■ FIGURE 4
flow rate, if you wish. In the circuit in
Figure 3, I added the three outputs and
divided by three so you only need one
voltmeter. When the indication is
slowed, you can switch the meter to the
individual outputs to see which one has
slowed.
I don’t know how fast the paddle
wheel will rotate in this system; I
assumed 300 RPM maximum in this
design, but the equations to change
the maximum RPM are:
C1 = 21.3/Fmax in microfarads and,
R1 = 6. 26*Voutmax in Kohms.
The Hall effect IC has a current
output, 6 mA minimum and 16 mA
maximum. It switches between
those values as the vanes rotate. I
used a 1N4148 diode to bias one
input so that 6 mA times 51 ohms
will be less than the diode drop for a
low input; 16 mA times 51 ohms is
more than the diode drop for a high
input. You could connect the
voltmeter to the LM2917 output, but
I wanted to add the three tach
signals and the summer inverts the
signal, so I needed an inverter in
front of the summer. The inverter
also converts the 0-to- 6 volt signal
from the LM2917 to a 6-to-0 volts
signal into the summer.
FLYWHEEL
RE-MAGNETIZER
QI built a re-magnetizer for
Vespa P series flywheels
that produces a strong
field at all six solenoid core
ends, or shoes, as I call them. But,
the rig doesn’t work on the flywheels.
I copied loosely a picture of a
factory machine. The Vespa flywheels
are about six inches ID. Six 2 x 1.5
inch magnet faces of alternate
polarity are exposed within the
inside diameter. The re-magnetizer
28 May 2007
has six solenoids wound with five
wraps of 12 gauge stranded copper
wire on 5/8 x 8 inch mild steel bolts
that end in one inch square angle iron
shoes fitting inside the flywheel
diameter to complete the flux
circuits on the business end of
the re-magnetizer. All solenoids
(electromagnets) are in parallel with
about half an ohm resistance
overall; 160 amps for two seconds
warmed the coils but a little. Assuming polarities are correct, what is
likely wrong with my contraption?
Are the solenoid shoes too small
and too loosely fitted against
the magnet faces? Do I need a
bolt through the flywheel center
connecting to the re-magnetizer
bottom, thus completing the flux
circuit in another way? The picture
of the factory machine hinted at
something like this.
— Melvin Schallot
AAny air gap will greatly reduce the magnetic field; the
fit should be tight. I would
connect the coils in series
because you don’t know how the
current divides when in parallel. You
will need 88 volts to drive 27 amps
through the six series coils. If you
charge up a 1,000 µF, 100 volt, can
type, high ripple electrolytic capacitor
and discharge it through the coils, that
should work; although bigger is better.
If all the coils are wound identically,
then you mount them around the
circle; the magnetic fields will cancel
unless you reverse every other one.
That may be the real problem. You only
have to reach a peak of magnetic flux
in order to re-magnetize. Time is not a
factor, as far as I know.
SAWDUST VACUUM
QI use a table saw with a
shop vacuum attached.
It would be more convenient
if the vacuum would
come on automatically whenever I
start the table saw. I think a
current/voltage sensing system
may have already been published.
However, would you please recommend such a system?
Thank you very much for any help
you can give.
— Frank Lemon
AI assume the shop vacuum
and saw both run on 115
VAC, so just connect the saw
motor and shop vac in
parallel. If you don’t want to mess with
the saw switch, buy a switch box and
switch at the local hardware store, and
while you are there, get an outlet box
and outlets. You can wire it up per
Figure 4 and plug both the saw and
vacuum into the outlet box.
GAME SHOW LOGIC
QI’m looking for assistance
in designing a hobby circuit
to use as a game show
quizzer for our church
youth. I’ve made several quiz boxes
over the years using SCRs as the
trigger and latch device per the
schematic I found in the Feb. 1977
issue of Popular Electronics magazine.
I need to update the design to
show which contestant came in first,
second, third, and fourth in a match
with four teams of five contestants
per team ( 20 total contestants). I’d
also like to select between individual
mode (where each contestant
competes against all others) and team
mode (where the fastest contestant
on a team locks out the rest of
his team).
Each contestant will hold
a normally open push button to
depress when they think they can
answer the question. This sounds a
buzzer (telling the quizmaster to
stop reading the question), lights
their signaling light on a remote team
light box, as well as lights in front
of the timekeeper official (color of
light or blink rate identifies if they
were the first, second, third, or fourth),
and starts a digital timer. The circuit
should not have any ties (response
rate of 0.1 millisecond or faster).
It also needs immunity to static
electricity since it will used be in a
carpeted room during the winter
with lots of spectators walking around.
I can accept a hardware or software
solution. I have some experience
