In this paper, we propose a PID temperature controller for passive solar water heaters by regulating an electric valve developed from a low-cost three-way valve that was integrated with a servo motor. An experimental model was developed to characterize the three-way valve and to find the parameters of the controller. The control algorithm was embedded in an Arduino Nano kit to control the opening angle of the designed valve for mixing hot and cool water from the inlet. By adjusting the hot and cool water mixing ratio, the mixed outlet water temperature can be maintained at the desired value. The response of the PID model was also compared to that of the fuzzy logic model. This study also determined the optimal value of the initial valve opening angle to achieve the required energy efficiency and response time. Experimental results show that the system response using the PID controller had an overshoot of less than 3%, a settling time of about 110 seconds, and a temperature error of ±1⁰ C. The research results provide an advanced feature for passive solar water heaters that contributes to energy saving, comfort, and safety for users.