Icom SM-20 Instructions page 2

Reprinted with the permission on ARRL. Copyright © ARRL
January 2016, QST p59
Figure 2 — PCBs for a pair of colored LEDs (top) and a single bi-color LED (bottom)
connect to the microphone by the red (+8 V), yellow (PTT), and black (ground) wires.
Figure 3 — The PCB fits into the right side of the microphone base. Note the connections of the
red, yellow, and black leads.
Color Blindness in Radio Amateurs
I was proudly showing off my single red/green LED PTT indicator to a fellow ham,
who pointed out to me that the single LED changing colors would be quite useless to
him, as either state would look the same! He is color blind. I was quite surprised, and
after a bit of research I learned that:
(1) up to 8% of males may suffer from red-green color blindness,
(2) color blindness to blue or yellow is less prevalent.
There are presently about 730,000 licensed Amateur Radio operators in the US.
If we assume that 75% of them are males, then there may be up to 44,000 hams
out there who would not appreciate this bi-color LED PTT indicator!
However, by using two LED indicators slightly spaced apart, one green and one
red, or one blue and one yellow, color blind hams can still make sense out of it, the
same way they do with traffic signals while driving, that is, by position or location of
the illuminated LED.
2 January 2016 ARRL, the national association for Amateur Radio
LED Preparation
The LEDs are connected to the PCB with
flexible #24 or #26 AWG insulated wires
(Figure 2). Keep track of the bi-color LED
leads to be certain which you should con-
nect then to get the colors right. If you get
it wrong, just reverse the leads. I shortened
the LED leads to about a half inch, then
slipped ⁄ -inch shrink tubing over the in-
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8
dividual red and green connection leads,
and finished with larger shrink tubing over
both wires.
If you opt for two separate LEDs, bend the
leads at right angles about 0.2 inches from
the LED body, as seen in Figure 2. Next, po-
sition both LEDs with their leads pointing at
each other in a soft-jawed vise at the desired
distance apart. I used 0.75 inches. Tack the
leads together with a drop of solder, then
position and solder the color-coded red and
green wires to the respective cathodes. Use
a drop of hot-melt glue to cover and secure
the solder joints. See the QST in Depth web
page for additional details.
LED Mounting
I strongly recommend using the small
and unobtrusive 3 mm LEDs. They can
be mounted with just the dome show-
ing. Make sure the area beneath is clear,
and drill a small LED mounting hole
( ⁄ inches) in a location of your choice.
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64
Carefully enlarge the hole with a small
reamer or round file for a snug fit on the
LED. Then use a drop of hot-melt glue
from the underside to secure it in place.
This is the only mechanical modification
needed to the microphone.
Connections to the Microphone
I color-coded the wires between my PCB
(Figure 3) and the microphone. The Icom
SM-20 microphone uses Pin 2 for +8 V
(red wire), Pin 5 for PTT (yellow), and
Pin 6 (black) for chassis ground. This may
vary according to your microphone and
radio combination. Use an ohmmeter to
verify continuity from the eight-pin round
plug to the solder lands on the microphone
internal PCB.
Printed Circuit Board
My printed circuit board layout and ad-
ditional construction details are available
on the QST in Depth web page. I will make
1 www.arrl.org/qst-in-depth
®
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