This circuit of IC 555 as adjustable duty cycle flasher is unique. It uses the trick of charging the timing capacitor C1 through two different paths of resistors R2 and R3, via two independent diodes D1 and D2.
When power supply is connected to the circuit (9V, PP3 battery) the timing capacitor C1 starts charging through resistors R1, diode D1 and resistor R3. The output of the IC becomes HIGH and the LED D3 lights up. The voltage across the capacitor starts building up.
When its becomes equal to or slightly greater that the 2/3 voltage of battery voltage i.e. 6V, the internal threshold comparator of the IC SETs the internal Flip Flop. So the output of the IC becomes LOW and the LED turns OFF.
For more details about the internal block diagram of IC 555 and its complete working refer on this link
Now the capacitor starts discharging through resistor R2 and diode D2 into pin-7 discharge pin. When the voltage across the capacitor drops slightly below 1/3 of battery voltage i.e. 3V, then the internal trigger comparator RESETs the internal Flip Flop and again the output of the IC becomes HIGH, the LED lights up and capacitor C1 starts charging.
Remember that in calculations, you must use resistor values in ohms and capacitor value in Farad. To convert resistor value from kilo ohm to ohm, multiply by 103, and to convert value of capacitor from microfarad to farad, multiply by 10-6.
The practically tested PCB layout of the above circuit is given below. The following image is NOT size proportional. If you want to get the PCB layout of this circuit for actual construction, then contact us. We shall send you the actual size PCB layout for free.