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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