K8JHR wrote:
I would like to add a pilot light to the front panel of my TenTec
RX-320D. I have SOME home brewing skills... having built several
kits, including a few TT receiver kits, and having designed some
small project circuits on my own. My experience suggests doing this
is not as easy as merely soldering a LED into the circuit, and
sticking it through a hole in the front panel.
You need a power supply, an LED, and a current limiting
resistor. Typically, you use a push-in plastic bezel to mount the
LED to the panel.
The schematic shows that a nominal 15 V enters the radio, is filtered
with an RF choke and a 100 uF capacitor, and is available as the
internal 15 V supply. I'd mount the current-limiting resistor on the
PCB near C7 (one end to the +15 V PCB trace with a very short lead,
one end floating). A wire (22-24 ga. stranded, insulated hook-up
wire) would run from the floating end of the resistor to the anode of
the LED (the one with the longer lead and without the flat on the
diode body). Another wire (also 22-24 ga. stranded, insulated
hook-up wire) would run from the LED cathode (shorter lead, flat on
diode body) back to the PCB to ground (also preferably somewhere near
C7). If you want to get fancy, make the anode wire red and the
cathode wire black. Doing it this way (resistor on the PCB +15 V
trace), if something goes wrong with the wiring you won't
short-circuit the +15 V supply.
LEDs have forward drops between 1.5 and 3 V, depending on technology
and color. Assume ~2 V, so the resistor will have ~13 V across
it. Since I = E/R, a 3.3k resistor will regulate the diode current
to ~4 mA. Since P = EI, the resistor will dissipate ~0.05 W (50
mW). So, a 1/4 W resistor is sufficient.
Use appropriately-sized heatshrink tubing at the resistor (run it
right down to the PCB, completely covering the resistor) and on each
LED lead (again, run it right to the body of the LED). Route the
wires neatly and use tie wraps as appropriate.
Of course, do all the work with the radio off, and check it visually
for correct polarity and for solder bridges or other shorts before
you turn it on.
Note that there are several regulated voltages in the 320 (+5
digital, +5 analog, and +10). I did not suggest using them for two
reasons: first, the lower voltages increase the uncertainty of the
voltage across the resistor, and thus the uncertainty of the LED
current. And second, there is no need to add a further current
burden (with increased power dissipation) to the regulators. If you
measure the drop of the particular diode you use, you can calculate
the resistor for the desired current with the lower voltage, and such
a small additional load will probably not adversely affect the
regulator, so you could probably use a regulated voltage if you
prefer. If you do, I'd use the +5 V digital supply.
Best regards,
Charles
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