Light is one of the most fundamental parts of our experience of the world. Without it our main sensor, our eyes, would not function. In this post, I want to explore various ways of creating light form the ground up. Light emitting diodes (LEDs) is where I will start. LEDs emit light proportional to the current flowing through them. Depending on the color, like other diodes, the voltage drop over them is exponential in the current according to

$$V = V_T ln(I / I_s)$$

Usually the forward voltage of a LED is given at its operating current. The forward voltage is different for different LED colors.

The simplest way to drive a LED is to connect a given voltage \(V_i\) over a current-limiting resistor R and the LED in series as shown in the figure below.

To calculate the resistance of R we subtract the forward voltage of the LED from the voltage of the power source and divide by the current we want to flow through the LED:

$$ R = \tfrac{V_i - V_LED }{I_{LED}}$$

The resistor now limits the current through the LED by burning the remainder of the energy that is not emitted as light (and heat) in the LED.

Better ways to generate light are with for example a transistor current source.