how about an orange? http://howaboutanorange.com/feeds/blog 2026-02-22T15:25:40Z Mats Mattsson Copyright (c) 2026, Mats Mattsson tag:howaboutanorange.com,2011:2026-02-22-11-00-00 LED Exponential Current Control 2026-02-22T11:00:00Z 2026-02-22T11:00:00Z Mats Mattsson Human vision is better at noticing a small change in brightness for darker objects than lighter objects. This is a good thing, but it makes it harder to create a brightness controller.

A naive circuit for brightness control will use most of its range for bright values and very little for darker values. So it is very hard to set a dim value.

Most models of human vision uses gamma-correction to fix this, but it is hard to do using an analog electric circuit. Instead an exponential correction can provide a similar result.

I put together an LED control circuit to test this:

LED exponential current control circuit diagram.

The circuit built on a breadboard:

LED exponential current control circuit on a breadboard.

Notes

I tried several resistors and potentiometers to the input range for the exponential amplifier.

The LMV358 operational amplifier minimum output level is typically 120mV. This prevents it from generating a 0V control signal and to turn the LED fully off.

Voltage ranges:

LocationRangeInverted
Potentiometer2.78 - 2.98 V
Exponential amplifier2.47 - 0.20 VYes
Inverting amplifier0.10 - 2.28 V
Current control input0.04 - 0.91 V

References

  1. Gamma correction, Wikipedia. https://en.wikipedia.org/wiki/Gamma_correction
  2. LMV358 Operational Amplifier, Texas Instruments. https://www.ti.com/product/LMV358
  3. 1N4003 Diode, Diotec Semiconductor. https://diotec.com/en/product/1N4003.html
  4. BC548 NPN Bipolar transistor, Wikipedia. https://en.wikipedia.org/wiki/BC548
]]> tag:howaboutanorange.com,2011:2024-10-18-10-00-00 Spectroscope Tear Down 2024-10-18T10:00:00Z 2024-10-18T10:00:00Z Mats Mattsson I bought two spectrscopes and have illustrated how I think they work.

One of them is using a diffraction grating to separate light into different wavelengths, while the other is using a prism to separate light using refraction.

Diffraction spectroscope

The spectroscope consists of an entrance slit, a collimating lens, and a diffraction grating. It is housed in an aluminium tube and the ends are press fit into the tube. The slit and lens are held in place with retaining rings and the grating is glued into place.

Grating spectroscope illustration.

Photo of the dismantled spectroscope:

Grating spectroscope tear down photo.

Refraction spectroscope

The prism spectroscope consists of an entrance slit, a collimating lens, and a stack of three prisms glued together.

The first and last prisms are made of low dispersion glass and the middle prism is made of high dispersion glass.

Prism spectroscope illustration.

Photo of the dismantled spectroscope:

Prism spectroscope tear down photo.

Comparison

The grating spectroscope shows a more uniform spectrum compared to the prism spectroscope It is also harder to find the correct position for the eye when using the prism spectroscope.

The prism spectroscope allows for adjusting the focus, which makes it possible to see more detail in the spectrum.

References

  1. Abbe number, Wikipedia. https://en.wikipedia.org/wiki/Abbe_number
  2. Sellmeier equation, Wikipedia. https://en.wikipedia.org/wiki/Sellmeier_equation
  3. M. N. Polyanskiy. Refractiveindex.info database of optical constants. Sci. Data 11, 94 (2024) https://doi.org/10.1038/s41597-023-02898-2/
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