Terminal Information

Project: Making a Simple LED Circuit

What is an LED?

A Light Emitting Diode

LED stands for Light Emitting Diode. A diode only allows current to flow though it in one direction. There is a wide range of diodes with a number of versatile functions and abilities. A Light Emitting Diode is one that also emits light when passing a current though it. Today they are quite common and can be found in items like the household lightbulb, or specialized items like traffic lights.

Turning it on

You can think of a diode sort of like a one way door for current. Pushing on one side will open it, but pushing from the other side wont. Unless of course you push really hard from the other side and you break it. This backwards breaking limit is called the Reverse Voltage Limit.

Also like our one way door, a diode requires a little push before it will open and allow current to pass though. This "little push" is called the Forward Voltage. This limit basically tells us at what voltage our circuit needs to be before the LED will turn on and open for the current to pass though. Below this there is still some "seepage" of current, but it is negligible and doesn't really need to be considered for our purposes.

You should be able to find the forward voltage limit in the documentation from your vender. Sometimes it can be with the product information but often it will be found on a document from the manufacturer called the data sheet or spec sheet. Sometimes, due to the inaccuracies of mass manufacturing, the forward voltage limit can be given as a range instead of a single set value. In this case you may need to make multiple calculations for how those upper and lower limits may affect your circuit.

The diode will resist any attempts to have its voltage increased beyond its forward voltage limit. Small increases in the voltage to the diode create drastic increases to the current that passes though. More current passing though the LED will make it brighter, but too much will burn it out! The current needs to be limited to prevent destroying the LED. This limit should also be provided by the vendor or on the data sheet from the manufacturer. It should be called the forward current or current limit.

Limiting the Current

The simplest way to limit the current though the LED is with a resistor. We can use the relationship in Ohm's Law to figure out how many ohms our resister needs to be. We just need to replace the variables with our our own limits and values, then solve for the resistance.

Ohm's Law

V = I × R

Let’s start with the voltage. We don't use the entirety of the power supply’s voltage when calculating the value the resistor, just the amount that exceeds our forward voltage threshold. You can think of this current limiting resistor kind of like a weir. A weir is a sort of underwater damn that constricts only part of the water’s flow in order to control it. The amount of voltage we need to limit can be calculated simply by subtracting the forward voltage of the LED from the voltage of your power supply.

Limiting the current for a LED circuit can be thought of a little like this diagram of the reservoir and weir. Just like the weir only needs to account for the depth of its stream and not the full depth of the reservoir, the resistor value doesn't need to account for the full voltage of the supply, just what is left over after the LED's forward voltage.
(Power_Supply_Votage - Forward_voltage) = I × R

Next add in the current we want to pass through our LED. You can figure this out from your forward Current or current limit. You won't want to use the exact given limit though, this is the threshold of what the LED can take before it starts to burn out. You will want to use a little less than this to create a little bit of a buffer as real world components can vary a little from the ideals of our calculations.

(power_supply_voltage - forward_voltage) = (forward_current - safety _buffer) × R

Now that we have all the information we know added into the equation, we just have to solve for the resistance. To do this we just need to divide both sides by the current.

(power_supply_voltage - forward_voltage)
(forward_current - safety_buffer)
= R

Now that we have the equation for the resistance, we just have to plug in our values and solve.

Tutorial

Materials

Before we begin, we will need to select and gather the different components needed to create the LED circuit. These include a power supply, the led(s), the resistors, wires, and something to connect them all together.

Wires/Connectors

Usually a circuit is soldered together using solder and a soldiering iron. This, however, can be dangerous and intimidating to beginners. Instead of solder, in this tutorial we will be using mostly female jumper wires as are easy to get and can be connected by simply plug a pin or wire directly into their connector. The jumper wires do restrict you to using a single strand or "solid core" wire (wire made of a single wire and not multiple strands) since it needs to be thin and rigid enough to be plugged into the connector.

Male (left) and female (right) jumper cables

You can make your own jumper cables using wire, Dupont Male and Female Connectors, and a Crimping Tool. You may even need to do this like I did for my own power supply, since its connecting wires were made from multiple strands.

A crimping kit (left) was used to attach jumper connectors to the power supply (right)

Power Supply

This power supply uses two AA batteries

I have selected a power supply that runs on two AA batteries. Each battery is 1.5 volts so the total voltage of the supply should be 3 volts. However, measuring the power supply we see that it is 3.18 volts! This is because I used two new AA batteries which are typically a little over 1.5 volts. You should always account for variances such as this and add in a buffer when making your calculations. I will consider it as a 3.3volt power supply, as future replacement batteries may vary even more.

LED

The red through-hole LED

LEDs come in many different shapes and sizes. I have selected a red, through-hole LED to use. Through-hole simply means it has small pins or wires that are meant to be placed through and soldered to a hole in a circuit board. These will be useful for us since they can be plugged into our jumper wires to connect them without soldering.

From the manufacturer I can see that my LED needs between 1.9-2.1 volts for its forward voltage. In my calculations I will use the lower limit of 1.9 volts to be on the safe side. If you do not have the information for the forward voltage from the manufacturer, you can use the table below as a general rule of thumb for the forward voltage by LED color.

Typical LED Forward Voltage by Color
COLOR TYPICAL VOLTAGE
RED 1.8v
ORANGE 2.0v
YELLOW 2.1v
GREEN 2.1v
BLUE 3.0v
WHITE 3.0v

We also need the forward current for our LED. The vendor of my LEDs posted that it is 20mA (0.02 Amps). If you can't find the forward current for your LED you can use 20mA as a general rule of thumb for through-hole LEDs. To be on the safe side I will use a safety buffer of 2mA for my calculations giving me a current of 18mA.

Resistor

Now that we have all of the values for our components, we can calculate the resistance value we need for our resistor. First start with the equation to find the resistance:

R =
(power_supply_voltage - forward_voltage)
(forward_current - safety_buffer)

Next, fill in all of the information that we have:

R =
(3.3 - 1.9)
(0.02 - 0.002)

Now we just need to solve the equation to find the value we need for our resistor:

R =
(3.3 - 1.9)
(0.02 - 0.002)
⇓
R =
1.4
0.018
⇓
R = 77.7Ω

Selecting a Resistor

Usually resistors will come at different set levels and we won't find the exact resistance we need. You can get around this by adding different resistors in series, or one after another. When you do this the resistance adds up and acts like one resistor. So I could select a 75Ω resistor and a 3Ω resistor to get a 78Ω resistor.

The closest resistors have values of 75Ω and 100Ω. Of these I selected the 75Ω resistors.

My resistors jump from 75Ω to 100Ω. Since I used the worst case limits in my calculations I will simply use the 75Ω resistors as it will still likely be under the 20mA limit. In fact we can use Ohm's law to find what the current will be using the 75Ω resistors:

1.4
75
= 0.0186Amps or 18.6mA

We can see that 18.6mA is still under our current limit of 20mA, so we should be ok using the 75Ω resistor.

Resistor Color Code Values

Most resistors do not have their resistance value written out on them. Instead they use colored bands and a color code to show their values. Each band of color represents a digit in the number of its value. The last band is a "multiplier", or essentially how many zeros come after the given digits. They have a final band separated from the first group to represent the error, or possible inaccuracy of the given value.

If your resistors get separated from the packaging that tells what their resistance values are, you can use the table below to identify their resistance values from their color code.

Resistor Band Value Code
Color Digit Ω Multiplier Tolerance
Black 0 1 Ω 100
Brown 1 10 Ω 101 ±1%
Red 2 100 Ω 102 ±2%
Orange 3 1K Ω 103
Yellow 4 10K Ω 104
Green 5 100K Ω 105 ±0.5%
Blue 6 1M Ω 106 ±0.25%
Violet 7 10M Ω 107 ±0.1%
Grey 8 100M Ω 108 ±0.05%
White 9 1G Ω 109
Gold 0.1 Ω 10-1 ±5%
Silver 0.01 Ω 10-2 ±10%

Example: 2570KΩ Resistor

Assembly

Now that we have all our components, we can begin to assemble our LED circuit. As we are using jumper wires, the circuit goes together and can disassemble fairly easily. Unfortunately it can sometimes come apart when we don't want it to, so you may consider placing a small bit of tape on the junctions to help hold it together if you want to use it for something else.


A small piece of tape can help to hold connectors together

First I will begin by attaching a female to female jumper cable to my resistor. It doesn't matter in the circuit which side of the LED that the resistor is on, but in my circuit I will be placing it in front of the positive side of the LED.


Step 1: Attach the resistor to the jumper cable

As I want to attach the resistor to the positive pin of the LED I need to identify which side is positive. The positive side of the LED is called the Anode. The negative side is called the Cathode. On a new LED you will notice that one pin is longer than the other. It is the convention that the anode is the longer pin and the cathode is the shorter one.

If you're not sure if your LED has been trimmed and you want another way to check, you can look close inside of it to find out. As it is clear to let light out, you can also look in to see what the anode and cathode attach to. You will notice one side is larger than the other. The smaller side is called the Post, and is usually shaped a bit like a chair or a nose. This is the anode.

The other side is called the Anvil and is shaped, well, a bit like an anvil with a cup in the top of it. This is the cathode. The cup on top is where the LED produces the light.

Diagram of a LED

Now that we've identified the anode, we can plug it into the jumper cable attached to the resistor.

Step 2: Attach the anode to the jumper cable

Next we can attach another cable to the other side of the resistor, as well as another cable to the other side of the LED. These cables will attach to the power supply, so depending on what kind of connector it has you may have to mix and match your jumper cables. My power supply uses a female connector, so I am using a male to female connector.



Step 3: Attach jumper cables to the other ends of the resistor and LED

A Note On Power Safety:

While the power levels we are working with are pretty safe (just a little over 3 volts for me), it is still important to practice good safety procedure. Even a single battery under the right conditions can be dangerous. There is even a survival technique to start a fire using a single battery and foiled gum wrapper.

Always make sure power is off when working with electronics!

The danger from these low voltages comes mainly from shorting the power supply, or connecting to the positive terminal directly to the negative terminal with little or no resistance in between. If you look again to Ohm's Law, you notice that with little or almost no resistance, our current becomes huge. That much current tends to heat things up fast! Not only is this not good for the power supply, but it can lead to burns or even start a fire under the right conditions.

Make sure to follow these safety procedures:

  • If your power supply has a switch, make sure it is in the OFF position.
  • If your power supply has batteries, make sure they are removed until the circuit is complete.
  • If you power supply runs on wall power, make sure it is unplugged until the circuit is complete.

Final assembly

We can now attach the jumper cables to the power supply. Since I attached my resistor to the anode of my LED, I will plug that wire into the positive terminal of the power supply, and the other wire to the negative terminal. If you are unsure which is which on your power supply, you can usually identify them by the wire colors. Red Wires are usually used for the positive terminal. Black Wires are usually used for the negative terminal.




Step 4: Attach the resistor's cable to negative and LED's cable to positive terminals of the power supply

Now that the circuit is assembled we can install our batteries and turn on the power supply. The LED should light up!


The circuit is now finished. The LED should light up when powered on!

Trouble shooting

LED Does Not Light Up:
  • The LED may be backwards or attached to the terminals backwards.
    • Try switching the wires attached to the LED.
  • Your batteries may be low or dead.
    • If you have a battery checker, check their power levels to see if they are good.
    • Try replacing them with new batteries.
  • The LED may be defective.
    • If you have more than one LED, try switching it out for another one.
  • The Resistor may be too big.
    • Check your calculations and that the values you have for your components are correct.
    • Try using a slightly lower value for your resistor and see if it lights or even lights dimly.
LED Only Lights Dimly
  • Your batteries may be low.
    • If you have a battery checker, check their power levels to see if they are good.
    • Try replacing them with new batteries.
  • The Resistor may be too big.
    • Check your calculations and that the values you have for your components are correct.
    • Try using a slightly lower value for your resistor and see if it lights or even lights dimly.
LED Lights Brightly, then Dies, or Immediately Dies

Note: If your LED lights brightly and then stops, it has likely been burned out and is dead. You will need to replace it with another one.

  • The Resistor may be too small.
    • Check your calculations and that the values you have for your components are correct.
    • Try using a higher value for your resistor and see if it lights and does not burn out.