[arm-allstar] Radio-less allstar node

Sun Nov 17 10:16:04 EST 2019

Hi Don,

I recently built a radio-less node using a RA-42. It was a fun project. 
As with any such project I learned things and had to make revisions 
along the way to get the results I wanted. I am proud of how it turned 
out. Here are some notes that might help you. I will also post a link to 
some pictures in the near future.

The RA-42 doesn't have enough gain on it's audio input for a typical 
mic, so I used a SSM2167 mic pre-amp board ($3.80 with free shipping 
from China) to provide the Mic pre-amplification, compression and a 
noise gate. Note, the provided documentation with mine had the two 
controlling resistor functions switched -- Knowing which one is the 
compression and which is the noise gate is important! I removed the two 
tiny SMD resistors and replaced them with tiny trim-pots, which fit in 
holes in the PCB. I figured out the correct resistor functions by 
looking at the traces and IC pin functions.

I first tried some cheaper audio amplifiers, but even running off the 
unregulated 12VDC power input, before my regulator circuit, the 1W 
TDA2822M audio amps I first tried didn't have enough kick, so I ended up 
using the TDA2030A board ($9.26) and the best speaker I could find that 
would fit in my case. I extended both line and speaker level audio to 
rear panel jacks, with the speaker jack switching the internally mounted 
speaker.

I first tried using "buck" switching voltage regulators, but couldn't 
keep the switching noise off the DC supply and out of the audio, so 
ended up to using the LM338K Linear (non-switching) regulator, with a 
trimmer from my junk-box to adjust the output voltage. Since I used a 
Pi3B+, it really needed 5.2VDC at the Pi, even with thick-as-possible 
wires directly to the Pi GPIO DC pin and ground. Note the LM338K did 
need to be mounted on aluminum chassis panel for heat sinking. I mounted 
the Pi so it's ethernet and one pair of stacked USB jacks are accessible 
from the rear of my device.

The HMN 1053A Motorola Mic I like requires DC bias, so a 470 ohm 
resistor to +5VDC worked. I changed the Mic connector to an RJ45, 
crimp-connector and used a panel mount RJ45 jack. I connected the PTT 
wire from the Mic to the COS input on the RA-42. I connected the PTT 
output from the RA-42 to a front-panel LED, through a current limiting 
resistor.

My Pi was mounted such that the SD slot was inaccessible, so I found 
using the 10cm cable to extend that connector up to top of the Pi with 
double-stick foam tape, makes the necessary occasional card access 
considerably easier. Be careful--longer extender cables I tried first 
meant my SD card kept getting corrupted. The trick was the super-short 
FPC cable.

I did not want to have to extend the HDMI connector from the Pi, and far 
prefer a serial port to HDMI for emergency Pi console access, so I used 
a tiny MAX2323 3.3VDC to RS-232 adapter, and configured the Pi config to 
use the serial port for console, mounted on the rear of my small chassis.

I mounted a row of small toggle switches and LEDs on the front of the 
chassis, to show power, Pi activity, extended heartbeat and other LEDs 
from the RA-42 PCB, and TX/RX status. I also left a couple switches and 
LEDs for future use via GPIO pins. LEDs also indicate RA-42 COS (TX mic 
key) and RA-42 PTT (RX) state.

I found the USB connectors, jumper cable, and DB9 made the Pi and RA-42 
footprint much larger, so I removed these connectors and used pin 
headers and dupont connectors to interconnect the USB between the Pi and 
RA-42, and for the mic and amp wiring to the RA-42.

E-bay parts list, for things not found in my junk box:
-------------------------------------------------------------------------

Micro SD TF Memory Card Kit Male to Female Extension Flat FPC Cable 10cm
https://www.ebay.com/itm/122110484793

1pc. 3-inch 4-watt 8-ohm Amp Speaker by PUI Audio
https://www.ebay.com/itm/183684534474

LM338K 1.2-30V Adjustable Voltage Regulator 5A TO-3
https://www.ebay.com/itm/163627843571

DYNACO HAFLER OTHERS TO-3 TRANSISTOR SOCKET  (Used for LM338K regulator)
https://www.ebay.com/itm/283424931901

BIVAR 5MM LED HOLDERS, CR174, CLIP & RING, NEW (200 pcs)
https://www.ebay.com/itm/282855732037

Motolola DTMF moble microphone HMN 1053A

NEW - Assmann WSW Components A-RJ45KU-R Waterproof Panel Mount Jack - 
Set of 5
https://www.ebay.com/itm/163454129578

5 x 5K OHM TRIMPOT TRIMMER POTENTIOMETER 3296W 3296 (Used to replace 
resistor on SSM2167 PCB)

5 x 200K OHM TRIMPOT TRIMMER POTENTIOMETER 3296W 3296 (Used to replace 
resistor on SSM2167 PCB)

SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 
3V-5V
https://www.ebay.com/itm/332468614881

US Stock TDA2030A TDA2030 Mono 15W 12V AC/DC Audio Amplifier Board 
One-channel
https://www.ebay.com/itm/391757170167

MAX3232 RS232 Serial Port to TTL Converter Module COM Serial Board With 
Dupont Cable
(works with 3.3V Pi GPIO TX/RX lines)

Mark  Thomas (KC3DRE)

On 11/13/19 7:04 PM, "Don Ritchie via ARM-allstar" wrote:
> I almost hesitate to ask this question, but I’m a 77 year old ham, 
> that has been licensed for 59 years and am still learning.
> I am planning on making a “radio-less” all star node, using one of 
> Kevin’s RA-25 interfaces and
> have some confusion.
> Looking at the pin-outs for the 9 pin connector, some things seem 
> obvious,
> while others are confusing.The way I think I see this …
> pin 1 will be connected to an amplified speaker ( other side to 
> ground, pin 8)
> If this were connected to a radio, this would be going to transmitter 
> audio.
> pin 5, PTT to ground, ( pin 8) ( maybe through a current limiting 
> resistor ??)
> OR, because this is “turned around” would it go to COS (pin3) ??
> But where does my “audio in”, mic input and touch tone pad, go ?
> My choice seems to be pin 6, Discriminator audio, again in a “turned 
> around” world,
> OR ??
> I know this isn’t rocket science …
> But it seems as though nobody that has built one of these, has not 
> documented it very well.
>
> _______________________________________________
>
> ARM-allstar mailing list
> ARM-allstar at hamvoip.org
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>
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-- 
Mark G Thomas <Mark at Misty.com>, KC3DRE