Studio switching solution?

[yestertech]

I'm after finding out if this is possible and some circuit ideas:
I want to switch off a mains powered device when a studio 'red light' comes on ( meaning mic live )
The red light is mains powered and contains a 15 W pygmy lamp. It is interfaced to the mixing desk via a 12v high to mains converter box. I need to do the switching 'non invasively' i.e without interrupting existing connections etc.
My thoughts are to somehow 'monitor' the current flow in the mains cabling, using inductive coupling (?), such that when the light is energised the albeit small current flowing can be detected, substantially raised in level and used to operate a NC relay to switch off said device. The 'add on' unit would be housed in a project box with mains in /out and self powered using a small internal transformer/wall wart.

How many turns of something would it be likely to take around a piece of standard 3 core 0.75mm cable to develop a suitable signal or is this a bonkers idea ?


Any thoughts most welcome ....Andy

[teetoon]

Hello, I was thinking of a hall effect sensor circuit as used in a volt stick ? Then feeding a relay ?
Just my first thought.
David.

[McMurdo]

You can get current detection relays if that's an option. You just pass the live conductor through the front mounted current transformer doughnut and set the front pot to the required current level.

[Refugee]

If the feed is twin and earth just add a bit of foil to the outside of the cable on the live side.
With the output of the bit of foil passed through a detector diode to produce a little bit of negative bias you can turn off a MOSFET that is biased on by a resistor of a couple of meg.
It will switch DC from 5 volts to 500 volts easily.

[mhennessy]

Yes, it's a bonkers idea.

Trying to sense current on the mains output of the interface box sounds like a nightmare from just about every angle (e.g., safety, complexity). And what happens when the red light fails? Or is replaced by an LED equivalent that draws a fraction of the incandescent bulb presently fitted? Detecting the field from the wire is similarly a non-starter - what if someone lays another mains cable next to it? You might as well tap into the existing mains and use it to control a double-throw relay with a 230V AC coil - but this relies on the red light interface box working (see below).

Instead, look at the non-mains side of the installation.

1. The output from the sound desk will just be a contact closure.

2. The input to the interface box will be the LED of an opto-coupler (plus a series resistor).

3. There will need to be a supply of volts to this - it might be a separate PSU, or it might be contained within the interface box.

Now, as circuits go, this is as simple as it gets. All you need to do is to add another interface box in parallel with the first. Or if you want to build something, you'll need an opto-coupler and series resistor - wire this in parallel with the same in the existing setup, plus a transistor to drive a relay on the other side of the opto-coupler. Job done - simple, reliable, safe.

Example interface box: https://www.canford.co.uk/Products/4...-SWITCHER-Mk.2 - it's just possible that this is what you are already using. The mk1 version of these are fairly unreliable, BTW. I have a dead one in front of me - one day I'll do a PM and work out if it's worth repairing the faulty ones we have at work.

I realise that this box has the inverse logic to what you require, but it could still form the basis of the project if money to buy another is available. All you'd need to add would be a double-throw relay with a 230V AC coil. But personally, I prefer low voltage relay coils on reliability/cost grounds...

For those who haven't come across this topic before, these interconnections are called GPIOs. General-purpose inputs/outputs. Strictly, the desk is a GPO, and the control box has a GPI. The inputs and outputs float, so you can wire them up in any way you need, and they are deliberately simple - which means they are easy to troubleshoot. You can make simple logic functions with them using nothing more than parallel connections and perhaps the occasional diode or two. For example, suppose you have more than one microphone in the studio: simply wire the fader-start switches in parallel before connecting to the mains interface box. Suppose you want a "red light recorder"? That can be wired in parallel with the input to the existing red light mains interface, just as I've suggested for your application. In the wild, the wiring for GPIOs is done on Krone frames, just like the audio circuits, and there is usually a central PSU for all functions. The play servers that we use have 16 GPOs and 16 GPIs - every one of those is used for things like fader starts, ready lights, RDS signalling, etc, in addition to red lights.

[GrimJosef]

Originally Posted by yestertech

... I need to do the switching 'non invasively' i.e without interrupting existing connections etc ...

When Andy says this I suppose it could be that he actually does mean it. If he does then no matter how 'bonkers' the idea of sensing the circuit's state remotely might seem, that is what he's going to have to do.

Personally I think that sensing the AC mains voltage on the lead, as per Refugee's suggestion, might be more effective than trying to sense the current. If Andy's not allowed to chop the lead open and single out the live conductor then the return current along the neutral is going to mean that the magnetic field will largely be cancelled. The foil on the outside of the lead, on the other hand, will have roughly the same capacitance to the live conductor as to the neutral and so will form a 50:50 capacitative divider (it might be more like 33:66 if it's a 3-core lead). Attaching an external circuit will put in additional capacitance to ground, especially if screened lead is used to avoid signals from other mains leads. But even if it's 1:100 that would still give 2.4V out of 240V and a 2.4V signal should be easy enough to detect.

Without wanting to get into too much detail, the best way forward might be to put a 'head amplifier' very close to the sensor foil. Then stray capacitance to ground could be kept to a minimum.

Cheers,

GJ

[Refugee]

I have got a capacitive hand held voltage detector from the 1970s and it only gives a reading if you place it against the live side of a bit of twin and earth and nothing on the neutral side. The device has a couple of transistors in it.
A length of wire a foot long clipped to the edge of the feed cable should work.

[GrimJosef]

OK, since writing the last post I've just been out and done the experiment. I wrapped a short length of 3/4" (19mm) wide self-adhesive copper tape right round a 3-core mains flex. I then connected a scope probe (1Mohm/95pF) to it and looked at the 50Hz pickup on the scope. With the live and neutral switched off the probe picked up 65mV p-p. Actually it picked up a good fraction of this even when it wasn't clipped to the copper foil, so that's largely just the background noise level in my workshop. When I connected the live and neutral to the supply the signal rose to 780mV p-p. It should be easy enough to distinguish between 65mV and 780mV. If more signal was needed then the tape could have been wound around a longer length of the mains lead. Or a sensor with less than 95pF capacitance to ground could have been used.

Cheers,

GJ

[mhennessy]

This approach would never, ever be done by any broadcaster, big or small.

GPIOs exist for exactly this purpose. GPIOs are trivially easy to work with. Off-the-shelf equipment exists to do exactly what the OP asks for, and will be certified to be safe for installation in a place of work.

Even if the OP has to build something because this is a small or voluntary setup with limited funds, then doing it the GPIO way will require fewer components than trying to sense the current or electric field, and will be much more reliable.

Incidentally, you might wish to consider that the existing red light interface box will almost certainly use a triac, and so will have snubber components across that. Hence there will be leakage. This might not result in much voltage across the 15W incandescent pigmy lamp fitted to the red light, but what about when that bulb is replaced for an LED equivalent? And hopefully the triac is in the live, not neutral, if you wish to sense the field.

Getting this home-made unit certified as safe - whatever its eventual MO - is vitally important. Hopefully that's a statement of the obvious, but you never know who might read these threads in the future.

Good luck!

[GrimJosef]

I really don't know the circumstances of Andy's job, so there's hardly any point in me speculating. But if I turned up at someone else's recording studio and said "It will be OK for me to mod your wiring a bit won't it ? I do know what I'm doing. You can trust me." then I can imagine that they might say "No. That's a rule we have here. No-one from outside is allowed to work on our kit.".

Cheers,

GJ

[mhennessy]

Then perhaps you'd be surprised to learn that this is what happens all the time where I work - virtually all installation work is done by 3rd parties, and a lot of the modifications are done by 3rd parties rather than local engineers (who are busy enough with their day jobs).

Documentation like method statements and risk assessments are supplied and signed off prior to work commencing. Change requests are raised, which include fall-backs and contingencies, contracts are signed, etc, etc. You wouldn't just turn up and start work. And if your proposal was to detect the electric field in a mains cable using home-made electronics when GPIOs are available to use with off-the-shelf equipment that is certified and supported, then you wouldn't make it past the tendering stage.

The companies that do this sort of work are usually called SIs (system integrators). For example, Dega, IPE and dB Broadcast. But something this simple might well be done by a contractor - though he or she will still go through the same processes. Indeed, that's the case even if it was done by staff. Due process sometimes feels like a PITA, but it exists for good reasons.

In the past Andy has given some background to this - IIRC, I believe that it's a very small operation that is perhaps run in part by volunteers, but no-doubt he can expand on the details if he feels that is appropriate, and I'm sorry if I've remembered incorrectly or developed the wrong impression. Anyhow, such a setup might not have the resources to appoint an SI or even employ a contractor, but that doesn't change the fact that the work has to be done properly and safely. I volunteer for our arts centre, and in-between running the audio for shows I do a fair amount of installation work for them. It's done to the same standards I would expect to see at work (luckily for them, perhaps ), and despite the tight budgets, there's no way I'd take chances with safety or install something that is not reliable - after all, our audiences have paid to see a show.

Sorry if any of that sounds a bit over the top for the OP's context, but it's the norm throughout the industry. I'm the first to be pragmatic where smaller (perhaps voluntary) operations are concerned, but it still doesn't change the fact that even on a near-zero budget, detecting current or fields for this application is utterly crazy. I accept that it's a bit of fun, and could work in certain situations, but here it is more complex, will use more components, cost more, and be less reliable. Sub-optimal in all ways.

And now I must depart with my calibrated measurement microphone and tune the processors on the PA system at the arts centre. There's no reason why their PA can't sound nearly as good as a studio monitor (it's almost there, frankly - this is just the final fine-tuning). Off-topic, I know, but shows that I practice what I preach - it might be voluntary work, but it's still done to the highest standards possible

Photocell next to the lamp? It would also tell you (in a roundabout way) if the lamp failed.

[GrimJosef]

Originally Posted by mhennessy

Then perhaps you'd be surprised to learn that this is what happens all the time where I work ...

I'm not surprised at all Mark. When I said I could imagine that some people might be restrictive about what could be done with their kit, that didn't mean that I couldn't imagine that other people might not. Clearly your employers are commendably flexible .

All I was saying was that Andy's statement about needing (his word) to do the switching non-invasively was unambiguous. So I assumed he meant it. But if that's not the case then fine .

I also have dealings with our local Arts Centre. I'm the technical rep for the town's film club which hires the place for a screening once a month. A few years ago, when they only had a small screen, we asked if we could help them source a larger one. They were polite, indeed their technical people were apologetic, but they said that the local council, who own the place, wouldn't allow anything to be installed for the long term which they didn't own and hadn't spec'd. This felt a bit arbitrary to us but, arbitrary or not, there was no way round it. Flexible they ain't !

Cheers,

GJ

[Andrew2]

Originally Posted by merlinmaxwell

Photocell next to the lamp? It would also tell you (in a roundabout way) if the lamp failed.

Ha, you beat me to it. Not only is it completely isolated, it would tell you if the lamp had failed too. As it would be looking for red light only, the photocell could be red-filtered to make it virtually blind to all other stray light.

the photocell could be red-filtered

Or blue/cyan filtered inside the red housing, even more discrimination.

[dseymo1]

Originally Posted by Andrew2

Ha, you beat me to it. Not only is it completely isolated, it would tell you if the lamp had failed too. As it would be looking for red light only, the photocell could be red-filtered to make it virtually blind to all other stray light.

Likewise!
As a general principle, if one is testing for a specific condition it's best to do so directly if possible.
In this case, the spec is 'when the red light is on', which is easy to test for.

[mhennessy]

Originally Posted by GrimJosef

I'm not surprised at all Mark. When I said I could imagine that some people might be restrictive about what could be done with their kit, that didn't mean that I couldn't imagine that other people might not. Clearly your employers are commendably flexible.

Well, as I said, it's the industry norm - I have contacts in many different broadcasters. The exact details might differ slightly, but it's all the same basic idea: operational requirements will always evolve and require changes to the infrastructure - no matter how much flexibility you try to build in at the start. These days, a lot of changes can be made by modifying the configs of the desks as - for example - a lot of the MML (Micky Mouse logic) that was traditionally done using GPIO wiring can be done by writing boolean equations in the config. Physical GPIOs are slowly being replaced by GPIO-over-IP, which in many ways in a shame. But I digress...

Anyway, the point of the process I outlined earlier is precisely to allow all of this to happen. Change is inevitable (cue the vending machine joke!), so it has to be allowed to happen in a low-risk, controlled fashion, with suitable contingencies in place, and clear lines of accountability.

Originally Posted by dseymo1

As a general principle, if one is testing for a specific condition it's best to do so directly if possible.
In this case, the spec is 'when the red light is on', which is easy to test for.

I agree with the first sentence, but here the specific condition is not "red light is on" - it's "mic is live". The red light coming on is a side-effect of that, but between the fader-open switch and the red light is an interface box which can fail (especially if it's the ubiquitous type I mentioned earlier) and a bulb which can also fail. I'd really not want to monitor the bulb, be it via the current that flows, the electric field that's generated, or indeed, the light it produces (nice bit of lateral thinking all the same ). Especially given that it's so trivially easy to connect to the existing GPIO setup - and certified devices already exist for exactly this purpose. (Sorry to repeat myself)

I'm really keen to hear back from Andy about this. Firstly, I'd love to know if/why he has ruled out connecting via GPIO. But also, I'm scratching my head wondering what bit of kit he needs to turn off when the mic is live. Of course, it's usual to mute any speakers in a studio when the mic(s) go live to avoid feedback or talkback or similar. But you don't do that by removing the mains from them - not unless you actively want to shorten their lifespan, and assuming they power up and down instantly and without pops and clicks. Away from lights, I really can't imagine what else you'd need to control this way...

[dseymo1]

I understand, of course, that in reality the required condition is probably 'mic is live', and agree with you if that's the case, but the OP specified 'red light on'

[GrimJosef]

Originally Posted by mhennessy

... But also, I'm scratching my head wondering what bit of kit he needs to turn off when the mic is live ...

Where I used to work it was quite often mobile air conditioners. We didn't like the way they disturbed the stillness of the air, but I can imagine that in a recording studio the noise they make could be the problem.

Cheers,

GJ

[red16v]

Perhaps it could be to simply switch off the loudspeakers in a control room when the mic is live to prevent howl round - who knows!

Coming from a broadcast engineering background myself, I fully concur with mhennessy's general comments. Hardwired GPIO is the way to investigate - it might be helpful if the OP could give us the model number of the sound mixer he is working with?