FM Instead of Wi-Fi
I would like to use a device such
as a Pi or Arduino to connect to an
Ethernet. I want to send/receive data
wirelessly via digital FM instead of
Wi-Fi to communicate with devices
where the Wi-Fi/internet connection is
unavailable, but still relatively close
(suitable for low power FM where FCC
regulations would not be an issue).
High speed data transfer such as
for streaming video is not a must, but
100-200 kps would be ideal. This
would, of course, vary depending on
the strength of my signal.
Can I buy a board with Ethernet
and simply I/O data to an FM
transceiver? Any suggestions to help
me get started would be great.
#6141 Gary McPherson
Delay Circuit Needed
I need a little help with an automatic above ground pool filler circuit I
constructed. It’s nothing complicated
— a water level switch with a power
supply. When the water level drops
below the set point of the float switch,
it completes the circuit which applies
12 VDC to a solenoid which activates
and allows the water to flow.
When the water level comes up
and the float switch raises up to the
preset point, the reverse happens and
the water stops flowing.
My problem is I need some sort of
delay before the power supply sends
12 VDC to the water relay. The float
switch — due to motion by the wind or
vibration — normally keeps the water
level switch bouncing. This, in turn,
keeps the water constantly turning on
and off. If there was a 15 to 30 second
delay until the float switch stabilized in
one condition — either on or off — it
would prevent the water switch from
constantly going on and off.
Anyone out there with a circuit
that can do this? It would be even
better if the same timer also supplied
the voltage for the water relay. Thanks
#6142 Gary Lichtenstein
Las Vegas, NV
Battery Charging Indicator
My truck only has a voltmeter for
battery condition indication. I don't
have room for an ammeter. Is there a
circuit I could build with a red and
green LED to indicate if the battery is
charging or discharging?
Telemarketing has gotten both
ultra sophisticated and out of control.
Do Not Call listings are a waste.
I'd like to stop all incoming calls
short at the point of entry, unless the
caller — on receiving a prompt — dials
an additional pass code to access my
stand-alone answering machine and
simultaneously ring any phones on
which the ringer is turned on.
Is a DIY circuit available? Is
anything off-the-shelf available? I
checked with AT&T marketing and all
they offer is blocking of all calls where
no telephone number or caller name
is available. That's not what I want.
#6144 Robert Wheaton
San Antonio, TX
Renewing Permanent Magnets
I have an old style hand crank
telephone generator. It works, but
output voltage is low due to the four
horseshoe magnets it uses being weak.
What is the best way to revitalize these
magnets to bring the generator back to
like-new operating condition?
Also, how can I tell when — as a
result of the revitalization process —
the magnets have achieved maximum
strength and won't get any stronger?
#6145 David LeFevre
[#3143 - March 2014]
Sweep Generator Range
I have a Heathkit sweep generator
1274 which I bought with the hope
I could quickly determine the
frequency response of amplifiers I play
with from time to time. However, there
is no way to determine the range of the
pattern on my scope — at least none
that I have figured out. I’m not very
advanced in such matters. Is there any
easy way to add blips to the pattern say
at 12 kHz and or 15 kHz?
One way to do this is to connect a
series L-C tuned circuit (high Q) across
the input of the amp under test. Since
capacitors are higher Q than inductors,
a high value cap and low value inductor
would be best. I chose a 470 uH inductor (Mouser 580-22R474C, $0.59) and
0.374 µF capacitor for a 12 kHz notch.
All questions AND answers are
submitted by Nuts&Volts readers and
are intended to promote the exchange
of ideas and provide assistance for
solving technical problems. All
submissions are subject to editing and
will be published on a space available
basis if deemed suitable by the
publisher. Answers are submitted
by readers and NO GUARANTEES
WHATSOEVER are made by the
publisher. The implementation of any
answer printed in this column may
require varying degrees of technical
experience and should only be
attempted by qualified individuals.
Always use common sense and
Send all questions and answers by email to firstname.lastname@example.org
or via the online form at www.nutsvolts.com/tech-forum
78 June 2014