electronic key-set - prevents multiple entries

prevents multiple entries This electronic key-set, although similar to its mechanical counterpart (switching, interlocking, reciprocal release, and so on) has one import- ant advantage over it: when two or more keys are pressed simul- taneously, all contacts remain open, which is by no means guaranteed with a mechanical set. The latter may actually allow several contacts Ub (V) 3 0.45 5 2.8 9 10.2 15 23.5 to close at the same time, which can have disastrous consequences. The circuit is simplicity itself: five standard or miniature push-button switches, two ICs, four resistors, six diodes, and five LEDs. The heart of the circuit is IC2, a BCD-to-decimal decoder type 4028. This guarantees that whatever the input information only one of its outputs is logic high (= switch closed). To start at the beginning: inputs A . . . D of IC2 are at low logic level via resistors R3 . . . R6 and the 0 output is therefore logic 1. This is in- verted by N1 which causes D1 to 7-55 mm R1 C IC1 1p 1...