This is the simplest electronic door lock that has a remarkable code implementation because it uses only one active component and at least 8 button switches.
How does the door code lock circuit work?
To activate the relay the 4 buttons S1-S4 must be pressed at the same time. If any of the 4 buttons S5-S8 are pushed the relay doesn’t switch so the door won’t open.
The supply voltage must be equal to the working voltage of the relay. One transistor like BD135 can switch currents up to 0.5A, at the same time 2N2222 can do only 0.2A.
Enjoy this electronic door lock, its easy to build and straightforward.