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An ordinary
automatic room power control circuit has only one light sensor. So
when a person enters the room it gets one pulse and the lights come
‘on.’ When the person goes out it gets another pulse and the lights
go ‘off.’ But what happens when two persons enter the room, one
after the other? It gets two pulses and the lights remain in ‘off’
state. The circuit described here overcomes the above-mentioned
problem. It has a small memory which enables it to automatically
switch ‘on’ and switch ‘off’ the lights in a desired fashion. The
circuit uses two LDRs which are placed one after another (separated
by a distance of say half a metre) so that they may separately sense
a person going into the room or coming out of the room. Outputs of
the two LDR sensors, after processing, are used in conjunction with
a bicolour LED in such a fashion that when a person gets into the
room it emits green light and when a person goes out of the room it
emits red light, and vice versa. These outputs are simultaneously
applied to two counters. One of the counters will count as +1, +2,
+3 etc when persons are getting into the room and the other will
count as -1, -2, -3 etc when persons are getting out of the room.
These counters make use of Johnson decade counter CD4017 ICs. The
next stage comprises two logic ICs which can combine the outputs of
the two counters and determine if there is any person still left in
the room or not. Since in the circuit LDRs have been used, care
should be taken to protect them from ambient light. If desired, one
may use readily available IR sensor modules to replace the LDRs. The
sensors are installed in such a way that when a person enters or
leaves the room, he intercepts the light falling on them
sequentially—one after the other. When a person enters the room,
first he would obstruct the light falling on LDR1, followed by that
falling on LDR2. When a person leaves the room it will be the other
way round. In the normal case light keeps falling on both the LDRs,
and as such their resistance is low (about 5 kilo-ohms). As a
result, pin 2 of both timers (IC1 and IC2), which have been
configured as monostable flip-flops, are held near the supply
voltage (+9V). When the light falling on the LDRs is obstructed,
their resistance becomes very high and pin 2 voltages drop to near
ground potential, thereby triggering the flip-flops. Capacitors
across pin 2 and ground have been added to avoid false triggering
due to electrical noise. When a person enters the room, LDR1 is
triggered first and it results in triggering of monostable IC1. The
short output pulse immediately charges up capacitor C5, forward
biasing transistor pair T1-T2. But at this instant the collectors of
transistors T1 and T2 are in high impedance state as IC2 pin 3 is at
low potential and diode D4 is not conducting. But when the same
person passes LDR2, IC2 monostable flip-flop is triggered. Its pin 3
goes high and this potential is coupled to transistor pair T1-T2 via
diode D4. As a result transistor pair T1-T2 conducts because
capacitor C5 retains the charge for some time as its discharge time
is controlled by resistor R5 (and R7 to an extent). Thus green LED
portion of bi-colour LED is lit momentarily. The same output is also
coupled to IC3 for which it acts as a clock. With entry of each
person IC3 output (high state) keeps advancing. At this stage
transistor pair T3-T4 cannot conduct because output pin 3 of IC1 is
no longer positive as its output pulse duration is quite short and
hence transistor collectors are in high impedance state. When
persons leave the room, LDR2 is triggered first followed by LDR1.
Since the bottom half portion of circuit is identical to top half,
this time with the departure of each person red portion of bi-colour
LED is lit momentarily and output of IC4 advances in the same
fashion as in case of IC3. The outputs of IC3 and those of IC4
(after inversion by inverter gates N1 through N4) are ANDed by AND
gates (A1 through A4) are then wire ORed (using diodes D5 through
D8). The net effect is that when persons are entering, the output of
at least one of the AND gates is high, causing transistor T5 to
conduct and energise relay RL1. The bulb connected to the supply via
N/O contact of relay RL1 also lights up. When persons are leaving
the room, and till all the persons who entered the room have left,
the wired OR output continues to remain high, i.e. the bulb
continues to remains ‘on,’ until all persons who entered the room
have left. The maximum number of persons that this circuit can
handle is limited to four since on receipt of fifth clock pulse the
counters are reset. The capacity of the circuit can be easily
extended for up to nine persons by removing the connection of pin 1
from reset pin (15) and utilising Q1 to Q9 outputs of CD4017
counters. Additional inverters, AND gates and diodes will, however,
be required |
