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    This simple and cheap circuit is not just for Christmas! There are just two resistors, a small-signal transistor such as a BC547, one ‘flashing’ LED and a string of ‘normal’ LEDs. The flashing LED works as an oscillator and switches the transistor on and off; and the transistor switches all the other LEDs. An (unregulated) 12 V mains supply can be used for power.

    No current-limiting resistor is required in the LED chain, because the forward voltages of the LEDs in the chain add up to the supply voltage. If red LEDs are used, with a voltage drop of 1.65 V, then 12 V will supply seven; alternatively, use six yellow (2.1 V each) or five green (2.7 V). You can of course always mix the colours.

    Flashing LEDs circuit diagram

    Fairy lights Variation:
    Alongside the NPN transistor add a PNP transistor with its emitter connected to +12 V, with another string of LEDs connected down to ground. The two strings will flash alternately.

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    9 Responses to "Fairy Flashing LED Lights"

    1. Shivam pradhan says: on June 16, 2010 at 5:21 pm

      Its work really awesome if you use 2 9v battery and in every 15 put them in sunlight for 1/2 day and it will recharge

    2. its very interesting though,,but can i asked what kind of transistor use in this diagram.thanks and more power!

      • Kerry Keel says: on December 7, 2011 at 3:56 pm

        The transistor is a NPN type. Use a 2N3055 for best power capacity. A TIP30 would also work. The big issues for the transistor is that it has the current capability to handle -all- of the current of all of the diodes being used. A x5 to x10 multiplication factor of current capacity against the total current draw of all of the diodes is a good idea in terms of longevity of the transistor. Having a current capability to close to the total current draw of al of the devices puts the transistor in danger of failure.

    3. if we add a PNP with another string of LEDs,but how to determine the flashing rate?Suppose you require 2 flashes/second?

    4. what color of the six LED to make that circuit?

    5. How it works in 70 led string…????

    6. what if we put 6 more leds in parallel to the string wil it work same as these 6 ones or not

    7. So, if I replace R1 with a 718 ohm resistance (100 + 620 ) it shoul work on 9v battery?

    8. Replacing the 1k resistor with anything from 620 to 720 is fine for conversion of this circuit to 9v. the LED string will need to total about 9v.

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