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More  Indicator Circuits

Active AC Power Line Indicator

The circuit below is designed to detect when a power cable is active (hot). When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow.  It is powered by a 3v CR2032 lithium coin cell.  It has no on/off switch and will function for years before the battery has to be replaced.  The circuit could be contained in a small plastic housing with a diameter of about one inch.
The circuit uses a LMC7225 ultra low power voltage comparator.  A number of resistors bias the device’s inverting input slightly higher than the non-inverting input.  This insures that its output remains low and the LED turned off.  A circular metal plate on the inside top of the housing forms a reference capacitor electrode.  When the device is held, the human thumb and therefore the human body becomes one large capacitor plate.  A second flat plate in the sensor end of the device forms a field sensing electrode.  When the sensor plate is held near an active 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 output on and off, which pulses the green LED on and off, indicating an active power line.  The value of the resistor R3 can be changed to increase or decrease field sensitivity as desired.

Click on Drawing Below to view PDF version of Schematic

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