Automatic Room Power Control

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
meter) 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 coming
into the room and the other will count as -1, -2, -3 etc when persons
are going 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-color 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-color 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) and 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 energize relay RL1.
The bulb connected to the supply via N/O contact o 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 to handle up to nine persons by removing the connection of pin 1
from reset pin (15 and utilizing Q1 to Q9 outputs of CD4017 counters.
Additional inverters, AND gates and diodes will, however, be required.

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