# Lamp Circuit - Need Help

Discussion in 'General Electronics Discussion' started by Roabish, Apr 21, 2012.

1. ### Roabish

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Apr 21, 2012
Hi Guys, I am still young and learning and have spend hours online looking for help.

I have an unusual project, I want to be able to make a 220v lamp fade in and out at a specific "speed", for example I set the POT to 10sec and the lamp will brighten to max and then take 10sec to go out and immediately start the cycle again. I also need this speed to be manually adjustable via a POT.

I ahve looked at other lamp dimmer circuits, but nothing I can modify or "tweak" to suite my needs.

Can anyone out there help me please.

2. ### poor mystic

1,067
31
Apr 8, 2011

Hi and welcome. I hope you are able to do this work safely; the circuits you need to build are lethally dangerous.

For ease of explanation, let's first see how to build a phase-controller without the requirement that the lamp dims in and out every 10 seconds.

The first thing you need is a reliable power supply for the low-voltage side of the electronics. Let's say you choose to build your equipment to run on a regulated 5VDC supply. The earth (0V) of the DC supply will need to be connected to the neutral of the 220V mains. (Is your 220V referenced to earth? Which country are you in, please?)

Next, you need to arrange a resistor-capacitor 'ramp' circuit which will produce a sawtooth wave in synchrony with the rectified 220VAC. The ramp voltage will climb to 5VDC, and reset to 0V, 100 times per second or 120 times per second if your mains is at 60Hz. The best way to accomplish the reset is to detect the zero-crossing point in the mains cycle and drive a transistor to drain the ramp capacitor.
The next thing to consider is a potentiometer which you can use to set a voltage between 0V and 5V.
Use a comparator to produce a 5V signal at the instant the ramp voltage exceeds the voltage on the potentiometer, and feed the output of the comparator to a triac or opto-coupled triac driver. This triac will be used to switch the lamp you wish to control.
So far, we have described a phase-controller for a tungsten-filament lamp.

Now let's see what needs to be added, to get the cycle of dimming and brightening you require.

The RC ramp we have described climbs to 5V and resets to 0V, but the ramp we need is a little different: we need to control both the rate of climb and the final voltage of the ramp. Therefore we use a sawtooth wave generator to provide the charging voltage on the RC ramp. We arrange for the sawtooth wave to have a frequency of 1/10 Hz, so that each cycle takes 10 seconds.

This description is merely an outline of one method of achieving your result. I do not say that this is the best way, and it is certainly not the only way. This method uses analogue electronics; we are in a digital age and it may be tidier and cheaper to use a microcontroller.

I have written this post off-the-cuff, there are bound to be errors and omissions. Please feel free to ask questions.

Mark

Last edited: Apr 22, 2012
3. ### Roabish

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Apr 21, 2012
Hi Mark,

Thanks for the info. Here is all the info.

I am busy with my son's project and need some help, at the best of times I have problems replacing the batteries in the remote so your patience is appreciated. After spending many hours on electronice forums I have a firm understanding of the technical jargon but lack the "design" experience to complete this project, school science is af far as it goes.

I require a 220VAC lamp to fade in and out in a continuious cycle and be able to adjust the fade time. The "in" and "out" time must be the same, so 10 s to fade in must also be 10 s to fade out.

I have looked at maybe using a saw tooth generator to trigger a 220V lamp dimmer circuit but it keeps blowing up (stop laughing), as I said before I lack the design insight, I tried to modify existing circuits to suit me needs.

Any help would be appreciated

4. ### poor mystic

1,067
31
Apr 8, 2011

So you really need a complete design? That's a very big job - any design suitable for a beginner would need to be proven at every stage.
Perhaps you'll need on-the-spot assistance from an experienced technician.

You could safely use an off-the-shelf 220V dimmer and move the knob back and forth under the control of a battery-powered machine you make with a toy car motor.

Mark

5. ### BobK

7,682
1,686
Jan 5, 2010
For safety I would use an opto-isolated triac to build this. That way the low voltage electronics are not connected to mains.

Bob

6. ### Roabish

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Apr 21, 2012
Thanks for your advice mark, might be an option. To design this from the ground up, what would it cost? any takers?

7. ### poor mystic

1,067
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Apr 8, 2011
Hi again Roabish
To help in this way would mean perhaps a week's work in a fully equipped workshop for a designer with no other jobs. That means say 40 hours at at least \$100/hr.