Here is low cost circuit that can be used for implementing a warning light on the top of tall structures as a warning for low flying aircrafts. Usually such lights are just stay glow type. Here our light is a flashing type, and surely this will draw more attention and of course add more safety.

When the AC mains is switched on the bulb will get supply and it will glow. When the bulb glows the LDR adjacent to it gets illuminated and it’s resistance drops. This will increase the voltage drop at the base of transistor Q1 and it goes on.When Q1 is on the relay is activated and trips the contacts to make the bulb off. Now the LDR resistance increases and the voltage drop across base of Q1 decreases to make it off. The relay will be de-energized and the bulb glows. This cycle repeats to produce a continuous flashing of the bulb. Since the capacitor C1 is connected across the relay the relay will remain activated for some more time even after the transistor Q1 is OFF, making the bulb to stay off for some more time.

Circuit diagram with Parts list.


  • The LDR should be mounted at some place near to the bulb so that when bulb glows the light falls on the LDR.
  • Set up the circuit as said above and power up. Then adjust the POT R2 so that the bulb starts flashing. Done! The circuit is ready.
  • Some parts of the circuit are live with potential shock hazard. Be careful!.
  • Bulb of any Watts can be used as load provided that a relay with suitable power rating is used.
  • All capacitors are electrolytic and must be rated 25Volts.


  1. may i know what is the value of bridge diode… plzz.. tnx..