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Circuits designed by David Johnson,
P.E.
Last Updated on:
Thursday, March 22, 2012 05:55 PM
Master Category List -
Dave's Circuits
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More
Alarms;
Power Line Circuits |
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Unplugged Appliance Alarm |
| I got a call from an
administrator at a large prison. He told me that he was having problems with
inmates who keep unplugging refrigerators, vending machines and microwave ovens in
a commons area. He wanted a battery-powered device, which could be mounted
to the outside of the power cord of an appliance. The device would produce a
loud alarm whenever the power plug of the appliance was unplugged from a power
outlet. He said that the prison had an emergency backup generator, so if
utility power were lost; their generator would kick in within a minute or two.
He therefore did not need any automatic timeout after the alarm started. I
gave this some thought and came up with the circuit below. |
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| A 9v battery
powers the circuit below. Its standby power is so low that a 9v battery should
last 5 years or so. A pair of copper strips pushed onto the surface of the power
cord by the “clamshell alarm assembly” collects a tiny bit of voltage from the
electric field emitted by the power line through capacitance coupling. A
low power voltage comparator converts that voltage into a square wave signal.
The open drain output of the TLV3401 comparator from Texas Instruments keeps the
capacitor C4 discharged, when the circuit detects AC from the power cord. When
the AC power is lost, the capacitor C4 is charged up through R5 and turns on the
transistor Q1. Q1 then turns on the alarm. A piezoelectric siren with a
sweeping tone is recommended as the alarm sound generator. Note that the circuit
will not go into alarm if the appliance is turned off, since AC would still be present
in the power cord. Only when the plug is removed from a power outlet will the
device go into alarm. I also show how to use a small reed relay in the circuit
during installation. By taping a magnet near the switch, on the outside of the
plastic alarm enclosure, the alarm will be kept off. |
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Click on Drawing Below to view PDF version of Schematic |
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