>>>READER-TO-READER QUESTIONS AND ANSWERS
Automated Pitch Control Of
Canard Wing On A Race Boat
I'm trying to figure out the best
(and cheapest) way to control a wing
(see diagram) on a race boat. It needs
to adjust the wing's angle of attack
against the boat's angle of attack given
varying speeds. I'm also trying to fit all
the mechanicals within the wing itself.
Servo speed and accuracy is very
important as things can go very wrong
very quickly at racing speeds.
#12089 Tom Nusbickel
Santa Ana, CA
Inverter Not Working
I have an inverter with a megatron
card that doesn't work. I’ve measured
the oscillator voltage and it’s okay.
What can I do?
Pocket PC Interface
I’m looking for a circuit to
interface my Pocket PC through the
SD slot. I am trying to come up with
an interface to use my HP iPAQ for
many different applications ranging
from data aquisition to robot control.
Also, regarding the data sync
cable and power charging, I have a
22.50V solar cell, max output 120 mA.
I would like to build a power supply
that connects the USB sync cable
from the Pocket PC to the solar cell.
I'm not looking at using the data lines
on the USB, only the power source.
What would be the best circuit design
to drop the supply voltage down to
the normal voltage that the USB port
from my desktop PC provides for
charging the Pocket PC?
#120811 Richard Forkner
Grand Junction, CO
On Air Digital TV
I am very confused. After the
switch to digital TV, can I use the same
antenna I am using now for analog
reception, or do I need to modify or
replace it to receive digital signals? It
seems that everyone thinks that all the
signals will be using the UHF band
and are selling UHF antennas. Is this
true or will some of the stations keep
the frequencies they are using now? Is
there a frequency listing of the stations
after the change over in February? I
was under the impression that the
VHF/UHF antennas that are being
used now would work for HDTV, but I
get conflicting advice regarding this.
Can someone please give me the
[#9085 - September 2008]
I need a way to find a break
and/or weak connection in a radio dog
Preferably some kind of signal
injector with a radio signal strength
meter that can help pinpoint breaks or
poor splices in a wire.
#1 There are several ways to
pinpoint the problem in the wire. The
easiest approach is to use a DIP meter
in absorption mode or an absorption
wave meter to find the spot where the
signal gets weak (the absorption wave
meter needs a "sniffer coil"). The
other way is to modify a receiver and
add a signal strength meter to it.
Another possibility is Time Domain
Reflectrometry (TDR), which is
frequently used for network cables
UTP and STP, unshielded twisted pair,
and shielded twisted pair applications.
But, it will work on coax, as well (it
should work on any medium). The
break, short circuit, or any discontinuity such as a change in line impedance
will show up as a reflection since the
antenna cable forms a "coax cable" of
some sort with the surrounding
ground. In theory, it will allow you to
pinpoint the break if the propagation
velocity is known. The soil conductivity
varies widely (more than three orders
of magnitude) which will affect the
impedance, but the approach works
well on coax, STP, and UTP. Joseph J.
Carr has a whole section on TDR in his
book Secrets of RF Circuit Design. The
DIP meter and sniffer measurement is
well described in Chapter 9 of the
ARRL Data Book, where you can find
a circuit for an amplified wave meter.
You may have to build an additional
coil for the DIP meter, because most
DIP meters only go up to 250 MHz
and your dog fence is probably higher
than that. I have seen absorption wave
meters go up to 450 MHz.
Walter J. Heissenberger
#2 Before doing a lot of testing, it
might be wise to replace the batteries
in the collar and transmitter and, if
possible, test the protected area with a
second collar receiver.
As we don’t know the operating
frequency and modulation scheme of
the original transmitter, a test “
transmitter” could be used. The matching
test receiver needed could be a simple
battery-operated AM radio tuned to
an unused part of the broadcast band.
The test transmitter in this case could
be a lab VFO (Variable Frequency
Oscillator) that covers the same
frequency band (around 1 MHz) and
has AM modulation (a 1 kHz tone, for
example). If you are unable to borrow
this gear, you can construct a very
simple VFO or even a crystal
controlled oscillator. Modulate its
power supply with an equally simple
555 IC type astable oscillator to create
your very own low power radio
beacon. Either way, the transmitter
signal must be made weak so that the
test receiver (radio) is not overloaded
or pushed in to its AGC limiting.
Start by removing the original
transmitter and attaching the dog
fence antenna to the signal generator.
The radio should easily pick up the
signal near this connection. Reduce
the signal level as needed so that the
radio cannot detect the signal as it is
moved by hand away from the
transmitter connection. To aid in
monitoring the signal level (sound
level in the radio’s speaker), try connecting an AC voltmeter (or DMM)
across the radio’s speaker terminals.
By following along the dog fence
boundary cable, any damage (breaks,
weak connections, or shorts) would
December 2008 93