Prompt determination of the positive or negative signs of charged objects promotes students' understanding of electrostatics and motivates them to learn more about this field of physics. We developed a low-cost (about US$5) and easy-to-use device for checking the signs of the static charges in objects. It consists of two easy-to-build circuits, one for positive- and another for negative-charge detection. Both are composed of a metal-oxide-semiconductor field-effect transistor (MOSFET), an LED indicator, a tactile switch (momentary contact switch), and a jumper wire. In this paper, we provide instruction on how to build this device. Junior high school students who used it successfully created a triboelectric series by testing various daily-life materials. A free-response questionnaire revealed that the device is highly useful for consolidating students' understanding of static electricity.