|I chose to
power the monitoring circuit with a small battery. With a simple battery supply,
there is less a user has to worry about. Using modern components, I was able
to keep the current consumption to a very low level. A small 3v lithium
battery cell will power the circuit for about 5 years. The two wires connected
to the circuitís transistor switch can be used to turn on a light or noise maker,
located some distance from the monitoring box.
The circuit uses a small unshielded 100mH inductor as
a current transformer. The voltage generated by the coil is fed to a single
transistor circuit, which is configured as a high gain, low frequency Amplifier.
The circuit has a gain over 100. The output of the Amplifier is connected to a
voltage comparator. The DC bias voltages at the input of the comparator set
the sensitivity of the signal from the Amplifier at about 10 millivolts. This
should be sufficient for most applications. If the coil is placed properly
against the outside of the power cable, a current of 100ma AC should be sufficient
to activate the circuit. The output of the comparator is a pulse train equal
to the 50Hz or 60Hz power line frequency. A simple diode rectifies the pulses
and produces sufficient DC voltage to turn on the transistor Q2. The
transistor acts as a switch, which closes when the circuit detects motor current
flowing through the power cable.
If the user wishes to power an indicator light from
the same 3 volt battery, he should use a flashing LED circuit, such as the one shown
below. This type of circuit will not tax a small battery. Of course, the
user can increase the battery size if desired. The circuit will work fine from
two 1.5 volt AA cells.