How to Work With Unknown LEDs
LED stands for "light emitting diode." It is a semiconductor designed to emit light at many different frequencies, such as red or yellow or green. LEDs may be purchased individually, or in bulk. They may also be salvaged from circuit boards, by desoldering.
An LED's voltage is usually found on the package it came in, or on the manufacturer's site. If this information is not provided, you may assume values that are 1.7 to 2 volts. The actual value can be found when the LED is lit. When using different voltages, assume an operating current of 15 to 20 milliamps, which is normally safe for an unknown LED.
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Things You'll Need
- LED
- 9-volt battery
- Battery holder
- Multimeter
- 1k resistor
- Solderless breadboard
- Jumper wires
Instructions
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Connect the battery, resistor and LED together in a series circuit. Do this by placing the battery holder and resistor into the circuit board. Using jumper wires, connect one end of the resistor to the battery holder's red lead. Connect the resistor's free end to the positive lead of the LED. Wire the negative side of the LED to the black lead of the battery holder, which is ground.
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Insert the battery into the battery holder. When this is done, the LED will light.
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3
Turn the multimeter onto a DC voltage setting of 10 volts or more. Measure the LED's voltage drop by placing the multimeter's red lead on the positive side of the LED, and the black lead on the LED's negative side. A representative reading is 1.9 volts.
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Use the measured voltage to find the value of other resistances with either an LED calculator or, alternatively, Ohm's Law. For example, use Ohm's Law with a battery source of 9 volts, a 1.9-volt LED and a targeted current of 20 milliamps. The minimum value for the resistance R is (Voltage Source -- LED Voltage) / LED current = (9v -- 1.9v) / 0.020 A = 355 ohms, which means that the standard resistor value of 390 ohms should be used.
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Tips & Warnings
Blue or white LEDs are usually 4 volts. The voltage source must always be much higher than the LED voltage for stable operation. High-voltage LEDs, such as 5 or 12 volts, normally do not need current limiting resistors.
To observe different ranges of behavior, a variable resistor equal to or above the calculated minimum resistance may also be used.
Never use a lower resistance than the minimum one calculated. Always choose a standard value that is the same or higher.
References
Resources
- Photo Credit led image by Albert Lozano from Fotolia.com
