Mystery lights redux, AC-line powered 68HC908, MIPVEF

This batch of Design Ideas includes yet another in an ongoing series of "mystery light" circuits. As faithful readers of Design Ideas will recall, the series kicked off with "Stealth mode LED controls itself," by Howard Meyers, which inspired "Single microcontroller pin senses ambient light, controls illumination," by Loren Passmore. Our latest entry, by Dhananjay V. Gadre and Sheetal Vashist, "LED senses and displays ambient-light intensity," explores an LED's photovoltaic characteristics.

Seeing three different approaches to the same design problem recalls what happened in the greater-Boston area in the late 1970s when a "scrambled" UHF pay-TV station went on the air. Not long after, bootleg decoder designs began appearing, each of which reflected its corporate origin. The minicomputer engineers used mixtures of analog and digital components, while the military-electronics guys thought nothing of dropping in an expensive double-balanced mixer and using two power supplies. After a while, one could make an educated guess as to where the latest decoder design originated. A few years later, the station became a free-to-air broadcaster, instantly rendering untold numbers of illicit decoders obsolete.

Moneymaking TV schemes come and go, but heat is always with us. In"AC line powers microcontroller-based fan-speed regulator," frequent contributor Abel Raynus (click here to see how frequent) shows how to control a cooling fan and derive power for a microcontroller directly from the AC line. Instead of using a stepdown transformer or switched-mode power supply to feed the controller's hunger for low voltage, Abel's design uses a series-connected capacitor's reactance to provide a nearly lossless voltage-dropping element. If you try this approach, you use a capacitor rated for AC power-line service. Also, recall that I = C (dV/dT). In addition to a smooth sine wave, the AC line carries nasty voltage transients that can deliver impressive amounts of current. To tame the transients, add a flameproof resistor in series with the capacitor.

Next, authors Ezio Rizzo and Vincenzo Pronzato show how "Simple circuits sort out the highest voltage." This circuit monitors three voltages and delivers the largest of the three as its output. As another way to think of it, call it an analog multiple-input positive voltage envelope follower (MIPVEF) that's roughly analogous to a digital-logic priority encoder. Circuits like this are simpler to implement in analog form than as mixed analog and digital circuitry, and as a bonus they require no messy microprocessor software.

By the time that you read this, America's 2006 election will be mostly over except for recounts, challenged results, and accusations of electoral fraud. Have we gone too far in our application of electronics and data-processing to elections? For an inside look at electronic voting technology and how it can go horribly wrong, see "How to steal an election by hacking the vote."

Speaking of voting, what's your favorite microprocessor? Visit the CPU Shack to review a collection of historically significant (or not so significant) processors, and don't miss the pages that list Soviet, East German, and Bulgarian knockoffs of western microprocessors. And did you know that the Hubble Space Telescope's last service mission upgraded it to an 80486?

Housekeeping matters: A Design Ideas reader suggested that authors who have used Linear Technology's LTSpice software to simulate their circuits should include listings of their circuit models as part of their Design Ideas submissions. Feel free to do so.

And finally, if you have too much time on your hands, check out Fun With Words, which includes useful reference material for Scrabble fans, plus useful tables of character counts, and letter and word frequencies.