Selective Dimmer for Two Light Sources

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Discussion Overview

The discussion revolves around designing a circuit for a selective dimmer that controls two light sources, allowing for equal dimming under slow adjustments and preferential brightness shifts under rapid adjustments. The focus is on the technical aspects of the circuit design, including potential components and methodologies.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant describes a need for a dimmer circuit that allows two light sources to brighten and dim equally under slow adjustments, but shift brightness towards one source under rapid adjustments.
  • Another participant questions the type of light sources being used, suggesting LEDs driven by PWM or AC mains powered incandescents, and inquires about the original poster's experience with microcontrollers or CPLDs.
  • A participant suggests that the project could be simplified with a few opamps and transistors, outlining a potential circuit design involving a buffer, differentiator, and current drive for each LED.
  • There is a discussion about whether the brightness shift should be persistent or only temporary during the adjustment, with the original poster clarifying a preference for persistence until the next rapid adjustment.
  • Another participant proposes that digitizing the control signal may be necessary for persistence, recommending the use of a microcontroller for the project.

Areas of Agreement / Disagreement

Participants express varying levels of familiarity with the technical aspects of the project, and while some suggest possible solutions, there is no consensus on the best approach or components to use. The discussion remains open-ended with multiple competing views on how to achieve the desired functionality.

Contextual Notes

Participants express differing levels of experience with electronics, which may influence their suggestions. The complexity of the project is acknowledged as relative, depending on the background of the individual attempting to implement the circuit.

Who May Find This Useful

This discussion may be useful for individuals interested in electronics, circuit design, or hobby projects involving LED lighting and dimming control mechanisms.

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A "selective" dimmer

Hi!
I need some help to solve a problem I can’t get my head around. I would be very grateful If someone could help me to get started.

I need a circuit for a dimmer connected to two light sources. When the rate of change of the current is within a pre-determined value, I want the system to behave as if the two light sources where just parallel connected to a power supply through the dimmer. => Brighten and dim equally.
But when the rate of change of the current in the “positive” direction passes this boundary I want the current to be shifted to make for example Light source 1 to shine brighter than Light source 2.
In the same way brightness is shifted to Light source 2 if the rate of change is passed the boundary value in the negative direction.

To make it clear…
If I slowly twist the dimmer in any direction both lights turns equally brighter or dimmer. But if I make a sudden move in one direction, the brightness is shifted towards the corresponding light.

I am pretty sure it can be done with Inductors, but a solid state semi-conductor circuit is preferred.
 

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Or Entity? said:
Hi!
I need some help to solve a problem I can’t get my head around. I would be very grateful If someone could help me to get started.

I need a circuit for a dimmer connected to two light sources. When the rate of change of the current is within a pre-determined value, I want the system to behave as if the two light sources where just parallel connected to a power supply through the dimmer. => Brighten and dim equally.
But when the rate of change of the current in the “positive” direction passes this boundary I want the current to be shifted to make for example Light source 1 to shine brighter than Light source 2.
In the same way brightness is shifted to Light source 2 if the rate of change is passed the boundary value in the negative direction.

To make it clear…
If I slowly twist the dimmer in any direction both lights turns equally brighter or dimmer. But if I make a sudden move in one direction, the brightness is shifted towards the corresponding light.

I am pretty sure it can be done with Inductors, but a solid state semi-conductor circuit is preferred.

Welcome to the PF. What are the light sources? Can they be LEDs driven with PWM DC voltatge sources? Or do they need to be AC Mains powered incandescents? Do you have experience programming microcontrollers (uCs) for this type of function? Or, do you have experience designing CPLDs or FPGAs for this type of function?
 


Thanks berkeman.

Cant say I have much experiece at all. Know some basic electronics; but I am a quick learner!
Had to search wikipedia a couple of times to understand your post..
Didn't think it had to be that complicated..

Sure it could be LEDs
 


Complicated is a relative term... For a person who has uC evaluation kits laying around, and is practiced at whipping out projects with them, this project would just involve some C code. Or for a person who is practiced at building CPLD-based state machines and logic, and has a CPLD evaluation board laying around, it would only involve some VHDL or Verilog code.

But if neither of those things fit, then no problem, there should be some ways to build what you want in a fairly simple way. Driving LEDs definitely helps simplify the whole power supply thing.

Since the rotary control will most likely be a potentiometer, and you want the circuit to be simple, I'll assume that we won't digitize the pot output voltage. You could build the circuit with about 3-5 opamps and a couple transistors, I think. The first opamp would be a buffer for the pot voltage (call its output V1), the next opamp would be a differentiator stage, to give you a voltage that is proportional to how fast V1 is changing (call its output V2), and the next two opamps would be used to convert V1 into a drive current for each LED, with an error term from V2 either added in or subtracted out of that LED drive. Make sense?

But even this solution involves some basic opamp circuit familiarity (or willingness to learn). Have you studied opamps at all?

Maybe others reading this thread would have even simpler suggestions... ?
 


Now that I think about it more, there may be an even simpler solution with just a few transistors and discretes...

But I need to ask what the context of your question is. If it's for a school project, then we are limited in how much help, and what kind of help, we can give. For school projects, you must do the bulk of the work. What is the context of your circuit question?
 


berkeman,
If this was a school project I would probably have solved it myself. I made the post because this isn't really my field. I study physics at the University of Stockholm; and can probably say some t the electromagnetic foundation of this stuff; but electrical engineering, circuits and stuff is somewhat a different path.. But as I said, I am a quick learner so; hit me!

I'm looking for a way to build a desktop lamp with two groups of LEDs where I can control the color temperature and brightness with the same dimmer. Its just a potential hobby project.
 
Last edited:


Fair enough. But maybe I misunderstood the use of the adjustment velocity. Do you want the relative brightness to only be different at the time of the adjustment (and dependent on how fast the control is turned), or do you want it to be persistent, where the difference is held after the control is stopped?
 


I want the shift to be persistent.. till the next {faster than predetermined value}-movement.
So for example with a slow twist I brighten up both groups of LEDs (relative brightness unaffected). With a quick twist the relative brightness is shifted to the corresponding group of LEDs.

Now let's say that one group contains white/blueish cold colored LEDs and the other group contains LEDs that are warm white; then both intensity and the color temperature of the light can be controlled with the potentiometer.
 


Well, for persistence, I think you will need to digitize the control signal (potentiometer signal), and control the LED brightness with either a uC or a CPLD circuit. I think the easist one for you (and your thirst for new knowledge and expertise) would be to get a uC evaluation board, and learn to program it in Assebly, Basic or C. We're discussing uC controlled projects in this nearby EE thread:

https://www.physicsforums.com/showthread.php?t=307526

.
 
  • #10


berkeman said:
I think the easist one for you (and your thirst for new knowledge and expertise) would be to get a uC evaluation board, and learn to program it in Assebly, Basic or C.
.

..A project for this summer then!
 

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