Keyhole or Door Keys Flashing Finder

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.

Schematic of Door Keyhole Finder Circuit Version 2

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.


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