Electronics newbie- controlling LED illumination

[salman]

Hello Everyone!

My knowledge of electronics is very basic so please bare with me. What I am wanting to do is to very precisely control the illumination of a set of LEDs.

I would like to be able to control the LEDs over a 4 log until range of illumination (ie be able to control the range of brightness from 0-1000) however I would only require access to about 256 steps equally spread out across this range.

Also I would like the LEDs to be pulsating as they do this.
How dim can LEDs go before they completely go off?

The project I am working on is a medical device known as a dark adaptometer for a university project which measures the rate at which a person's eyes adapt to the dark by progressively making a pulsating light dimmer and dimmer.

Thanks for taking the time to read this. I would really appreciate any advice on what the best way to go about this task cheaply is.

 

[kcoer]

Use a 555 timer for the "pulsation"
Use a 200k potentiometer for the dimming

How many LED's are you talking about?
Do you know how to set up a 555 timer?
What are the tech specs for the LED's?

Heres the Schematic for a 555 timer in astable multivibrator (aka oscilator)

http://www.google.com/url?sa=i&rct=...v6EoCeIGy0kpb63SO6f4sIZA&ust=1387818764355158

 

[salman]

Use a 555 timer for the "pulsation"
Use a 200k potentiometer for the dimming

How many LED's are you talking about?
Do you know how to set up a 555 timer?
What are the tech specs for the LED's?

Heres the Schematic for a 555 timer in astable multivibrator (aka oscilator)

http://www.google.com/url?sa=i&rct=...v6EoCeIGy0kpb63SO6f4sIZA&ust=1387818764355158

Thanks for you reply

I don't think with the suggestion you've made that I'd have precise enough control of the LED illumination. Would it be possible for me to do something using pulse width modulation?

 

[kcoer]

Salmon ( i like your name ),

Use Two potentiometers. Look up 555 timer astable oscillator diagram. For R1 use a pontentiometer and for the resistor leading to your LEDs use a potentiometer. That Pot1 will control the length of the V high. I could be wrong though.

 

[(*steve*)]

Let me answer some of your questions that have not been answered and ask you for clarification in some places...

I would like to be able to control the LEDs over a 4 log until range of illumination (ie be able to control the range of brightness from 0-1000) however I would only require access to about 256 steps equally spread out across this range.

A possible question is how will you set which of these 2656 levels you want? Is it from a microcontroller? Or do you want to dial it up?

If you are going to dial it up, is a continuous adjustment a problem?

And when you say precise, do you mean accurate? Do you want to be able to set the same level again (accuracy), or the same level on another LED (precision)?

Also I would like the LEDs to be pulsating as they do this.

How do you define pulating? Is it pulsing (flashing - i.e. 0% then 100% of your desired intensity, then 0%, ad so on) or do you want the intensity of the LED to vary a little (say from 50% to 10% of the nominal brightness?) ?

How dim can LEDs go before they completely go off?

There is no limit to their dimmness. You can get to the point where you can statistically determine the probability of them emitting a photon if you wish.

The project I am working on is a medical device known as a dark adaptometer for a university project which measures the rate at which a person's eyes adapt to the dark by progressively making a pulsating light dimmer and dimmer.

Taking that into account, I'd probably go an arduino using PWM to control the intensity of the LED. This will allow you to set the intensity (under program control) in a very repeatable manner.

By picking a suitably slow PWM frequency and with the use of a low pass filter, you could use the resulting variable intensity as your pulsing (perhaps)

A similar thing could be done with a 555, but since you want something (presumably) very repeatable, you'd have to go down the route of digital or even microcontroller control of it, so just using a micro would eliminate the 555.

The problem with using a slow PWM to vary intensity is that it has greater effect near 50% duty cycle, and it tails off either side. You could also dynamically change the PWM duty cycle, however this would require more skill in programming.

 

[(*steve*)]

The off topic argument about 555s has been moved here: https://www.electronicspoint.com/555s-terrible-no-theyre-not-yes-they-no-theyre-not-t266388.html

 

[salman]

Let me answer some of your questions that have not been answered and ask you for clarification in some places...



A possible question is how will you set which of these 2656 levels you want? Is it from a microcontroller? Or do you want to dial it up?

If you are going to dial it up, is a continuous adjustment a problem?

And when you say precise, do you mean accurate? Do you want to be able to set the same level again (accuracy), or the same level on another LED (precision)?

Also I would like the LEDs to be pulsating as they do this.

How do you define pulating? Is it pulsing (flashing - i.e. 0% then 100% of your desired intensity, then 0%, ad so on) or do you want the intensity of the LED to vary a little (say from 50% to 10% of the nominal brightness?) ?



There is no limit to their dimmness. You can get to the point where you can statistically determine the probability of them emitting a photon if you wish.



Taking that into account, I'd probably go an arduino using PWM to control the intensity of the LED. This will allow you to set the intensity (under program control) in a very repeatable manner.

By picking a suitably slow PWM frequency and with the use of a low pass filter, you could use the resulting variable intensity as your pulsing (perhaps)

A similar thing could be done with a 555, but since you want something (presumably) very repeatable, you'd have to go down the route of digital or even microcontroller control of it, so just using a micro would eliminate the 555.

The problem with using a slow PWM to vary intensity is that it has greater effect near 50% duty cycle, and it tails off either side. You could also dynamically change the PWM duty cycle, however this would require more skill in programming.

Thanks a lot for your response, it's really appreciated!

I only the need the range of brightness to be 0-1000. Out of these 1000 levels I need access to 256 levels that are equally spread out.

The pulsating needs to be completely off and then on again so flashing down to 0% and then to the desired intensity.

Ah brilliant, yeah an arduino sounds like a fantastic idea, I've put a post on their forum and I'm about to order one now. A bit of a steep learning curve but it will be fun hopefully.

Thanks again for all your help.

 

[(*steve*)]

I only the need the range of brightness to be 0-1000. Out of these 1000 levels I need access to 256 levels that are equally spread out.

OK, some technical details are required.

Assuming that the 256 levels are such that 0 is off and 255 is 100%, do you want 1 to be 1/1000th of 255?

I ask this because 0 might be 1/1000th of the full intensity, and 1 might be about 5/1000. Alternatively 0 might be off and 1 might be about 4/1000 (which completely negates the 3 orders of magnitude issue).

Do you want a linear change in brightness, or a logarithmic one (considering the eye's response is closer to logarithmic).

The pulsating needs to be completely off and then on again so flashing down to 0% and then to the desired intensity.

That's easy

Ah brilliant, yeah an arduino sounds like a fantastic idea, I've put a post on their forum and I'm about to order one now. A bit of a steep learning curve but it will be fun hopefully.

Have fun. If you need any more help, we're always here.