IC 555 as adjustable duty cycle flasher

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.

ic555 adjustable duty cycle flasher Circuit
Practically tested circuit of ic555 adjustable duty cycle flasher. Very simple to construct as a mini project submission in your school or college

Working

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.

Note: In this circuit, the charging time t1 is more, and discharging time t2 is almost half of t1. You can calculate t1 and t2 as per your requirements, using the following formulas.

Formulas

ic555 adjustable duty cycle flasher formula
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.

PCB Layout

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.

ic555 adjustable duty cycle flasher PCB
The practically tested PCB layout of ic555 adjustable duty cycle flasher. For actual size PCB layout contact us

Video of actual working

ic555To understand the working using animation, you can download the Circuit Wizard file of the same project. You can run this circuit to actually see the voltage levels, current direction, etc. For complete details of IC555, refer to our most popular article on IC555.