Icom SM-20 User manual
QST®– Devoted entirely to Amateur Radio www.arrl.org January 2016 1
Desk Microphone
Power-On and
PTT Indicators
Know when power is applied to
your microphone, and when the
PTT is engaged.
Don Dorward, VA3DDN
Often one cannot tell just by looking at a
powered desk microphone whether it is
connected to dc power, or if the push-to-
talk (PTT) is engaged. You also can’t tell
if the PTT switch actually works, or worse,
if it has been accidently locked in transmit,
unless you are looking at the radio or the
RF output power meter. I decided to add
telltale LED indicators to my Icom SM-20
microphone that would glow in one color
when power is provided to the microphone,
and glow in a second color whenever the
PTT switch is engaged.
Guidelines
Don’t risk ruining the value of your micro-
phone — modifications should look pro-
fessionally done. Don’t risk shortening the
life of the PTT switch by running LED cur-
rent through it. Microphone PTT switches
are often just small tactile switches rated at
less than 20 mA. Don’t overload the radio
microphone power source; it is typically
rated +5 or 8 V dc at 25 mA maximum.
Finally, I kept my circuit small and simple
for anyone to build and install.
The Circuit
I built versions of my circuits using either a
two-lead bi-color LED, shown in Figure 1,
or two individual reverse-connected LEDs
with colors of your choice (see the sidebar
“Color Blindness in Radio Amateurs”).
I chose green for the normal power-on
condition, and red to indicate that the PTT
is engaged. This circuit requires a micro-
phone with a PTT switch that connects to
radio ground for transmitting.
QS1601-Dorward01
LED1*
(red) (grn)
R1
220 Ω
R3
270 Ω
R5
100 Ω
Kr Kg
R4
4.7 kΩ
R2
3.3 kΩ
C1
C2 0.01
D1
1
2
3
Q1
BC547B Q2
BC547B
16 V
2.2 μF
+8 V (5 V, R5=0)
(red wire)
To Radio PTT
(yel wire)
com/gnd
(blk wire)
1N5819
*Bi-color red/grn
LED or two
separate LEDs in
parallel.
C1 — capacitor, 2.2 mF 16 V tantalum
C2 — capacitor, 0.01 mF 50 V ceramic
D1 — Schottky diode, 1N5819
LED1 — (see text), bi-color 3mm LED Jameco
#94553 or Digikey #160-1058-ND
Q1, Q2 — NPN transistor, BC547B, 2N2222,
2N440, 2N3904, or similar
Figure 1 — Schematic diagram for the microphone PTT indicator. LED1 is external to the printed
circuit board and may be either a single bi-color red/green LED, or two separate cross-connected
LEDs. Connect the cathode for the red LED to Kr, and the cathode for the green LED to Kg.
R1 — resistor, 220 W5% 1⁄4W
R2 — resistor, 3.3 kW5% 1⁄4W
R3 — resistor, 270 W5% 1⁄4W
R4 — resistor, 4.7 kW5% 1⁄4W
R5 — resistor, 100 W5% 1⁄4W
Transistors Q1 and Q2 do the polarity re-
versal. R1 and R3 set the individual LED
forward currents. Normally Q2 is
ON
and
Q1 is
OFF
. When the cathode of diode
D1 is grounded via the microphone PTT
switch, Q2 turns
OFF
and Q1 is
ON
. Con-
nect the green LED cathode to the collector
of Q2 (point Kg), and the red LED cathode
to the collector of Q1 (point Kr).
R5 sets the LED forward current for the
level of brightness you prefer. Icom radios
provide +8 V to the SM-20 microphone,
so I set R5 to 100 Wto supply a nominal
10 mA of LED forward current. R5 can be
replaced with a wire jumper for radios that
supply only 5 V.
Building the Circuit
The choice of components is not critical.
You can build the circuit on a piece perf-
board or on a PCB like I did (Figure 2).
There is limited space inside the SM-20
microphone. My PCB measures 1.2 by 0.73
inches and fits nicely into a corner (right
side of Figure 3) of the microphone base.
Reprinted with the permission on ARRL. Copyright © ARRL
January 2016, QST p59
2 January 2016 ARRL, the national association for Amateur Radio®www.arrl.org
1www.arrl.org/qst-in-depth
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.
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.
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 1⁄8-inch shrink tubing over the in-
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.1
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
(7⁄64 inches) in a location of your choice.
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
Figure 3 — The PCB fits into the right side of the microphone base. Note the connections of the
red, yellow, and black leads.
Reprinted with the permission on ARRL. Copyright © ARRL
January 2016, QST p59
QST®– Devoted entirely to Amateur Radio www.arrl.org January 2016 3
For updates to this article,
see the QST Feedback page at
www.arrl.org/feedback.
Photos by the author.
ARRL member Don Dorward, VA3DDN, has
been a licensed Amateur Radio operator since
2002, with basic and advanced certification. He
graduated Ryerson Electronics Technology in
the Gerber files available at no cost to any-
one who would like them. There are also a
limited number of finished PCBs available
at my cost plus postage. Contact me by
e-mail for details.
1963. His career positions included manage-
ment of R & D, ISO9001, and ISO13485 Quality
Systems, Regulatory Affairs, in the areas of
technical support for electronic components
and materials, environmental testing and instru-
ment calibration, automotive electronics product
development, switch mode power supply de-
velopment, medical electronics, UL/CSA and
EU product safety testing and certification, and
EMI compliance. Don developed programs for
accelerated life testing methods such as HALT
and HASS, in-house training for Quality Sys-
tems, ESD prevention, and IPC Workmanship
Standards for the Acceptability of electronic
equipment. He is the inventor on two patents,
and retired in 2006. Don is a Life Member of
IEEE, Radio Amateurs of Canada, and Ten-Ten
International. You can reach him at va3ddn@
arrl.net.
Reprinted with the permission on ARRL. Copyright © ARRL
January 2016, QST p59
Additional Information
Note: Pin-out outline of
Q1,Q2, as shown on pcb
matches 2N4401 &
2N3904.
BC547B & 2N2222 may
require mounting 180 deg.
Please verify pin-out of the
transistors you have per
manufacturer data-sheet.
Please note, the illustration of the bi-color LED in the QST drawn circuit above is
incorrect. It should be as shown below:
Revised Jan 23, 2016VA3DDN
Kg
Kr
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