T&L Publications

Simulated Lighthouse Lamp
Q I am trying to build a circuit for a small lighthouse lawn ornament.
The "lighthouse" has an existing fixture for a 60 watt light bulb. I would like to vary the bulb brightness to simulate
the rotating light of a real light-house. The light should start at full brightness, slowly dim until it is completely
off, and then slowly brighten to full brightness. This cycle should repeat continuously. My plan is to use
a triac and vary the voltage to the light bulb, but I am having trouble deter-mining how to cycle the gate voltage
to keep in step. Any help would be greatly appreciated.
Ted via Internet
A. The solution is an X-10 12 A dimmer switch (a.k.a., Levitron 2208) that normally mounts in the wall in
place of the toggle switch (don't mistake this for the ultra-cheap WS467). If you hold it down, the light dims to
nada, then brightens to full, then dims back down, up again, down again ... forever. The time period is about
seven seconds per cycle. If you buy one of these Decorator switches brand new, it will cost you a lot more than
if you were to find one on eBay.

Simulated Lighthouse Lamp
Q I am trying to build a circuit for a small lighthouse lawn ornament.
The "lighthouse" has an existing fixture for a 60 watt light bulb. I would like to vary the bulb brightness to simulate
the rotating light of a real light-house. The light should start at full brightness, slowly dim until it is completely
off, and then slowly brighten to full brightness. This cycle should repeat continuously. My plan is to use
a triac and vary the voltage to the light bulb, but I am having trouble deter-mining how to cycle the gate voltage
to keep in step. Any help would be greatly appreciated.
Ted via Internet
A. The solution is an X-10 12 A dimmer switch (a.k.a., Levitron 2208) that normally mounts in the wall in
place of the toggle switch (don't mistake this for the ultra-cheap WS467). If you hold it down, the light dims to
nada, then brightens to full, then dims back down, up again, down again ... forever. The time period is about
seven seconds per cycle. If you buy one of these Decorator switches brand new, it will cost you a lot more than
if you were to find one on eBay.

I think a light house effect is more complicated than that....You have some low brightness as the ligth swings around just until it poits directly at you and then its brightness Peaks for an instant. I think you will have to experiment to get something that works effectively.<p>Probably have to use a micro and run some equation..<p>Todd

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by cato:
The pot in a light dimmer just sets the voltage going to the gate of an SCR. When the voltage is changed so does the point in the AC wave form at which the SCR goes conductive and applies power to the lamp.<p>You just want to replace the pot with a voltage signal.<p>Pay careful attention to all the potentials involved. I believe a proper circuit would include opto or other kinds of isolation.<hr></blockquote><p>
Hi again, Have been thinking about this problem. I guess the trouble is that the "voltage signal" that you talk about is synced with the AC signal. The RC component of the circuit serves to delay the voltage to the diac so that it turns on later and later in the cycle. <p>For me to sense the zero crossing and manually apply a voltage after a certain delay seems a little complicated. I think I can do this with a micro and some carefully timed code. <p>I wanted a more simple solution to this but maybe there isnt' one. <p>I've been thinking about using opamps to generate a triangle but I still need to sync to the AC waveform.<p>I've seen "zero crossing" triac drivers that automatically turn on at zero crossing (these are generally used for ON/OFF circuits..not dimmers. <p>So far I can only think of a miroprocessing solution to this.

Velleman makes a kit that will control the brightness of a lamp with a DC voltage of from 0-10 volts. The P/N is K8003. Jameco Electronics, WWW.Jameco.com has them in stock for about $22.00.
I have used it, and was able to use a slow speed triangle waveform to do artificial sunrise and artificial sunset. This module just might do what you want to do with some interfacing.

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by k7elp60:
Velleman makes a kit that will control the brightness of a lamp with a DC voltage of from 0-10 volts. The P/N is K8003. Jameco Electronics, WWW.Jameco.com has them in stock for about $22.00.
I have used it, and was able to use a slow speed triangle waveform to do artificial sunrise and artificial sunset. This module just might do what you want to do with some interfacing.<hr></blockquote><p>
Thanks, I might just have to buy one and reverse engineer it. I did buy a double fader that just cycles (one up and one down) in a repeating pattern (like what I want to design). I need to open it up and see how they did it..its not perfectly smooth so whatever they did probably is a hack (maybe).

A simple 555 timer could supply the sawtooth ramp voltage to a MOSFET.

Thanks for your reply<p>Using a IGBT in the linear region I think there's a lot of heat to deal with. And this cant be use with a motor. If we were trying to switch the AC (like a light dimmer) then theirs the zero crossing to worry about. If we didnt' then we could get random brightness levels as we switched on at different parts of the cycle. <p>After lots of thought I think a triac is and the switching of the waveform is the way to go. (just like a dimmer circuit).