I initially tried some a differential amplifier circuits, but found
that a simple comparator would work better since it consumes less
power, and can directly drive the digital output that used to be driven
by the Hall-effect sensor. Unfortunately, the comparator I
had in
my parts bin is an LM339, a quad unit, causing three of them to go
unused.
Circuit that converts the analog output to digital pulses for reading
by the
Power Node.
Test setup for the above circuit in my basement. Note the old
sensor on the coil of wire to the right.
The above circuit was
constructed on
some 'perf-board', and pluged into the Power Node for power and data
interface to the
Main Home Control
computer.
The LED is useful for adjusting the threshold of the sensor.
R3
is adjusted until the LED blinks when someone is using water in the
home. I was concerned that the small signal amplitude would
cause
the circuit to be prone to drift, but that has not occurred.
The finished project. Note that the sensor is conveniently
packaged on the end
of a rod, which allows it to be held as close as possible to the
meter. The
strongest magnetic field will be right at the interface between the
water turbine part (bottom
brass part), and the
meter dials (top black part).
The above picture shows
the finished
result. The sensor rod was fastened near the interface
between
the dial counter (top) and the water turbine (bottom). It
results
in a very clean interface.
In October 2009, Bryan Mumford, whom I assisted in building his own
sensor setup, sent me some information on his sensor. By
opening
up his unit, he found that the sensor chip is the
Sentron
CSA-1VG.
Screen snap of the web page for the sensor chip
that is used in the field strength meter.