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 ----- > From: Wagner Lipnharski > To: > Sent: 16 July 1999 16:11 > Subject: Re: Lamp Dimming, DAC's and other Jiggery-Pokery > > > 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. > > 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?