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The objective of the research was to develop a programmable environment controller using old personal computer such as PC/XT, PC/AT286 and 386 for the greenhouse environment. The controller was designed to maintain temperature, relative humidity and water availability in a desired range. The controller consisted of ten temperature sensors and two probes (water sensor). The relative humidity was measured using wet bulb temperature sensor. The outputs of controller operate a sprayer pump to increase humidity, a dripper for water supply, a fan to let air in and a fan to ventilate the air. The signal processing consisted of multiplexer, amplifier, analog to digital converter and interface card. The GEC (Greenhouse Environment Controller) software was compiled under DOS that works on any PC that has at least one expansion slot for interfacing. Using the software, operation of controller could be set in manual or automatic mode with changeable setting of parameter ranges.

Performance of the controller was evaluated by installing it in a model greenhouse. The desired relative humidity was set within the range of 80% to 95% and temperature was set within the range of 30 o C to 33 o C. Results of the test indicated that RH exceeded the setting to 97% after sprayer was switched off. Temperature also exceeded the desired lower limits to 29.1 o C. These set parameters were exceeded due to delay in response time of controller system including sensors. Based on the overshoot in humidity and temperature their set values were adjusted. The lower limits were set slightly higher and upper limits were set slightly lower. The overall tests indicate that the controller worked satisfactorily.