7-Digit Code Lock

EE JULY/AUGUST 1987 This code lock provides a high degree of security whilst being a very simple design. At the heart of the circuit is the Type 4022 octal counter. In the non- active state, C2 is charged via Rs, so that the reset (R) input of the counter is kept logic high. This causes output Qo to be actuated, while all other outputs are logic low. When Si is pressed, Ti is switched on via debouncing network R2-C1, and IC, receives a clock pulse. Also, C2 is discharged via R4-Di, ending the reset state of the counter and enabling it to advance. The time required for R5 to recharge C2, i.e., to reset the counter, is the maximum time that can lapse before the next key is pressed. The above cycle is therefore repeated only when S7 at the Qi output is pressed in time. When all keys have been pressed in time and in the correct order, Q7 goes high for about 4 seconds to enable driving the unlock cir- cuitry, e.g. a relay driver for an automatic door opener. The code for the lock shown...