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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 |
