henries. Layout is critical in
high frequency filters; in particular,
the coils should be at right
angles to each other. I designed
this filter using Philip R. Geffe’s
book Simplified Modern Filter
Design, John F. Rider Publisher,
1963.
FISH TANK
TACHOMETER
QCould you please suggest
a circuit and component
design to monitor the
flow of water through
a pipe? I envision a paddle wheel
with possibly two small magnets
on opposite sides (to balance the
spinner) and with a pickup coil
outside the housing to sense the
impulses as it rotates. I am capable of
making up the mechanics of the unit
but not capable of designing the
electronics.
I have three filters on a fish pond
in parallel with a common outlet.
When one needs cleaning, there is
no way to tell which one has slowed.
The paddle would just dip a little
into the stream so as to not slow the
output and be sealed in each unit.
The frequency might be displayed
on a small screen. The speed
indicated would simply be compared
to each filter to tell which one
needed cleaning when it was clean
or fouled.
Thank you for your help.
— Charles Forman
■ FIGURE 1
file/0643.pdf. It has the magnet
built in so your measuring vanes only
need to be made of ferrous material.
The enclosure should be brass,
aluminum, or non-magnetic stainless
steel. Mount the Hall effect switch
on the outside and connect it to a
frequency-to-voltage converter. The
maximum spacing between vane
and IC is 0.1 inch, as shown in Figure
2. You can use a digital voltmeter to
read the voltage and calibrate it to the
■ FIGURE 3
AI will leave the mechanics up
to you. I found this Hall
effect IC, made by Allegro
Microsystems; the datasheet
is at: www.allegromicro.com/data
■ FIGURE 2
May 2007 27
