Running led's using transistor

The main function of the circuit is to glow led's one after the other and create a running effect .

Now,let's us take a look at the circuit. It consist of 6 monostable multivibrators (MMV's) and clock generator circuit made by IC555. All of them are connected to a common 12V power supply.The square wave signal generated by IC555 is given to all 6 MMV's. The circuit is simple to understand.

To be frank it's just 6 MMV's with different RC time constants. let the time constant for 1st MMV be R1C1 ,for 2nd be R2C2 and so on. Then the relation between time constants is R1C1<R2C2<R3C3<R4C4<R5C5<R6C6

This tells us that C1 will first discharge turning on 1st MMV then C2,C3,C4,C5 & C6 at last. This gives us a running effect of led's but the story doesn't ends here. ⦁ The resistor is not the only common component in series with the capacitor which affects the time constant but also there is also a PNP transistor BC557 in common base configuration which acts as a variable resistance controlled by base current (Ib). ⦁ The base of every transistor is shorted, this makes the base current common for all transistors. ⦁ The base current (Ib) is controlled by 1Mohm pot which is connected in series with output of IC555. ⦁ Varying the potentiometer varies Ib. ⦁ varying Ib varies transistors resistance. ⦁ This varying resistance not only allows us to change the time constant of all the MMV's but also it allows us to trigger all MMV's at the same time as all the bases are shorted .Thus the speed control of running led is achieved by a single potentiometer.