NE555 is an IC which is widely used in timers and control circuits. A circuit for independent testing this IC is given here.

Here the NE555 is wired as an astable multivibrator. When the push button switch S1 is pressed the LEDs D1 & D2 will flash alternatively. That is when output is high D2 will glow & when output is low D3 will glow. The rate of flashing will depend on components R1,R2 & C1.


When push button S1 is pressed,C1 will start charging through R1&R2.When the voltage across C1 rises above 2 of 3 is the supply voltage the internal Flip Flop toggles . The pin 7 becomes low & C1 starts discharging. When the voltage across C1 goes below 1of 3 of supply voltage the internal Flip Flop resets & pin7 goes high. The C1 again starts charging.All this will take place if the IC is healthy.


According to the frequency of this charging & discharging D1&D2 will flash. From these observations we can conclude that IC NE555 is faulty or not .

Circuit diagram with Parts list.



  • Assemble the circuit on a good quality PCB or common board.
  • Power the circuit from a 9V radio battery.
  • If D1 and D2 flashes on the pressing of S1,we can assume that the IC is working.


  1. hi setharam ,manish is right the leds are not lighting alternatively

  2. i had connected a electrolytic cap for the value 10uF and ceramic cap for 0.001uF,nd then,the first LED blinks nd second bulb glows continuously…pls help.I think 0.001uF cap is not working for ckt…