Noise gate

88 65 deceptive lock This circuit offers a means to fool all EE July/August 1986 but the cleverest burglar, but, although it is a clever design, it has been kept simple, as a glance at the diagram will show. On the surface it looks like a simple operating panel with ten push buttons. However, anyone trying to open it illegally is in for a surprise! It is not just a matter of keying in the correct code, it is also necessary to keep one of the keys depressed for about 10 to 15 seconds. The circuit is based on a single Type CD4093, which contains four NAND gates with Schmitt trigger inputs. Gates N, and N2 form a bistable that contains the status of the lock. Assuming that the circuit has been off for some time, switching it on causes network R,-C2 to set the bistable to the "lock" position, that is, the output of N2 is logic low. Capaci- tor C 3 is discharged, and the only way the circuit can toggle is by recharging this capacitor. This is done by pressing key S. long enough for the...