# How to connect LED's directly to MAINS

Posted on Aug 16, 2012

A LED is defined as `light-emitting diode: diode such that light emitted at a p-n junction is proportional to the bias current; color depends on the material used` [Princeton University]. As the LED is a diode, it doesn't conduct electricity in both ways and it is not a resistor, so it needs an additional resistor. The value of the resistor is defined by: R = (Circuit Voltage - LED Voltage) / LED current. That means, If I need to use a LED connected to 110VAC and the led is a Red one, it needs only 1.8 Volts with 25mA then the formula will be: R = (110 - 1.8) / 0.025. R = 108.2 / 0.025. R = 4,328. In theory, A 4.7k resistor is enough to connect a LED to 110VAC, but, to be honest, I never follow formulas and I go with the experience that I have, so here is my schematic to connect a LED to 110VAC and some interesting applications.

The first circuit shows a single LED using 110VAC. I used this circuit to detect if voltaje is present at the AC outlet. A reversed diode is needed because the LED have a limit on reversed voltage. The second circuit labeled as "Night light" is basically the same circuit but it uses two LEDs. I use it to create a soft light in my room and walls to avoid accidents when I walk around the house during the night. By the way: I don't walk sleeping. Also, it can be used to know if an unknown voltage is AC or DC. AC voltages will lit both LEDs. The third circuit is used in my power supply so I can detect when I need another fuse. The last circuit, the "LED light" is an experiment to illuminate my room with LEDs instead of fluorescent light or light bulbs. It works fine as a 13 watts fluorescent light but the light emitted is not even. Anyway, it is perfect to save electricity.

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