LED Dimmers (2): It doesn't always have to be digital

摘要

From the many analogue LED-dimmers that Roel Arits sent to us, encouraged by our small Elektor Labs challenge [1], we have already put two in the spotlights in the previous issue [2]. In this second and final instalment we will present two more. In the previous issue we already noted that for an observed linear increase or decrease of the brightness of an LED (or an incandescent lamp, but we limit ourselves to LEDs here) the current through the LED does not increase or decrease linearly. but exponentially. This is all related to the more or less logarithmic sensitivity characteristic of our eyes, as was already described in the 19th century by the Law of Weber-Fechner [3]. We covered this extensively in the previous instalment already, so here it will suffice to state that we need an exponentially increasing or decreasing current through the LED to compensate for this logarithmic characteristic so that we observe a linear change in brightness.