The circuit and switching system is simplified by the use of a multiplex system. S1 inputs pulses to the decade counter 7490. The resulting BCD is decoded by the 7442. lt is the decimal output of this which carries out the multiplexing via the AND»gatesi S2 inputs pulses which are trans- ferred to the other 7490 decade counters by the AND-gate multiplex system.
The BCD output from the 7490% is taken to the AND-gates whose outputs control the Alarm ‘Disable' and ‘Enable' switch system. The 'Disable’ function effectively prevents TR2 from being biased on and hence prevents the 'Enable' Reedrelay from working.
This electronic combination lock circuit has several advantages over conventional electronic com- bination locks as only two switches need
The alarm is triggered it any of these digits is exceeded in value. While the circuit is capable of directly driving an actuator it is recommended that it is only used to disable an alarm system — conventional locks doing the actual — locking. (To operate the example the switch sequence would be: S1, S2, $2, S2, S1, S2, S1, S2, S2, S2, S2.}
Courtesy – ETI Circuits