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: > > 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...) 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 ) > > Thanks in advance > > Tom Feist > > Tombistiglobalnetcouk > > 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