LED circuit -Lighting a Light Emitting Diode

Lighting an LED is a simple circuit that teaches a lot about electronics.

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LED basics

Indicator LED Anode and Cathode schematic symbole illustrated diagram by electronzapdotcom of youtube and electronzap
Indicator LED Anode and Cathode schematic symbole illustrated diagram by electronzapdotcom of youtube and electronzap

  • LEDs need to be inserted in the right direction to light up. This is called the forward biased (FB) direction. That’s when the Anode is more positive than the Cathode.
  • Reverse biased (RB) is when an LED Anode is more negative than it’s Cathode. It won’t light up, and somewhere above 9V (whatever it’s breakdown voltage is) the LED will be forced to conduct while RB, and it will be destroyed.
  • You still need a certain amount of voltage, called forward voltage, while the LED is forward biased before it will conduct and light up. Commonly this is a minimum of 1.5V to 2.5V depending on color.

Basic properties of an LED protected by a resistor circuit fragment for learning electronics

Circuit on a prototype breadboard:

Prototype circuit boards (commonly called breadboards) let you quickly put together and take apart temporary circuits to test out and learn more about them.

Resistor protecting LED from 9V battery with battery snap on a breadboard drawing by Electronzap Electronzapdotcom
Resistor protecting LED from 9V battery with battery snap on a breadboard drawing by Electronzap Electronzapdotcom

Here is how you could wire an LED in series with a 470Ω (four hundred and seventy ohm) resistor on order to protect the LED from a 9V power source. The 9V battery has wires in the drawing thanks to a 9V battery snap attached to the battery.

  • Usually the LED cathode will be connected directly to the negative side of the power source. The negative side of a power source is usually called ground, and can also be called the zero volt reference point.
  • Limit current (usually with a series resistor) to no more than 20mA for most LEDs. You can go as low in current as you want by using higher value resistors. The LED will just be less bright at lower current. The resistor is most often connected between the LED Anode and the positive side of the power supply. 470Ω is a good minimum value to protect an LED from 9V, and to prevent the resistor from overheating. Most resistors are rated for 1/4 watt maximum wattage rating, which should still be limited to no more than 1/8W.
Current limiting resistor to protect LED basics diagram by electronzap electronzapdotcom
Current limiting resistor to protect LED basics diagram by electronzap electronzapdotcom

5V Breadboard power supply

Modules that snap onto a breadboard in order to provide power are typically called breadboard power supplies. They often come in electronic component kits, but they also need to have a 9V adapter with a barrel plug plugged into them, which isn’t usually included in kits. The adapter plugs into the 120V AC outlet (in the US) and outputs 9V DC.

Breadboard power supply module illustrated by electronzap electronzapdotcom
Breadboard power supply module illustrated by electronzap electronzapdotcom

You can use 220Ω or more to protect an LED from 5V.

Simple LED circuit Schematics
Resistor rectifier diode LED battery single pole single throw and push button switch schematic symbols by electronzap electronzapdotcom
Resistor rectifier diode LED battery single pole single throw and push button switch schematic symbols by electronzap electronzapdotcom

Circuits are almost always explained through symbols instead of pictorials. Here are three common ways that a simple LED circuit with a 9V power supply of some kind may be drawn. A battery symbol for a power source does not mean that a battery has to be used. Any power source of the proper voltage can be used.

Three common ways to show the same simple LED protected by a resistor from 9V schematic diagram by electronzap electronzapdotcom
Three common ways to show the same simple LED protected by a resistor from 9V schematic diagram by electronzap electronzapdotcom

The schematics often include extra information for educational purposes. Below is a lot more than you will usually find, but is good information about the circuits to know.

Voltage across LED and protective resistor schematic diagram by electronzap
Voltage across LED and protective resistor schematic diagram by electronzap
  • A red LED being protected by 5V with a 220Ω 1/4W (one quarter watt maximum) resistor will have approximately 13.6mA of current flowing through the circuit. The resistor will be dissipating approx. 0.041W of power, which is well below the recommended 1/8W (0.125W) limit for a 1/4W rated maximum resistor.
  • Red LED with a forward voltage of about 2 volts, being protected by a 470Ω resistor from 9 volts will have about 15mA of current flowing through the circuit. The resistor will need to dissipate about 0.104W of heat. So it will be pretty hot, but still a bit below the recommended 0.125W limit of a maximum 0.25W resistor.

Note:

  • Series components all have the same amount of current flowing through them.
  • Wattage/power (P) is voltage (V) in volts across a component times current(I) in amps through it. P=VI
  • LEDs have a maximum current rating instead of a power rating due to their forward voltage being relatively stable.
  • Resistors have a wattage rating because they don’t limit voltage across them.



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Other topics:

555 timer is an integrated circuit (IC). Being an IC, it has complex circuitry combined in a single package with external pins/terminals to connect to other circuitry. You can easily make all kinds of fun circuits with just a 555 timer and the components covered above, so I think it’s a good component to learn next.

Transistors will probably be the most challenging components to learn. Understanding them will help you understand all of electronics much better, and help you the most in being creative while designing your own circuits.

Other topics:

These pages are still being compiled.

Circuits covered more quickly series:


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