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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
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