Active AC power line indicator

The following circuit is designed to detect when a power line is live (live). A bright green LED on the top of the housing illuminates when the device's flat sensor end is brought near a live AC line. It is powered by a single 3V CR2032 lithium button battery. It has no switch and can operate for years before the battery needs replacing. The circuit can be housed in a small plastic casing approximately one inch in diameter.

This circuit uses the LMC7225 ultra-low-power voltage comparator. Multiple resistors bias the inverting input of the device slightly higher than the non-inverting input. This ensures that its output remains low and the LED is off. A circular metal plate inside the top of the housing forms a reference capacitor electrode. When the device is held, the human thumb, and thus the human body, becomes a large capacitor plate. A second flat plate at the sensor end of the device forms a field-sensing electrode. When the sensor plate is near a live power line, the electric field from that line induces a small AC voltage at the non-inverting input of the comparator. This signal toggles the switching state of the output, causing the green LED to flash, indicating a live power line. The value of resistor R3 can be changed as needed to increase or decrease the field sensitivity.