Each of the 12 keys are brought out to separate wires, and each wire is connected to a different pin of a 24-pin socket (SO1).
The four-digit access code must be entered in the proper sequence.
When the correct access code is entered (in the proper sequence),
positive voltages appear at pins 3, 4, 5, and 6 of Ul. With Ql
conducting, its collector is pulled to ground potential, energizing
relay Kl. Capacitor C2 controls the total time that the output of Ul at
pin 13 is positive after the release of the first key.