Stage two, is a switched break out box

Thread Starter

Revanvulkar

Joined Nov 22, 2021
14
Okay so my next task is a switched break out box (and that means what?) There is a Motorola XTL5000 installed in the bike and I need to be able to switch the headset between the UHF radio and the PA system.

I’ve been doing some research on how best to switch the audio signals (relays, BJT transistors, MOSFET, JFET) and now I have a migraine.

The four signals are audio left, audio right, mic+ and GND; this means that relays are out because they generate too much noise the distortion blocks the signals, so I was gonna go and grab some BJT transistors (4X NPN and 4X PNP) that would be hooked up to a DPDT switch supplying V+ On pole one and NEG on pole two.

Can I get some feedback please on what folks think about using BJT transistors? While having my oscilloscope plugged in I did see spikes as high as 10AMPS or more when loud noises were made through the mic but talking loudly or normally saw no real deviations.
 

Thread Starter

Revanvulkar

Joined Nov 22, 2021
14
how did you measure 10 Amps on your scope?



Why are relays out? Seems like the easiest, cleanest, distortion-free option (just like high end audio uses to pre-check shorted speaker wires).
I have a dual channel touch screen scope, it’s a workshop scan tool that has component identification and testing. allows AC coupling, up to 60 amps and blah blah, monitoring the circuit to identify EMI noise and I could see AC spikes and DC spikes. Once I make circuits I like using it to visually inspect circuit operations because thats usually when I pick up things I didnt think of or planned for.
 

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Thread Starter

Revanvulkar

Joined Nov 22, 2021
14
Where's the Schematic ?
I would probably use "Back-to-Back-MOSFET" Solid-State-Relays.
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Havent got that far yet, putting feelers out for component advice through you wonderfull folks before I dive into circuit design. I have a really good “sandbox” app which is where I do all my playing around in the virtual world. It’s were majority of my self learning comes from because I need to interact with components to really understand their functions.

I’ll run with your suggestion of MOSFETS and have a play with them tonight. The finished design will be printed on a circuit board then installed in a half case/ enclosure and filled with clear potting compound so visual inspection is still available should a fault happen, with Deutsch or Bulgin connectors into the housing to maintain watertight connections.

Thanks for your help I’ll post up a schematic when I’ve done one
 

RPLaJeunesse

Joined Jul 29, 2018
196
For simplicity and reliability I would use a good 4PDT relay. Make sure its contacts are gold and crossbar or self-wiping. Anything rated over 1A is probably too big and likely has the wrong contacts. Your "10A" reading is because of some oddness that definitely has nothing to do with the 4 signals involved. FWIW I did the audio for this: http://www.wb6nvh.com/CHP2009/CPVE002.JPG
 

RPLaJeunesse

Joined Jul 29, 2018
196
Just looked at page 131 of https://manuals.repeater-builder.com/mo-files1/XTL5000 Basic service manual 6881096C73-O.pdf and I'd say you have your signals wrong. Mic connector uses 2 pins for audio, 2 pins for PTT, and a different connector provides 2 pins for speaker. DO NOT in any way make a galvanic (DC) connection between speaker pins and mike circuit ground. Use a N:1 isolation transformer for the speaker signal if you want to use a 3.5mm connector that uses the signals called out in your original post (which ignores the PTT connection, by the way).
 
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