The circuit is drawn with a 12-volt supply – but it will work at anything from 5 to 15-volts. All you have to do is choose a relay suitable for the supply voltage you want to use. Replace the SPCO/SPDT relay with a multi-pole relay – if it suits your application.
Do not use the "on-board" relay to switch mains voltage. The board’s layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage – mount a suitably rated relay somewhere safe – Away From The Board.
Choose the four keys you want to use as your Code – and connect them to "A B C & D". Wire the common to R1 and all the remaining keys to "E". The circuit will power-up with the relay energized. To de-energize it – you must enter your code. To re-energize it – press any of the keys connected to "E".
To reverse the operation of the circuit replace Q2 with a BC547. With an NPN transistor in this position – the circuit will power-up with the relay de-energized. To energize it – you must enter your code. To de-energize it again – press any of the keys connected to "E"
Any keys not wired to "A B C & D" are connected to the base of Q1. Whenever one of these "Wrong" keys is pressed – Q1 takes pin 1 low and the code entry sequence fails. If you make a mistake while entering the code – simply start again.
The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13 terminals. The matrix type with 7 or 8 terminals will NOT do. A 12-key pad has eight "Wrong" keys connected to "E". If you need a more secure code – use a bigger keypad with more "Wrong" keys.