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Single bi-colour LED signal present / overload line level meter, adjustable

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New member
Jun 17, 2012
I built my second GSSL bus compressor and decided to incorporate a single bi-colour LED signal present / overload meter on the inputs to save space on the front panel.  Not being able to find anything like this on eBay, I decided to design and build one myself.  Here's the result:


It uses four quad comparators to measure both sides of the AC cycle for both left and right channels, for the overload and signal present indicators, the LED switching controlled by a logic gate. 

The threshold for the overload indicator is adjustable with a single trimmer, and there are two switchable sensitivity modes which cover standard line level equipment, and attenuated for hotter inputs up to around 25dbu. 

As left and right channels are measured separately, there are no issues with summing - an overload on left or right or both with trigger the indicator at the same level.  The meter can also be run in mono by connecting just the audio to left or right input and grounding the other.  The overload reference voltage set by the trimmer is mirrored + and - via an opamp, so the meter is very sensitive to sudden peaks in either polarity, and should be very accurate.

It runs from a single DC supply of at least 12V with a current barely larger than that supplying the LED.  Despite a lot of filter capacitors, the power supply should be fairly clean to prevent accidental triggering of the 'signal present' indicator.

I have several spare PCBs and components so I can put together a few more and can sell pre-built for £40 (not including postage).  This includes the LED (and mount), but the buyer would need to solder this onto the PCB themselves if they wanted flexibility in placing it in a chassis.

I'd be happy to sell the PCBs for £10 each, or provide the schematic / gerber files for anyone who would like them for free.

Please PM me if you're interested.

Jez Burns.

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