close proximity to a tuned circuit of the same frequency.
However, the operation of the circuit is such that the
meter shows a decrease in voltage.
I did this deliberately since most amateurs are used
to seeing and hearing about grid DIP meters, so I wanted
to keep the usage the same.
The first stage allows the user to adjust for the DC
operating point of the drain of Q401 using the offset
control. The second stage allows the operator to adjust
the gain of the circuit. My suggestion is that you begin
operation with both controls set to maximum and then
turn down the offset until the meter is at about 3/4 scale.
I have found that I always keep the gain at maximum so
it is entirely possible to replace the pot with a fixed
resistance of 10K.
Notice that I have not specified the value of R507.
This is because I do not know what meter may be used.
The meter in my unit (found in my parts supply and is
probably at least 30 years old) is 4 ma full scale, so I
used a 1.3K resistor. Since the maximum current is
5V/R507, this ensures that the meter will not be
damaged no matter what the adjustment settings. The
meter listed in the Parts List is 50 µA, so a value of 105K
would be appropriate.
The development of the oscillator circuit and
programming the PIC were both very much learning
experiences. Using LTspice to analyze the oscillator was a
first for me and was quite educational. I would
encourage anyone interested in building this project to
get the LTspice files and try it yourself. Learning circuit
simulation via computer program is a skill which you will
probably find very helpful if you want to do any linear
Although assembly language is not nearly as popular
as C or C++ for programming, I find it a good language
for smaller microprocessors. For one thing, it gets you
very close to the hardware. I have always liked assembly
since I started programming almost 45 years ago when it
was a necessity. The source code for the project is
available at the article link and is, I believe, quite well
These notes are not in any particular order ...
• Circuit Board ordering information will be available on
my website; email me for details.
• Coil Boards: When C2 is not used (shown as S in the
table), H2 must be shorted.
• Enclosure: I used epoxy to fasten 4-40 nuts to the
inside of the bottom piece of the enclosure so that I can
use bolts instead of screws to fasten the two pieces
together. This allows easier access if a battery is
• Enclosure: The connectors for the External Input and
Power do not have to be the ones I used.
• If you use the same method I did for the tuning
capacitor, you can use a 1/4” dowel to extend the shaft.
• Main Board: The final PCB is slightly different from the
one shown in the photos here and has some minor
modifications that make assembly easier.
• Main Board: Most headers and their sockets (H103,
H104, H201, H402, H501) do not need to be used — you
can simply solder the appropriate wires into the holes
for them. I like to use the headers since they make
assembly and disassembly much easier.
• Main Board: You may need to trim the sides of the
board near the top in order to not interfere with the
screws/bolts that tie the two halves of the enclosure
together. There are rectangles in the silk screen showing
the cuts that should be made.
• Main Board: R101 has its two mounting tabs cut off.
• Main Board: Use two stacked sockets for the display in
order to get it high enough to match the height of the
• Main Board: Note that all the SMD resistors are 1%.
This is because the 1% values are only 3¢ more per
quantity of 10: 18¢ vs 21¢.
• Main Board: Also, for the resistors, the Parts List
shows quantities of 10 for all the SMD resistors. This is
because ordering two is only 1¢ less than ordering 10.
32 July 2016