Electronic Design

# Bob's Mailbox

Hi, Bob: I have a problem and could use your help. It initially sounded easy to me, but proved not so. I need to find an analog circuit for the following problem: I have 13 resistors with different values. I know the list of values and need to randomly pick two resistors from those 13 to put into the circuit.

The circuit should be able to tell me which one is the larger resistance value. If possible, please provide some advice on this problem that has puzzled me for a while.

Tiejun (via e-mail)

Pease: Hello, Tiejun. It might be of interest for you to study the range of those resistors. If you load a 0.1- and a 0.2->‡ resistor onto a 5-V bus, you might get a stupid answer if they are destroyed by overheating. There are many ways to compare resistors' ratios. Some use an op amp, while others use a comparator. Most use a matched pair of resistors as a divider, so if the unknowns are close together in ratio, you won't be fooled. This is usually called a bridge, such as a Wheatstone Bridge. Did they ever teach you about bridges, which were used for 110 of the last 120 years? Set up your unknown resistors as a voltage divider. Set up the matched resistors as a voltage divider. Provide a bias voltage. If the resistors are ill-matched, it is very easy to see: The comparator or op amp tells you which R is HIGH.

Dear Bob: I read your Pease Porridge column all the time and love it. Now I have a question for you. I have a component that I have to source out and I'm having trouble finding it. The logo on the part is of a pair of hands doing a handshake. Do you know if this is an old National Semiconductor symbol? (I sure doubt it. /rap) The part is some type of video amp for a CCD array. Any info would be greatly appreciated.

Steve A. (via e-mail)

Pease: Man, I never heard of this nor saw this. I'll ask around. And perhaps some of our readers might know?

Dear Bob: Regarding "What's All This Magazine Stuff, Anyhow?" (electronic design, Nov. 15, 2004, p. 18). The word "magazine" is derived from the Arabic word makhzan, meaning storehouse per www.bhag.net/word/word24aug2003maga.html and my memory as someone (with some Arabic) who lived in Morocco for two years.

Fred White (via e-mail)

Pease: Hello, Fred, I am a bum for neglecting to include the etymology. I always like to do that, and I can't think of why I neglected to include it in my column. Thank you for mentioning this. But if you went back to Morocco, what do you think a storehouse is now called? If I went back 500 years to old England, what do you think a storehouse was called?

Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

TREKKING WITH BOB PEASE
Anybody interested in a great bicycle trek around the Annapurna Circuit? It was pretty tough when we did it in 2002, but I got a couple more friends who want to do it, so I might just go again. We'll be starting up the trail on June 1, 2006. Read about that trek at www.national.com/rap/trekking/bek/index.html, parts 14 and 15. Only tough, strong, mountain bikers AND good hikers should apply. But it will be a lot easier for us than our 2002 trip, as we know when and where to hire porters, and how to keep our weight down. Inquire to [email protected].

Is anybody interested in joining us on our next hiking trek in November 2005? This is an easier hike, up from Pokhara to the Annapurna Sanctuary—just 14,200 ft, with the great Annapurna peaks rising 10,000 ft above us, all around, and the full moon overhead on November 15. Our highest camp is at 12,200 ft. Then we traverse to Ghorapaani and Poon Hill for more great views. This is just a 25-day trek. Pricing is around \$1600 plus ~\$1400 for airfare from the west coast. For more information, go to www.instantweb.com/p/peterowens/Annapurna%20Sanctuary%20Camping2004.htm. Save up for your vacation now. Inquire to rap. Do it early because space is limited to about 12 people. /rap