED witha forwar d voltage \( V_{f} = 1.8V \) that is connected to a supply voltage \( V_{s} = 3.3V \), the voltage across the resistor is \( V = V_{s} - V_{f} = 1.5V \).

>>> I = 0.02     # 20 mA == 0.02 A
>>> Vs = 3.3
>>> Vf = 1.8
>>> V = Vs - Vf
>>> V/I          # R = V/I
75               # 75 Ohm

So to connect LED with a forward voltage of 1.8 V to 3.3 V, it should be protected by a 75 Ω resistor.

Wiring schematics

Schematics
Multiple LEDs connected in series.
Proper way to connect multiple LEDs in paralellel, each LED has its own current limiting resistor.

Pinout

LED
LED pinout2.
Anatomy of a 5mm green LED3

• The cathode leg (\(-\) or \(K\)) is the shorter leg (usually \( 17 mm \)).
• The anode leg (\(+\) or \(A\)) is the longer leg (usually \(19 mm\)).

RBG LED
RGB LED pinout4

Potentiometers

Potentiometer
Pinout of a regular potentiometer.

References