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How to Design a Simple LED Circuit
time2011/09/16
1.The first step is to choose how many LED's you want to light and their type (color, brightness). With this simple circuit you will be somewhat limited by the amount of voltage that can be safely provided for your project
2.Calculate the amount of voltage you need from your power supply.
Look for the "diode forward voltage" specification from the manufacture's website or packaging. If you do not have this information you can use the following to estimate:
Red or Orange 2.0 V
Yellow 2.1 V
Green 2.2 V
True Green, Blue, White 3.3 V
Blue (430 nm) 4.6 V
Say you want to have 3 Red LED's in your circuit. Only use the diode forward voltage value. So, 3 x 2.0 = 6V, needed.
3.Once you have the required voltage, choose how you want to power the circuit. Do you want to plug it into an outlet and not have to change batteries or do you want the freedom to move around?
DC voltage works easiest. AA, AAA, or 9V batteries are cheap and easy to find for a simple project. The simplest way to use AC as your source is a wall plug (wall wart), the large black plugs that come with most every electronic device you purchase. You probably have a box full of these sitting around. You will have to cut the plug off the end of the cord. Use a multimeter to determine the positive and negative lead.
AC voltage can also be designed in the circuit. AC will only drive the LED half of the time since the voltage travels in waves. A full-wave bridge rectifier can be used to fully power LED's. This is essentially what the wall wart is doing for you.
Find a power supply greater than the calculated diode forward voltage value from step 2. In our case we calculated 6 volts. A power supply value greater than 6 volts will be needed. If you are driving a large number of LED's, current may also be important.
4.Calculate the required LED resistor value.
LED's cannot be connected directly to the battery or power supply. The LED will be instantly destroyed because the current is too great. The current must be reduced. The easiest way to do this is by using a resistor. Calculate the LED resistor value with the following formula:
LED Resistor Value, R=(supply voltage - LED voltage) / LED current
In our example:
Say we use a 9V battery, then supply voltage = 9V.
LED voltage for red LED's, from Step 2 is 2.0 V
LED current is 20 mA (this is a typical value if not provided by the manufacturer)
If the resistor value is not available, then choose the nearest standard resistor value which is greater. If you want to increase the battery life you can select a higher resistor value to reduce current. The reduced current will result in a dimmer LED.
R = (9 - 2.0) / 20 mA = 350 ohms, use the next higher standard value = 360 ohms
5.Wire your circuit together.
You can soldier the wires directly together, use crimp connectors, or use a small circuit board. Choose the best method based on the size of your project.
6.The final step is to mount the LED's in your project. Radio Shack and others sell chrome or plastic LED holders that make a professional looking mount easy. You can add momentary push buttons or on/off switches to your LED circuits.
Designing and building a simple LED circuit is an easy project.