Inverting analogue control, and scaling it

[pritch]

After much fiddling about and an exchange of e-mails with the incredibly helpful Keith at Zero88, I've got the Sirius desk we managed to inherit back up and working.

 

I've just been trying to figure out how to patch it into our dimmers, when I realised something that's going to make this somewhat awkward.

 

The desk will put out 0v to +5v, +10v or +15v

 

Our dimmers work on -10v to -15v, with -10v being fully off and -15v being fully on.

 

I'll be amazed if there is a commercial product that will do this for me. I'll be even more amazed if it's within our tiny budget. I know that there was a negative output board available for the Sirius, but I can't imagine they're easy to get hold of, and even then it'll only solve half the problem.

 

I'm therefore thinking about whether or not there's anything I could home brew. I'm wondering if op-amps are the way forward here, but I could do with a few pointers. I'm not even sure if the Blue Room is the right place to be asking this, or whether I should go and find some friendly electronics types on another forum.

 

The real answer is, of course, new desk and dimmers, but it's going to take us a while to get the money together for something like that. *sigh*

[Liam FitzG]

There is a negative output kit for the sirus available from Zero88 (as least I think they still sell them - we have a few in stock). Check with Keith again.

 

Liam

[bruce]

A basic inverter, with or without gain, is simple to build - you need an op-amp per channel, and 3 resistors.

 

I built one a few years ago, using quad op-amps and veroboard. But it went on Ebay a while ago...

 

To get the -10V offset requires a little more thought...

 

Edit: basic inverter circuity is HERE.

 

You may also find THIS THREAD relevant.

 

 

As a matter of interest, what sort of dimmers are they that need -10 to -15?

[pritch]

Thanks Liam, I'll be dropping Keith a mail shortly anyway, as I need some replacement fader caps and buttons for the desk anyhow.

 

Bruce, great links, thanks. I have a feeling that it's possible to use opamps to scale the voltage, I just wasn't sure about inverting it. I think you may well have the answer there.

 

As a matter of interest, what sort of dimmers are they that need -10 to -15?

 

Tongue in cheek answer: Very odd ones

 

Somewhat more serious answer: The desk and dimmers were custom built for us by an electrical engineering company, who specialised in control circuitry for off-shore applications. Apparently this sort of control wasn't too rare off-shore in its day!

[Liam FitzG]

How many chanels are you using? Four dimmers may be cheaper than a new desk if it came to it. It sounds like the dimmers are the odd part.

 

Liam

[Ynot]

The real answer is, of course, new desk and dimmers, but it's going to take us a while to get the money together for something like that. *sigh*

I will have a couple of 6-way racked dimmers that'll work off +10v that'll be going up on Ebay very soon. They're Eurolight dimmers, boxed with Lectriflex outputs, and have the standard Strand-type octal analogue signal inputs.

 

If you're interested, PM me for more info.

 

Tony

[Ian Ferguson]

You could also make them universal positive or negative inputs by putting a bridge rectifier on each control line

[Seano]

If you're going to end up making something, it would be worth looking a bit more closely at the control circuitry on those dimmers too. If you're lucky, it may be relatively simple to modify them to take 0 to -5 instead of -10 to -15.

[pritch]

Again, thanks everyone.

 

Seano: Nice idea about modifying the dimmers, would probably save some effort.

 

Everyone else: Yes, I'd really like to replace the dimmers with something else. Unfortunately, the money to replace the dimmers and do the rewiring of the dimmer bay that would go with it (the current dimmers are hardwired) isn't likely to appear any time soon.

 

Ultimately I want to run several dimmer packs and a DMX desk but, having spoken to a sparks about this, we're going to need a large amount of electrical work done before we get anything else installed.

 

That said, I'm not sure DMX is the way to go. I want the next person to make changes to have as many problems as I'm having, if not more. Can you get desks and dimmers that communicate using semaphore?

[David A]

I have got a box of inverter boards but they are in Australia and would take a week to reach you and you would need someone with electronic skill to fit a power supply and pin out connections so how urgent is your need?

[bruce]

You could also make them universal positive or negative inputs by putting a bridge rectifier on each control line

 

Can't see how that would work, unless you could guarantee that 0V wasn't a common ground? Maybe I'm missing something - can you explain?

[pritch]

I have got a box of inverter boards but they are in Australia and would take a week to reach you and you would need someone with electronic skill to fit a power supply and pin out connections so how urgent is your need?

 

Thanks for the offer, David. We don't need this sorted in a hurry; we've got a two preset board that's been in use for the last however many years anyway.

 

I think I've found a way to invert and do the offset with one op-amp and a handful of resistors, but I'm not 100% sure. I think I'll just solder a load of stuff to a veroboard and give it a go with a couple of channels before I go any further.

 

If it works, I'll post the results here so that anyone else in the same boat (unlikely, I know) can benefit. If it doesn't work... well, we'll see.

[J Pearce]

There's an inverter schematic here.

[bruce]

Jon - that schematic was referred to in the 3rd post in this thread...

[J Pearce]

So it was.

 

I blame the endless paperwork and online forms; does odd things to your mind.