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October 25, 2018 12:18

Matching a red LED to a cr2012 battery to create a terminator light on sunglasses

I am trying to create a red light behing a lense on sunglassed for a terminator project,

so I need to match the voltage from a red LED to a small battery possibly a cr2032,

because of it being near an eye, safety is really important,

would i neeed a resistor?

and what would be the best way to wire it?


thanks for your help



November 1, 2018 09:14

Forward voltage (Vf) specifications for red LEDs are usually somewhere between 1.6 to 2.8 V, with 2.2 volts being the most common. A CR2032 battery puts out 3 V. So yes, you need a resistor in series to reduce the voltage to the LED.

You can find the Vf specification on the manufacturer's data sheet for the diode.
To calculate an appropriate resistor, you use the formula E = I times R, where E is the voltage that you need to drop (the battery voltage minus the diode Vf), I is the current through the resistor in Amperes, and R is the value of the resistor in ohms.
The resistor will be in series with the LED, and the current is the same in all parts of the series circuit. The current specification for indicator LEDs is usually 10 or 20 milliamps (0.010 to 0.020 amps). The formula can be rearranged to R = E / I.
Plugging in the 3 V battery voltage, 2.2 V LED Vf, and current of 0.010 (a conservative value), we get R = (3 - 2.2 volts) / 0.010 amps, or R = 0.8 / 0.010, so R should be at least 80 ohms. Looking at standard resistor values, 5% tolerance resistors are available in 82 ohms. 80.6 is a standard value for 1% tolerance resistors. Either of those would be fine. The resistor needs to be rated for the power it will be handling. The worst case situation would be if the LED has shorted. If that happened, the full three volts would be across the resistor, and the power would be V * V / R.
That is 3 * 3 / 80, or .1117 Watts, which is less than 1/8 Watt. So a 1/8 or 1/4 Watt resistor should be fine. You can use a battery holder like TE Connectivity part number 120591-1. For safety, make sure the LED is pointed away from the eye, and that the entire unit can be quickly and easily removed.

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