This circuit counts and indicates up to nine persons present in the room and automatically locks the door and switches off lights as soon as they leave. It automatically switches on the lights when the first person enters the room. The circuit can be used as a power-saving device and a security device to prevent unauthorised entry in the room, especially in a business meeting room. Circuit and working The circuit, shown in Fig. 1, is built around three NE555 timer ICs (IC1, IC2 and IC6), an up/down BCD decade counter 74LS192 (IC3), a 7-segment display driver 74LS47 (IC4), 4-input NOR gate HEF4002B (IC5) and a common-anode 7-segment display LTS542 (DIS1). S1 and S2 are foot switches. S1 is installed under door-mat of the door used to enter the room, and S2 is installed under a door-mat just inside the room. The outputs of IC3 change state synchronously with the low-to-high transitions on the clock inputs. Separate count up (CPU) and count down (CPD) pins of IC3 are used here. Its parall...