Low cost indoor air purifiers are our best defense against ambient air pollution. This paper integrates the design of a Corsi-Rosenthal box (CR box) method-based indoor air purifier with a data logger that measures, stores, and displays real-time PM2.5 concentration. The CR box method uses a set of HEPA filters and a fan to draw air through the filters. Our design adds a sensor system to the existing CR box, which consists of four HEPA filters and a fan. The sensor system measures PM2.5 concentration in the indoor environment and uses an algorithm to control the fan speed based on sensor feedback, thereby automating the fan control. The sensor box includes an AC Dimmer, a microprocessor, and a PMS7003 PM2.5 sensor. The algorithm regulates the fan speed: as PM2.5 concentration increases in a room, the fan operates at a higher speed and as the PM2.5 concentration decreases due to purifier operation, the speed gradually decreases until the fan/purifier stops operating, reducing unnecessary noise. With re-entrainment of the pollutants in the room, the purifier resumes operation. The maximum clean air delivery rate for this DIY indoor air purifier is estimated at 1500 m3/h and its single-pass efficiency is ~ 90%, based on parallel measurements at the inlet and outlet of the air purifier. This paper presents the detailed design and the working prototype of a low-cost, customizable DIY automated indoor air purifier, which can be tailored to specific room or user requirements. This work should inspire the inclusion of smart controls in low-cost air purifiers, potentially guiding future DIY designs.