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'Lamp Dimming, DAC's and other Jiggery-Pokery'
1999\07\16@104710 by Feist Family

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Hello all.

Just recently, I have been asked to try and build a computer system for
control of some stage lighting.  Having spoken to a friend of mine who is
reasonably knowledgable in the field, he thinks that the best idea would be
a PIC to interface to the PC serial port, and output to a DAC connected to
the low voltage control side of the lights.  As I understand it (being a
student and all...<g>) the analogue output of the DAC would be a variable
voltage. The way in which the existing system works is to have a main switch
block (no idea whats inside it) with a Low voltage control panel connected
to it. The panel is just a set of variable resistor sliders as far as i can
tell.
Anyway, back to the bit i'm confused about...depending on the internal setup
of the lighting controller, would a variable voltage be compatible with a
variable resistance?
If not, is there a simple way to control what is effectively a variable
resistor from a PIC? (low cost solutions would be nice, as speed and
accuracy isn't really that important, and it needs 30 channels)
Feel free to point out any errors in this, as I don't have much experience
with electronics other than A Level physics (well, I suppose I have learnt
to pick up the soldering iron by the cold end <g>)

Thanks in advance

Tom Feist

Tom<AT>bisti<DOT>globalnet<DOT>co<DOT>uk


PS. I think there are probably nice simple ways of doing it - a bulb of
varying intensity by mark-space ratio and an LDR instead of the control box,
but I really have to justify to my parents why they're buying me the
PICstart pack, and I'm not sure what to start with yet....so I'd like to use
a PIC if possible <g>

1999\07\16@111215 by Wagner Lipnharski

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A TRIAC device triggers by a specific voltage at its gate. Once the
triac is gated it short circuit itself as a switch. It stays conducting
while exist current through. The AC line switch phases 120 times per
second, so if the triac receives a pulse at its gate, it would enter in
conductance status and still conducting until one of those "120 phase
changes per second" happens.  If you keep the gate voltage constant, the
triac will still conducting (in real it restart conduction 120 times per
second).

The DAC output is a variable voltage.  Suppose you rectify (without
capacitor filtering) the extremes of a center tap coil of a 6Vac
transformer, to have only 120 positive semi-senoids. Then compare this
120 positive half senoids signal to the DAC voltage output, it will be a
time period when the AC voltage would be higher and then lower. If you
increase the DAC output the "AC higher time" would be shorter, if you
reduce the DAC output voltage, the "AC higher time" would be longer.
The output of this comparer circuit (LM339 for example) could be
directly tied to the triac gate, so the time the triac would be
conducting would be longer or shorter, controlled by the DAC voltage
output, controlling the lamps power.

This is the general idea.  Not exactly the final circuit.
Wagner.


Feist Family wrote:
{Quote hidden}

1999\07\16@123207 by Harold Hallikainen

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       Theatrical lighting control is generally either 0 to +10V analog
control (a wire per channel) or DMX512.  DMX512 sends up to 512 lighting
levels with each level represented by an 8 bit value.  The data is sent
at 250 kbps using EIA422 balanced line transmission.  Because of the
speed required, a PC serial port is generally not fast enough (they won't
go 250 kbps).  There ARE some systems that receive data from a PC serial
port and convert to DMX, but these are limited in the number of channels
or speed.
       I've designed several lighting controllers (and dimmers) based on
the PIC16c74.  One of these (the StarPort) takes data out of the PC
parallel port and converts it to DMX (actually, there's continuous
transmission and reception of DMX to a buffer RAM in the StarPort.  The
PC then reads and writes this buffer).
       For more info, see http://www.dovesystems.com .

Harold



Harold Hallikainen
spam_OUTharoldTakeThisOuTspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

On Fri, 16 Jul 1999 15:43:25 +0100 Feist Family <.....bistiKILLspamspam@spam@GLOBALNET.CO.UK>
writes:
{Quote hidden}

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1999\07\16@124230 by Feist Family

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----- Original Message -----
From: Wagner Lipnharski <wagnerlspamKILLspamEARTHLINK.NET>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: 16 July 1999 16:11
Subject: Re: Lamp Dimming, DAC's and other Jiggery-Pokery


{Quote hidden}

Thanks, but what I was actually looking for (apologies if I worded the first
post wrong) is just the variable
resistor bit.  I already have the power switching gear, and I don't really
want to start working with high
voltage/current for a first project.  The PIC is only there to replace one
of the control sliders.  From talking
to someone who works with stage lights, at switch on they draw 100+ amps,
which tends to fry the triacs
of homemade systems...no idea how the commercial ones work, but to me it
makes sense to use the
stuff I already have rather then start from scratch.   What would be helpful
is any information / websites
that deal with PC serial port (RS232?) interfacing to pic.  Just flicking
through an electronics catalogue
now, there's a digital volume control made by Dallas which might be
usable...any ideas/thoughts?
Are there any other commercial digital-analogue variable resistors
available?

1999\07\16@132953 by Adam Davis
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The only problem with using the digital pot is that it has a limit to the
current what can be drawn through it, and that limit is fairly small.  You'll
find that the dimmers don't need much current, but through the patch board you
could have several dimmers through one pot.  You'll note that the variable
resisters on the console either have large carbon tracks, or are wirewound.

The kind of system you describe sounds like the systme I worked on in HS.  The
dimers were all 0-10v, and we had a huge console with a thick cable going down
to behind the stage for the dimmer racks.  While it is best to make, use or buy
a DMX-512 to analog converter, that is not always an option.

What you'll end up doing is either
1) Use a DAC and circuitry to support the DAC so it goes between 0 and 10V, and
can handle the current
2) Use the PIC to fake a DAC through pulse width modulation.  The idea is that
if you switch the logic gate on and off at a high frequency, say 100kHz, and
have it on only half the time, and off the other half, then you'll end up, after
some filtering, with 2.5V out.  If you use a transister to switch a 12V source
on and off the same way, you can easily go between 0 and 10 volts by changing
the on to off ratio appropiately.  This would be fairly easy to implement with a
PIC.  I don't have any good references on implemention PWM with a PIC though,
but there is a lot out there.  Anyone have any relevant resources?

Then you need to find the input of the dimmer.  I never did look at the actual
schematic of the dimmer console, but one way to do it is to tie one end of the
pot to ground, the other to +10v, and the wiper will show between 0 and 10
depending on it's position.  The wiper would then go to the dimmer box.  This is
an inefficient way, mind you, but you wouldn't believe some of the things they
did 'in the old days'. ;-)  But the console I'm speaking of was made eons ago.

Either way, you'll still need some circuitry to protect and pump the signal up a
bit, current wise.

I hope this helps!

-Adam

Feist Family wrote:
>
> {Original Message removed}

1999\07\18@111623 by paulb

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OK then, noted that you want an emulation of the 10V analogue control
voltages.  This should be quite easy as you generally don't need very
fast changes (faster than say, 100 ms).

 PWM or duty-cycle modulation is definitely the way to go.  Take the
output from the PIC for each channel, filter it by a simple R-C filter
(perhaps two-pole) with a calculated cutoff of about 20 Hz, then apply
this to an op-amp driving a PNP power transistor (Darlington perhaps)
supplied at 12V. The op-amp multiplies the 0-5V signal generated by the
PIC by 2 using a 2:1 feedback divider.

 You'll have to scale the power transistors according to the load
provided by the dimmer bank (each channel).  Presumably the input
impedance is purely resistive and returns to ground, so can be checked
with a standard multimeter on a resistance scale and confirmed by
hooking up a variable supply and measuring the current drawn at 10V.

 If you have many channels, or you want to "remote" all this from the
PIC, you may wish to use latched shift registers as output expansion
from the PIC.  Mmm, maybe that's another story!
--
 Cheers,
       Paul B.

1999\07\19@132037 by Harold Hallikainen

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On Mon, 19 Jul 1999 01:13:44 +1000 "Paul B. Webster VK2BZC"
<EraseMEpaulbspam_OUTspamTakeThisOuTMIDCOAST.COM.AU> writes:
>OK then, noted that you want an emulation of the 10V analogue control
>voltages.  This should be quite easy as you generally don't need very
>fast changes (faster than say, 100 ms).
>
>  PWM or duty-cycle modulation is definitely the way to go.  Take the
>output from the PIC for each channel, filter it by a simple R-C filter
>(perhaps two-pole) with a calculated cutoff of about 20 Hz, then apply
>this to an op-amp driving a PNP power transistor (Darlington perhaps)
>supplied at 12V. The op-amp multiplies the 0-5V signal generated by
>the
>PIC by 2 using a 2:1 feedback divider.

       Any reason to use the transistor?  An op-amp will typically
output 20 to 25 mA, which is generally enough for 0 to 10V control of
light dimmers.

>[deletia]

>  If you have many channels, or you want to "remote" all this from the
>PIC, you may wish to use latched shift registers as output expansion
>from the PIC.  Mmm, maybe that's another story!


       I used a single PWM output of a 16c74 and demuxed it out to 48
channels.  A single resistor formed the R part of the RC filter (between
the PIC PWM output and the demux input).  Each demux output drives a
capacitor to ground which serves as the C in the RC filter and also
serves as a holding capacitor in a sample and hold.  This is followed
with a noninverting op-amp amplifier (gain of 10 or so to allow for
higher control voltage outputs... there's also a pot as a voltage divider
reducing the PIC PWM output voltage as necessary between the PIC and the
previously mentioned R).
       That's the basis of the 48 channel DMX decoder at
http://www.dovesystems.com .

Harold




Harold Hallikainen
haroldspamspam_OUThallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\07\19@184423 by paulb

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Harold Hallikainen wrote:

>   Any reason to use the transistor?

 Lack of knowledge in advance of the load impedance ;-)

>  An op-amp will typically output 20 to 25 mA, which is generally
> enough for 0 to 10V control of light dimmers.

 Hats off to you.
--
 Cheers,
       Paul B.

1999\07\25@181548 by mkinga

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Is the system you're controlling made by a company called "Zero" ?
Alternatively can you tell whether it uses 0-10v dimming (One of the
industry standards) If so there are chips available that will do most of
the hard work for you, try http://www.artisticlicence.com , they do a range of
stuff that may be useful to you.

Martin.

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