This circuit is innovative in more than one way and therefore belongs per se in Elektor’s Small Circuits Collection. Firstly, it demonstrates how the combination of a blue and a white LED can be used to give a realistic imitation of a camera flashlight. Secondly, the good old 555 IC is used in a way many of you may never have seen before — alternately monostable / astable — without too much in the way of external parts. Initially C3 will be empty, pulling output pin 3 to +12 V and causing the blue LED, D1, to light via R3. Next, C3 will charge up via R2. Meanwhile C1 has been building up charge through R1 and D3.
If the voltage on C3 reaches about 8 V (twothirds of 12 V), pin 3 of the 555 will drop Low. So does pin 7, causing the white LED to light, pulling its energy from C1. This energy drops quickly, causing D2 to dim in an exponentially decaying fashion, just like a camera flashlight. Now, because the 555’s output has dropped Low, the voltage on C3 will decrease as well. Ad soon as a level of 4 V is reached (one third of 12 V), the above cycle is repeated. Resistor R4 limits the current through the 555 to safe levels.
You may want to experiment with the latest hyper-bright white LEDs. SDK’s AlInGaP LEDs, for example, are claimed to light three times as brightly as regular white LEDs. A number of blue LEDs may be connected in series instead of just one as shown in the circuit diagram. Unfortunately, that is not possible at the ‘white’ side. For the best visual effect, the white blue LEDs should be mounted close together. When fitted close to the extra brake light in your car, the bluish white flash is sure to make even persistent tailgaters back off. Note however that this use of the circuit may not be legal in all countries.