Circuit Essentials - LED's

Light up your projects! This week's Circuit Essentials reviews the fundamentals of Light Emitting Diodes, or LED's. By the end, you should be confident you can make that random LED from the bottom of the component drawer do what you want it to.


Polarity

Diodes, and Light Emitting Diodes, are polarised components, meaning that there is a right and wrong way to connect them in your circuit. One side must be connected to positive (higher potential), and the other side to negative (lower potential).


Keep an eye out for the short leg, and the flat side of LED to identify the negative (cathode) terminal of the LED.



Forward Voltage

LED's have a forward voltage, which is the minimum voltage required to get current to flow in the component. Since LED brightness is directly related to the current flow, it's important that your supply voltage exceeds the forward voltage of the LED. Think of it as the "turn on" voltage.


Some common forward voltages for different coloured LED's are (source):


Red: 1.6-2.0V

Orange: 2.0-2.1V

Yellow: 2.1-2.2V

Green: 1.9-4V

Blue: 2.5-3.7V


The forward voltage can be found in the datasheet, or most multimeters have a diode mode allowing measurement of the forward voltage.


Current Limiting Resistor

LED brightness is based on the amount of current flowing through it - more current means more light. Except, too much current and the LED will burn out. Hence, a current limiting resistor must be used. Again, check the datasheet for the typical operating current, though 10-20mA is typical.


The value of the current limiting resistor can be found using Ohms law. Effectively, we're finding the voltage drop across the resistor, (KVL tells us this is the source voltage minus the LED forward voltage), and dividing by the desired diode current, to give the resistance.


So for example, for a typical red LED, with a forward voltage of 1.8V, a desired current of 10mA, and a source voltage of 5V, we would choose a resistance of (5-1.8)/.01=320 Ohms. Pretty simple, right?


PWM

An alternate way to modify the brightness of the resistor is to turn it on and off very fast! For this, we use something called pulse width modulation, or PWM. This is a square wave generated by a microcontroller or IC, that alternates between on and off very quickly. The longer the "on" cycle compared to the "off" cycle, the brighter the LED will appear. This all happens faster than the eye can resolve, and thus just appears at varying levels of brightness, and not a flickering light.


Fancier LED's

Once you've mastered a single LED, these fundamentals can be extended to multiple LED's (should ee connect them in series or parallel?), RGB LED's, or addressable LED's such as neopixels. RGB all the things!





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