Designing a simple circuit that lights up a typical LED is not very hard. To light it up you need to pass about 20 mA of DC current through the LED. from either a battery or some other DC source that is higher than the LED's voltage is needed. For instance, if the LED voltage is rated at 2.1V then a 5 V supply would do nicely. A resistor is connected in series with the LED to limit the current to about 20 mA. The value of this resistor can be calculated by subtracting the LED voltage from theand then dividing this result by 0.020. The closest to the result can then be used. Make sure the polarity of the LED is correct and when you connect it all the LED should shine brightly. If you want it dimmer then use a higher resistance for the resistor because that will reduce the current through the LED.