Pease Porridge Oct. 24, 2007

Oct. 25, 2007
More from Bob's Mailbox

Bob: As we were landing at O'Hare Airport last week, I was observing the runway and taxiway lighting. I immediately noticed that the lights are now LEDs. Would you believe that even these are designed to operate off the standard 6.6-A constant current system? (See 1.1/1.1.2.html.) This fixture is rated at only 3 W, which means it apparently drops 0.5 V at 6.6 A (perhaps 1.0 V or more with transformer losses?). There is still a power saving because they can supply the current loop from a lower voltage. (Yeah, but the transformer makes it a game, as you have seen. And they have to be prepared to put incandescents back in-in case the LEDs get flaky. /rap) There are two ways I could tell these were LEDs. First is the very pure blue or yellow color. But I can also instantly tell when a light source such as an LED is operating from rectified ac or pulsed dc. If you let your eyes scan from left to right, you can see a crude oscillograph view of the lights, with a dashed-line trail instead of a continuous trail, indicating a 60- or 120-Hz rectified waveform. You can also see this effect on LED car tail lights, which are apparently pulsed to dim them. I estimate the tail lights are flashing at a few hundred hertz to a kilohertz. (Would you expect this to be audible? I'd be surprised if it didn't make an annoying buzz. /rap) Someday I would like to build a spinning mirror device to view this more accurately.

â?¢ Ken Lundgren

â?¢ Pease: Any simple spinning disc or light chopper could confirm this. A motor and wheel (black paper) with a notch cut into it could be much easier than a mirror to rig, or a black-and-clear plastic foil disc. I bet you could rig up an electric demodulator too, but not a big deal.

Dear Bob: Seeing Christmas cards for sale in the local bookstore now gave me pause to realize that the November issues of ED are being set up. In all my many years behind the desk, only once in my 35-year career have I worked for a company that actually distributed a Christmas bonus-and even then, it wasn't cash! The better part of a decade fresh out of school was spent at Litton Aeroproducts, located near Los Angeles. Litton made inertial navigation and radio navigation equipment for a vast majority of the world's airlines. An occasional customer of Litton Aero was Lot Polish airline, the state-owned airline of Poland, then under a loose Communist rule. I don't recall all the legalities (and perhaps behind-the-scenes maneuvering) that went on, as I was just a design wonk. But I do recall that because of some trade restraints, it was illegal to accept cash from Poland. The average cost of an inertial nav system at the time was somewhere in the neighborhood of $85,000, maybe more. What Lot Polish did was to pay Litton in ham. Yes, the little-piggy-that-could kind. You could tell when LP bought something, as perhaps a half-dozen refrigerated trailers would show up in the parking lot-and always, it seemed around the end of December. And so, with your paycheck, you would receive a stub in the same envelope that would grant you one ham. If you didn't eat ham, you could have the stub exchanged for a voucher for a 10-lb turkey from the local supermarket. Litton is gone and never did any profit sharing, but it makes me wonder what our share would be-ham steak?

â?¢ Karl Strauss

â?¢ Pease: Hi, Karl. That is funny!

Hi Bob: I had to chuckle when your correspondent first wrote about filling the oil lamp well ("Bob's Mailbox," Sept. 1, p. 18), since I'd been mulling over the possibilities of detecting the water level in my 280-ft drilled well on a real-time basis by either organ-pipe-like resonance or acoustic time of flight. (Water can come as close to the surface as 30 ft, according to the label placed on the wellhead by the driller in 1994. Or to be more precise, 30 ft is listed as the "static water level.") (Either of your schemes would work okay, presuming there is enough accuracy in each range to suit your needs. My scheme with a capacitance detector between adjacent wires might work, too. /rap) But considering the original problem, isn't the use of a self-heated thermistor a standard, quite low-tech solution? (It might work, but it will not give you a gradual indication of when you are approaching the stop point, will it? Besides, if a little of the in-pouring oil splashed onto the thermistor, it will cool off, not so? So it would give a false reading. Would you be able to hide the thermistor so incoming oil would not hit it? Maybe, but life ain't as simple as it seems! /rap) Didn't 100 people suggest this? (No, thank heavens! /rap) Placing the thermistor in a 555 timer circuit driving a small loudspeaker would be my approach-no need to look at an indicator. (Will a 555 drive a thermistor in a self-heating mode? Maybe so. /rap) Digikey has tiny brick-shaped thermistors with millimetric dimensions and dissipation constants in air of 1 mW/°C for less than a quarter.

â?¢ Jim Hayden

â?¢ Pease: Maybe your idea would lead to a good solution. Hey, put two thermistors in parallel (or in series?). The first inflection tells you to slow down, and the second tells you "Stop!" Go ahead and build it and send it in as an Idea for Design. If you put them in a 0.25-in. ID tube, properly vented at top and bottom, they could detect the rise of fluid and avoid splashes of incoming oil.

Comments invited! [email protected] -or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

BOB PEASE obtained a BSEE from MIT in 1961 and is Staff Scientist at National Semiconductor Corp., Santa Clara, California.

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