LEDs, or Light Emitting Diodes, turn electrical current into light. They can create light in many different colors, including colors our eye can't see. The more current an LED takes, the more light it creates.
Inside an LED are two types of crystals, called N type and P type semiconductors. N types have a crystal structure with extra electrons, and P types have "electron holes", places in their crystal where electrons can fit. Usually, electrons can only flow from the N type to the P type. A diode is made by placing an N and P type semiconductor together.
When an electrons jump from the negative N type semiconductor to the positive P type semiconductor, it lets off a fixed amount of energy. This fixed amount of energy is emitted from the LED as a photon, a particle of light. The LED in our theremin kit creates green light.
Because the electron needs a certain amount of energy to jump from the N to P type semiconductor, LEDs need a certain amount of voltage to turn on. At low voltages, no or almost no light is created. But when the LED reaches its threshold voltage, it lights brightly. A small increase in voltage creates an exponential increase in current flowing and light created.
To keep the LED from burning itself out, a current limiting resistor is used. By itself, the LED would draw exponentially more current from a small increase in voltage. But because the current that goes through the LED also goes through the resistor, the total current is reduced.
An LED’s threshold voltage depends on what color it is designed to emit. A green LED might need 2.1 volts. But because blue photons have more energy, blue LEDs need 3 volts. LEDs only work when current flows in the correct direction. If you install an LED backwards, it won’t turn on.