Facebook post at https://www.facebook.com/groups/electronichobycircuits/permalink/3743513689006646/ advertises this circuit idea:
Amazing Idea With Relay & BC547 Transistor
I see this circuit much less than amazing.
Voltage level indicator idea is feasible. The “short circuit protection circuit” does not live up to the promises – can work on some conditions (enough resistance from power source to circuit) and fail catastrophically on other (circuit directly connected to 12V car battery).
Seems ro be basically workable idea. The 12V LED is pretty sensitive to overvoltage, meaning if input voltage rises just few volt above planned 12V, that LED will be soon fried.
The power is only cut if the voltage coming in to the circuit drops due short circuit enough for relay coil to “dop”. If there is no such voltage drop, the “protection” circuit does not work and will pass short circuit current to load until it fails catastrophically with smoke.
If you try to to turn on power to short circuit, the small start switch will “let the smoke out” easily.
The use of BC547 makes it pretty much useless component here and is not properly protected agains inductive kickback from relay coil.
Earlier related post: