Shake coil voltage source is fed into a bridge rectifier that charge a capacitor. The LED is controlled by a current source set which is set by the 40 ohm resistor (current through the 40 ohm resistor causes a voltage that turns on the lower NPN transistor which turns off the upper transistor. when the upper transistor turns off the lower also turns off which turns the upper back on keeping a constant current flowing through the LED.
