single-button code lock - to thwart would-be housebreakers

to thwart would-be housebreakers The disadvantage of almost every available code lock is that some sort of keyboard must be mounted at the door for keying in the (secret) code to open the lock. These keyboards attract attention with the result that all sorts of undesirables are likely to be found tapping on the buttons. The circuit here uses the normal doorbell push button to feed in the secret lock code. The code consists of a series of long and short pulses, just like a sort of Morse. At the centre of the circuit is an eight-bit shift register (IC1). The outputs of this IC are connected, via switches S1 . . . S8, inverters N3 . . . N10, and diodes D1 ... D8, to the base of transistor T1 which controls the lock relay. This transistor can only conduct if there is a "1" on the cathodes of all the diodes. In all other cases the base current supplied by R3 will be "carried away" by one or more diodes. The switches are used to select "normal" or inverted output from the register. This...