- 1). 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.
- 2). Insert the battery into the battery holder. When this is done, the LED will light.
- 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.
- 4). 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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