Original Prototype
The original prototype worked fine. clear loud audio…. and one size fits… one… cabling. I would have to reconfigure wiring for any radio – and currently that would require a soldering iron. On top of that, it’s a squid box… cables just dangling out. Not ideal for flexibility. So let’s fix that.
First Steps
I decided that the best way to handle this was to design a PCB, and include standardized connectors, as well as some way of reconfiguring without changing cable harnesses. Enter KiCAD. I had it installed from a project last year, but I didn’t really take it to the finish line on that effort, because requirements changed. KX0STL pushed me enough to re-learn it and use more of its capabilities (note – it’s a LOT for a free program).
Based on the initial run, my initial requirements included:
- RJ9/RJ22 “4P4C” modular connector for the headset
- PTT connector, RCA connector
- Radio Interface… something modular and common. I settled on an RJ45, because it could be a straight Ethernet cable to many radios, and it’s easy enough to get an Ethernet cable to cut in half if I needed to build something custom for something like an XTL or APX mobile.
- This led to the need to add a secondary audio input, preferably a 1/8″ mono jack – to connect audio from a radio which may not have audio on the RJ45 microphone connector (and use straight-through cables).
- Provisions for a PJ7
- Ready to install in some form of off-the-shelf housing, with minimal needs for cutting or customization
- Reconfigurable for different radios
- For radios using the RJ45 connector, I wanted an internal jumper system, like some of the digital interfaces (MFJ) have used in the past.
This really wasn’t a bad list. I wanted the ability (for a radio like my Icom or a Yaesu) to simply use straight-through off-the-shelf cables for radio connection, because they’re easy to replace in case one gats damaged. The reconfigurability was something I had to work on, though. I ended up coming across a DIP-sized “Header” which has prongs to solder wires and components to, which would allow me to build jumper sets for a specific radio, and just drop in the correct plug for what I need. Additionally, I think I can use breadboard jumpers to configure as needed, omitting the plug-in carrier.
With this in mind – I set forth laying out my first PCB layout:
My initial thought was to use a 4 port surface mounted Ethernet jack enclosure, available from many sources online – but I realized that none of those have provisions for mounting a PCB. I could have come up with some way to make it work, but those types of enclosures can change without notice, so long as they still support the keystone jacks they’re designed for.
To counter this, I started searching for enclosures, and found one on Hammond – it had provisions for a PCB, and looked easy enough to punch out openings for connectors (image is a link to the manufacturer’s site)
Since Hammond is good enough to include engineering drawings for the enclosure, I could create a blank PCB (with a lot of attention to detail) that fits the boundaries of the box. The KICad files are included below
This footprint allowed for a lot of options for connector layout. I went through a number of possible options, before settling on “input on one side, output on the adjacent side” layout. I did measure the PJ7 connector from Switchcraft, and determined it would fit – but just barely. For this effort, I would, however, need to make some changes. Here’s the board:
The connectors are labeled. RV1 is an optional potentiometer to adjust audio volume on the headsets.
The broad lines over the hole mark the boundaries of the PJ7 connector. If a PJ7 is to be used, J3 and J4 are not populated, in favor of wiring the PJ7 to the J5 header connector.
I did make a mistake – the J6 pins are not labeled here… I need to fix that.
- Pins 1 – 8 correspond directly to J2 pins 1 – 8
- Pins 9-10 correspond to the speaker input tip and shield
- Pins 17-20 correspond to pins 1-4 of the RJ9 jack for the headset
- Pins 15-16 correspond to the shield and tip (respectively) for the PTT connection
- Pin 14 is not used
- Pins 11 – 13 are routed to the potentiometer, if used.
Originally I had planned on an RCA jack for PTT, but I opted for a 1/4″ mono headphone jack. This allows a Heil footswitch to be used, or simple audio adaptors to be used to change to 1/8″ or RCA cords (I use an RCA plug, but many off-the-shelf cables also have 1/8″ plugs). These audio adaptors also seemed to have the most mechanical strength if I needed them, compared to other options.
Here’s the final schematic
And the really cool part… KICad includes a rendering plugin. With KX0STL’s help in selecting models, we got a great image of what the final product should look like:
I ordered board from JLPCB a couple days ago… just for fun, I had them done in Red FR4, so they’ll look a bit different, but I’m looking forward to populating the first one to see how this all goes.
The final cost of parts is about 20 dollars, plus shipping. The PCBs were much cheaper ($12 for 20 boards)- though the shipping on those was $20 by itself. Still not a bad BoM price, when you consider that the Plantronics SHS series PTT buttons are upwards of $300 new.
More to come when I get the final product built.
Next revision… constant improvements!
It is not a major functional change, but one change I plan on making is to arrange connections to place the RJ45 facing inward, with a cutout for cable routing. This would allow the radio cable to either pass out of the casein the side, or, if it makes an install cleaner, through the bottom of the case. At this point, I’ve decided that version 2 will eliminate the PJ7 connector for now. This allows more real estate for the circuit, and better allows for cable routing in the box, if I wanted the connectors to be inward-facing for the radio interface.
One thing I noted that I had forgotten was labelling for the jumper pins, so I added that. I may still add that to the version 1 design (for future revisions).
I may put together a V1.1, which would have the PJ7, and add labelling for the interconnect. Right now, I think this is a reasonable branch at this time… V1 can be either, V2 is RJ9 only, and then V1.1 (or V3) would be PJ7 (only?). At the moment, I just don’t see the PJ7 as a necessary option for my purposes.
At this time, the entire design is intended to be passive. I have noted that some portable radios don’t have enough drive for the headset. A mobile radio will drive the headset well, because it has sufficient drive to excite the 150 ohm earphone with little trouble. A portable doesn’t have enough drive from the 8 ohm source. Time to find some low power amplifiers – it’s been a while since I used an LM386. That will mean the whole system will need power, which does present issues with a portable, but most mobiles have some level of power available.
More to come on that front. For now I’m still waiting on the PCB to arrive, so I can build up V1 (modular connections only – no PJ7).