The paper presents the design, construction and testing of an Automatic Room
Population Monitor. It is used to determine the number of people in a room or other
enclosure. It has both logistic and security applications. The design is realized using
digital circuitry and light emitting diode (LED) light sources. When the light beams are
broken in the entry direction, the counter increments. When broken in the exit direction,
the counter decrements. The number showing on the display unit at any point in time is
the number of people currently remaining in the enclosure. The design uses latches to
clock the Up/Down counters in the circuit and hence determine the direction of motion.
It has an alarm that produces a high tone for forward crossing (entry) and a low tone
for exit, as well as LED directional indicators. For some security purposes, the alarm
can be switched off. A control oscillator is used to reset the circuit after a reasonable
time to prevent the circuit from responding in the event of someone crossing halfway. It
also has a ‘zero detector’ to prevent the circuit from responding when the count is
already at zero and a reset button in case the counting has to be restarted.