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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, June 01, 2021 03:06 PM
Hobby Circuits List
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More
Indicator Circuits
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Active AC
Power Line Indicator |
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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. |
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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. |
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Click on Circuit Below to view PDF of Schematic |
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