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 bicolor 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 in-stalled 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 bicolor 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 any person leaves 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 bicolor 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 through N4) are AND-ed by AND
gates (A1 through A4) and then wire OR-ed (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 energies 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 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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