Download humidity controller

Project Description
Summer in India is very hard to bear. Everyone wants to have a breeze of
cool air, whether at office, workshop or at home. It is also a well known fact that
everybody cannot afford an air conditioner. In cities, people use mostly desert
coolers that provide some relief from the scotching heat.
In many houses rooms are quite small and ventilation is also inadequate,
and if coolers are used continuously for a long time, excessive humidity is
caused. This can cause serious diseases related to lungs and skin.
In many coolers fan speed controllers are provided, but there is no provision for
humidity controller, which can be easily installed in any desert cooler, and the
user can get the desired level of humidity.
Circuit Description
This controller is designed to switch on/off the motor of the water pump
automatically. A commonly available 555 timer IC is used as the basic building
block of the controller. The timer is configured in astable multivibrator mode, and
generates a train of square wave pulses at its output.
A dual potentiometer (tandem type) is used to control the duty cycle (on
time /cycle time ratio) of the square wave generated by the timer. An NPN
transistor (BC548) is used to interface the relay to the timer IC, to avoid
overloading of the output stage.
Working
The on time and the off time depend on the values of resistor R1 and
capacitor C1 used in the charging and discharging paths, respectively. Here, two
silicon diodes D1 and D2 (1N4007) are used to split the charging and discharging
currents through VR1(1M) and VR2(1M) respectively, so that capacitors C
(100uf) and charge through + Vcc, R1(15K), Vr1, D1 and up to two third times the
value of +Vcc during the time Ton , and then discharge pin 7 of + Vcc, during the
time Toff, given by the equations:
Ton = 0.69 x (R1 + VR1 ) X C1 seconds
Toff = 0.69 x (VR2 ) X C1 seconds
Also, the cycle time of the square wave is given by:
Tc = T on + off = 0.69 x (R1 + VR1 + VR2 ) x C1 seconds.
In this circuit the cycle time is set one minute approx. The dual potentiometer
(tandem type) is wired in such a way that if VR1 is increased, VE2 is decreased
by the same amount, and vice – versa. This increases the on time by an amount
( say +t sec.) , and simultaneously decreases the off time by the same amount ( t sec. ), keeping the cycle time constant. In other words, the pulse width of the
square wave is controlled by the position of the dual pot.
The relay switches the 230V AC supply to the motor of the pump, which supplies
water only during the on time, when the time output (at pin 3) is high. For the rest
of the cycle time, the pump motor remains off. This action is repeated
automatically every minute. In this way the excessive humidity is controlled. The
humidity control action can be set anywhere between 10 to 90 percentage of the
maximum humidity obtained without a controller.
INSTALLATION
Considering the lack of space in the front panel and excess of water vapor inside
the cooler its recommended to install the controller in a small box outside the
cooler however the potentiometer can be fixed with in the front panel
PART LIST
DIODES
1N4007 - 5 Nos
I.C.
NE 555 (SGS-THOMPSON)
TRANSISTOR
BC547
RESISTANCES
15 K - 2 Nos
VARIABLE RESISTANCES
1M - 2 Nos
CAPACITORS
1000 uf / 25 V
63 uf / 25 V
.01 uf
MISC
TRANSFORMER 6-0-6 V / 500 mA
RELAY 6 V / 100 ohm SPST
CIRCUIT DIAGRAM