Slow, Automatic Light Dimmer

OK 407. So. D'yer know how a dimmer works? Go find that bit out. Then you'll discover you need a ramp on the control voltage what triggers the SCR (you _do_ know about the SCR?) That ramp is gonna be real, real ... slow. So you need to figure about high value resistors and low leakage capacitors.

Am I going too fast?

PN2222A fsubt 250MHz.

Um, yeah, I do know about the conduction angle for firing an SCR to control the rms voltage you jackass, you don't have to patronize me. FYI, I focus on large power systems for my degree but I was hoping to start exploring electronics a little more, thanks for all the help.

PN2222A wrote:

How about a binary counter with a R2R network on the output, and use the FF state of the counter to inhibit the clock input.

It will be better than a RC type time constant. The clock could be mains derived. One of the problems is that the normal incandescent lamp does not have a linear relatioship between volts applied and light output

martin

Thanks Martin, I'll look into it. I really appreciate sincere help.

mart> >

It depends on what kind of effect you are trying to achieve, the timing is not so much an issue as setting the triggering angle. If you want the brightening ramp to be barely perceptible ("slowly") then there is something called the Weber-Fechner Law which relates actual light output intensity to changes to human perception. This roughly means that if the bulb drive is increased in discrete increments, then the increase between increments should be no more than 10% of the intensity corresponding to the preceding drive. Since the circuit most likely will use binary counters, this adds other constraints. If say you decide to use 64 drive levels, as derived from a simple resistor ladder off 6-bits of counter, and the increment is set to 8% , then this gives you (1.08)^64~ 120 to 150 range of intensity for the ramp. The 64 drive levels also implies 15 x 60 /64 = approximately 14 seconds of dwell at each level, and this is 15 x 60 x 60/64= 844 line cycles, the most readily available time base. So your basic circuit will be to initialize for an RMS output of Vmax/150, count cycles, increment the DAC counter, servo the bulb drive to Vmax/150*1.08, and so forth until the drive produces Vmax. I'll leave it to Jim Thompson to work out the best configuration of parts for you- he has plenty of time on his hands and claims to relish a challenge.

WRT Fred's good info, you could use a photocell to monitor the light output, and effectively put lamp in a feedback loop, this might give you a linear response, or it might just be a bit OTT for a simple device

martin

Sorry Phil I thought this was a serous newsgroup of course got the joke, still it sounded good if you could of got one? i thought for a minute you where referring to something else I had read about on the net where they actually read 1 period and do some maths ect... never mind thanks for your help any way.

407, man, you cut me to the quick! J***A***? This is usenet,dude. We do have our standards.

What I'd be looking to do for this problem is this:

Zero-crossing circuit starts a timer when the line voltage is zero. Prolly the timer's a 555 or so. At the beginning of time (your 15-minute interval) the timer waits 8.32msec and sends out a quick pulse. That pulse fires the triac, turning the lamp on for a bit, until the next zero-crossing.

As the time runs out of the hourglass, the delay drops from 8.32msec to zero, or nearly so.

So I want a circuit that drops the trigger voltage steadily (more or less) from some initial voltage down to zero. Typically an R-C network. Maybe with a current source to make the ramp linear, and some low-input current buffers. Depends on how accurate, and how complex.

At this point, somebody will insist that it's easier to do with a PIC. They'll be right.

PN2222A pinout ebc, looking at the package.

Ok, I realize I may have been a little harsh, but when a person is patronized without a request for any clarification, I believe that this is lowering oneself to a sub-par professional standard and comes off as mere ignorance to me.

Anyway, as for your solution, that was actually the direction I was originally headed in to begin with as I have some experience with 555 timers and RC circuits. I just stated the problem broadly as to not limit myself to any particular foundation for design. However, as you stated, there will likely be many people that will claim that a PIC solution is much simpler, and I will probably succomb to the peer pressure and follow suit, but I'd like to keep my options open at this point.

407

PN2222A wrote:

Something like ww1.microchip.com/downloads/en/AppNotes/40171a.pdf

Use a PIC or a BASIC STAMP of course.

Not using a microcontroller for this ap is fundamentally insane.

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martin griffith wrote in sci.electronics.design:

15 min at 60 Hz gives a count of 54000. A 16 bit counter would fit nicely.

Anno