Using LEDs to build a simple polarity detector

Abstract Here we explain a straightforward method to measure the polarity of charged objects for introductory experiments of electricity. The device consists of two LEDs connected in parallel but opposite directions. The legs at one side are joined by twisting them and act as an antenna, and the two legs at the other side are twisted to one side of a 470 kΩ resistor. To detect polarity, the free leg of the resistor is hold by one hand and the antenna is approached to the charged object. The object will discharge only through the LED with the correct polarity, producing a dim light flash that is generally difficult to see in normal lighting conditions. To improve the detection process, the LEDs are hidden from direct light sources inside a 3D printed case that is also used to insert the components.