Design an Open Source in-line volume meter

[SmittyHalibut]

The idea is that this device is a piece of test equipment, not needed for full time use. A club could have one for everyone to use to help them adjust their mic levels. Then, once adjusted, remove it from the circuit.

• Has two RJ45 sockets in parallel. It's just a straight passthrough.
• Uses power from Vcc and Power ground.
• Has a VU meter on the Mic level, with fixed set points.
• Has VU meters on Left and Right headphones, with fixed set points.
• Include a power LED, maybe with a volt meter (which is basically a DC VU meter).
• Include a PTT LED. Don't add an appreciable load to the PTT line, though. So control a MOSFET, or a comparator, or something similarly high impedance input. Don't just drive the LED off the PTT line directly.
• Bonus points: Test points to measure: Mic/MicGnd, HP:L/HP:G, HP:R/HP:G, Vcc/Power Gnd/PTT

[gx1400]

Love this, thanks for the consideration.

I think VU meter for headphones would be unnecessary, since those are are going to be set by the end user with a volume knob regardless.

While having this in-line does provide power, I was envisioning this to be a stand-alone device to test the headset side of the exchange exclusively. Including a 5.5 x 2.1 mm power jack would allow it to be standalone without touching a radio.

Suggest:

• mic is driven into a fixed resistive load (600 ohm? 1k ohm?) to set a consistent level.
• I like the power and PTT indicators.
• I could see the benefit of a cheap tone generator (even something as simple as an 555 timer with adjustable level just to verify headphones are wired correctly on the user adapter and functional). Maybe two tones, one for left, one for right. Bonus feature, add a pot to adjust the tone freq for left and right independent.
• thru-holes for an SMA or BNC connector for Mics, and HPs to simplify hooking up to a scope

[SmittyHalibut]

So, interestingly, the device you describe is almost exactly something I built in the very early days of developing OHIS. It's very large and expensive to build, and never worked properly (I screwed up several things in the design and never iterated on it. It was early in my design of VU meters.) So at the very least, I have a starting point.

I can see definite value in adding the DC bias source and load for testing the mic without a radio, and that's simple enough to add that I probably will. The oscillator and headphone driver are a little more involved, but I'll see what I can do. The sockets for o-scope connections are one of the things that made my original design so damn large. I tried to make everything edge mount on a single edge, and it turned into a 19" rack mount board. Having said that, I REALLY like the idea of being easily connected to a scope or signal generator.

tldr: I have a first draft of exactly what you're asking for already. I'll see about iterating on that.

[SmittyHalibut]

Just for kicks and giggles, here's that board:

User adapter connects on the left, Radio adapter on the right. Large pushbutton 8P2T switches next to both ports to select whether that port is connected to local test equipment, or through to the other port. Then all the visibility stuff in the middle: VU meters, BNC taps, some switches to select what a given BNC is measuring, a local PTT, etc.

[gx1400]

Opportunity for reducing real estate: use edge mounted SMA connectors.

I have about six other projects hanging that I need to close out before starting something new, but if you keep this ticket open, or open another issue for a standalone piece of headset test equipment. I can try to start a KiCad circuit with some ideas to prototype in the future

[SmittyHalibut]

RP-SMA!! Eww! ;-)

Believe me, I know all about edge mount SMAs. I literally have about 1000 in stock right now for various projects. :-)

Since most o-scopes are BNC, I figured BNC was the right connector for this board.

As for you developing something, by all means! Please do! What's the whole-repo version of "I accept pull requests?" ;-)