LED needs stable current to correctly work. This circuit will demonstrate the differents between Constant current supply and Direct power supply.
The yellow led is connected directly to battery. The cyan one is powered through 2 npn transistors system.
The battery is a oscillator to demonstrate how led's current react to fluctuating voltage. As the result, the yellow led's current is more fluctuating than the cyan one's.
As you see, the transistors work like a smart current controller. They are so intelligent! :)
