Coulometric respirometry is a highly sensitive method for measuring O2 consumption in small organisms but it is not in widespread use among physiologists. Here, we describe a coulometric microrespirometer based on a digital environmental sensor inside a sealed glass chamber and controlled by an Arduino™ microcontroller. As O2 is consumed, exhaled CO2 is removed, causing pressure to decrease in the chamber. The sensor detects the decreased pressure, and the controller activates electrolytic production of O2, returning pressure to the initial value. O2 consumption is calculated from electrolytic charge transfer. The effects of developmental stage, body mass and temperature on O2 consumption of Tenebrio molitor beetles were easily measured by the apparatus. This straightforward design is a significant innovation in that it provides continuous data regarding environmental conditions inside the experimental chamber, can be fabricated easily, and is adaptable to a wide range of uses.