I have multiple lights being powered by a 28V battery. There is some circuitry, I am not sure what it is, that is used to dim them by decrease the voltage provided to it. One of the lights is a LED that dims at a slower rate then then the other lights. With just the LED and a 1500 resistor the TLF is 3lm at 28V, 1.5lm at 19V and 0lm at 10V. I want to design a circuit so that the TLF is 3lm at 28V, 1.5lm at 25V and 0lm at 22V. I could do this with a few op-amps however I was hoping to find a more elegant solution.

 

A voltage controlled resistor that had 0R at 28V and a very large resistance at 22V should work but I can't find anything like this that exists.

 

LED brightness is roughly proportional to current passing through it, over a fairly wide range until you get near maximum brightness, where the curve of brightness versus current lays over.

So, you want a constant current controller with smooth control over its range. The most common way to achieve this is a controller set at whatever maximum current is appropriate for the LEDs, and then you pulse-width-modulate PWM that current down from 100%. The PWM signal is at that maximum current at the peak of each cycle, but the duty cycle of those peaks drops as you turn it down. The LEDs spend more time off. These very fast pulses are perceived by a human as constant light.

 

There is some circuitry, I am not sure what it is, that is used to dim them by decrease the voltage provided to it.

so you aren't so sure about this "dimming circuitry" but you are damn sure that it dims the led by decreasing the voltage? doesn't that sound like a contradiction to you?

unless you want to completely scrape it, you will need to figure out how this dimming circuitry works.

 

so you aren't so sure about this "dimming circuitry" but you are damn sure that it dims the led by decreasing the voltage?

I used a DMM and when I turn the knob labeled dimming the voltage acrosss where the light fits in drops.

 

I used a DMM and when I turn the knob labeled dimming the voltage acrosss where the light fits in drops.

what you see may not be what you get.

 

Once upon a time, I was part of an evening's entertainment. The audience saw an empty box; they could see through in any direction, including over and under. They saw me place a woman inside and close it. Then, I fired a pistol through it and when the box was opened, the woman was gone.

In this story, your DMM is playing the part of the audience. What it sees, is not real.