gets through its initialization.
The brightness control is read by the A/D and is used
to control the on-time of the segments of the LED display.
The program uses only the four most significant bits,
yielding 16 brightness levels. With the control fully
counter-clockwise, the display is effectively turned off. I
selected the values of the current-limiting resistors (R301-
R308) experimentally by seeing how bright the display got
using a few different values.
The display is multiplexed in that only one digit is
turned on at a time. The on-time is varied by the software
within an Interrupt Service Routine (ISR) based on the
voltage at the center of R101. I selected the update
frequency by viewing the display and increasing the
frequency until there was no flicker. In order to get the
display to the same height above the board as the
switches, I had to use two stacked 16-pin IC sockets. The
sockets have to be cut in half lengthwise since I could not
find any sockets the correct width for the display. Not all
the pins of the sockets are used — note the position of pin
1 of the display on the PCB (printed circuit board).
Oscillator (Schematic 4)
and Coil Circuits
I believe I spent more
time on this circuit than all
the others combined. The
oscillator is essentially a
configuration except for the
location of C402. I decided
to implement a source
follower using the same
transistor as the oscillator in
order to ensure that the
circuits which follow would
not load the oscillator.
For those of you who
are interested in circuit
synthesis, I have included
my files at the article link
for LTspice which I used for
analyzing the circuit. There
are two files which are
almost identical: one for the
low end of the tuning range
for a particular coil, and one for the high end. I
found that it was simpler to have both modeled
and viewed simultaneously so I could compare the
results. I made the circuit designators in the LTspice
models the same as those in the coil schematic.
Photo 2 shows the analysis of the low end of
the 8 MHz– 20 MHz coil. If you look carefully at
the lower left corner of the graphic, on the Status
line you will see that it indicates a frequency of
7.95 MHz. I used LTspice to determine values for
C3 and C4 that would work well and not have a
very long settling time. I did not try to find
“optimum” values since I’m not sure I would have
recognized them as such.
It was very satisfying to see that the circuit
analysis and the formulas discussed next worked
out (mostly) in the real world! I say “mostly”
because the upper two bands did not work out
quite as calculated; they were fairly close, but not
as close as the lower bands.
28 July 2016
■ SCHEMATIC 4. Oscillator.
■ SCHEMATIC 5. Coil.