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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Mahaya
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
its very interesting though,,but can i asked what kind of transistor use in this diagram.thanks and more power!
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.
if we add a PNP with another string of LEDs,but how to determine the flashing rate?Suppose you require 2 flashes/second?
what color of the six LED to make that circuit?
How it works in 70 led string…????