This is a more oldskool design using conventional analogue electronics and relays. Tthe transistor switch and relay is used is so that many LED displays can be switched from a very small signalling current, and use of a PNP transistor means that the relays are triggered by putting an earth on the signalling wire.
apologies for wonky hand drawn circuit as I was doing it in a rush at work earlier whilst waiting for some files to copy, and I find it quicker than using any computer software
There is another circuit (not shown in diagram) that is the same as the red light circuit but switches an amber light. Those are connected to the relay contacts from the mixer. The blue/green light is triggered by the toggle switch that switches the audio for the headphones, eventually I am going to use a relay for this and have it set up so instead of the toggle switch this feed can be flipped by pressing a button either near the main desk or the midi controller (but I have to remember how to make a bistable circuit which I have not done for about 25 years)
Although I previously was switching the 12V to the LEDs directly, I decided to build this as it has the advantage that if a cable comes adrift, the worst case is there is a false or random signal shown (or none at all) rather than studio battery being shorted via the mixer relays – it wouldn’t do them or the channel strip PCB much good, and fixing that means taking out the whole channel strip and working on it at component level, as it is I managed to blow 5 1A fuses in a row the week before last as I’d got myself confused with my own wiring colour scheme and had also put the breadboard and the studio battery feed in an awkward confined space below the console
Eventually the finished version will have those units in boxes rather than on the metal shelf), when I find the time and space to make up the soldered version.
The relays are “British Telecom Type 47” – widely available worldwide but beware of some models that may already contain the flyback diode, this makes them polarity sensitive which confused a lot of people who understandably don’t expect a relay on its own to be polarity sensitive..
