This led series will Blinking alternately, The way it works is determined by IC NE555 and transistors are used as reinforcement for each section (20 upper, 20 lower) work optimally. 555 circuit below is a flashing bicycle light powered with four C, D or AA cells (6 volts).
Two sets of 20 LEDs will flash alternately at about 4.7 cycles per second using RC values shown (4.7K for R1, 150K for R2 and 1uF capacitor). Time intervals for two lights about 107 milliseconds (T1, LED on) and 104 milliseconds (T2 lower LEDs).
Two transistors are used to provide additional time beyond the current limit of 200mA from the NE555 timer. A single LED is placed in series with the PNP transistor base so that the 20 LED 555 is turned off when the output is high during a time interval T1. High-level output timer 555 is 1.7 volts less than the supply voltage. Adding the LED forward voltage increase is required for the PNP transistor to about 2.7 volts, so the 1.7 volt difference from the supply to the output is not sufficient to activate the transistor.