This 3 volt Lithium cell operated LED flasher can be used as a keyhole finder in darkness or to find your door keys. A small lithium button cell can power the circuit more than 6 months continuously with day and night flashes. The circuit can also be used in key stand to search key in darkness.
The circuit is a simple oscillator comprising two complementary transistors BC 547 and BC557. These NPN and PNP transistors are wired as a simple oscillator with components C1 and R1 so that the LED flashes based on the charging and discharging of C1. Current consumption of LED is very low so that a normal 3 volt lithium battery can power the circuit for long time. A miniature 12 volt battery used in car remote can power the circuit more than 2 years continuously.
Use a high bright transparent 5 mm Yellow LED and fix the unit near the keyhole.
Note: the previous version designed by D Mohankumar was not working, as many of you mentioned in the comments, so I’ve tested it and came to the same conclusion. I had to test different versions of it and the end result is the circuit that you see above. I’ve added the R6 resistor between the base of T1 and minus. R6 value can be between 3MΩ and 10Ω, LED blinking frequency will be higher with smaller values.