Electronic Design

Bob's Mailbox

Dear Bob:
A few years back, you wrote a column about an earthquake detector (Electronic Design, Oct. 24, 1991, p. 119). Just recently, I was asked by a friend in Mexico if I knew of a cheap sensor that could detect small events and possibly warn of an impending quake. (Sounds like wishful thinking...Normally, if a quake is several miles away, the first arriving shocks are no more than you'd get from a truck driving down the street! /rap)

I looked around and couldn't find anything that would fit the bill. Soooo...To make a long story short, here is what I came up with: Take your garden-variety electret/condenser mike element; I used a Panasonic unit (Digi-Key p/n P9931-ND, $1.40 each). Carefully remove the baffle or screen and open up the top of the can with a sharp, pointy X-ACTO knife. By open up, I mean enlarge the opening of the top hole to gain access to the Mylar electret membrane.

Once you have exposed this membrane, you can attach a small lead ball that's 10- to 100- mils in diameter with a small drop of super glue. Where do you get this small ball, you ask? Just melt solder on the end of your iron and shake it over a glass of water. You can get any size you want.

Size is important because it impacts the frequency response and the output. Anyway, when finished, select the proper bias resistor for a 9-V supply (6 to 12 k), tune for max, and you have a whopping output. When looking at the signal on a scope, you can tell the direction of the movement (REALLY?? /rap) and lots of other things. A small amp, threshold comparator, and alarm finishes the thing. (I bet this would be pretty good compared to the conventional approach with a LONG pendulum and a HUGE coil of wire! Cute!! /rap)

Bob, I really enjoy your work and love your column. I have 30 years in the biz as an analog/digital/RF designer. I tutored under Dr. Frank Wanless in the early CMOS days at LSI Systems, and paid my dues as a photo products designer at Fairchild. Much fun and many jobs later, I find myself working in Mexico as a test engineer and looking forward to retirement. I plan on writing and working on the many projects, all analog, I have saved up over the years. Keep up the good work.
via e-mail

P.S. I still miss the good old days in the '70s at the Wagon Wheel.

Yeah. I went there a few years ago. Very quiet. Best regards.—RAP

Dear Bob:
Re: "What's All This Logarithmic Stuff, Anyhow?" (Electronic Design, June 14, p. 111). At the end of the article, where you were mentioning the CA3046, you missed a very useful feature of the device: The two "spare" transistors can be used as an on-substrate oven, with one transistor doing the heating and the other doing the sensing. Alternatively, a power resistor can be placed on top of the chip to do the heating, while retaining the on-chip temperature-sense transistor. I used this technique several years ago to stabilize a log amp, and it worked well. The idea is not original. I think I got it from a Jim Williams application note. Cheers!
via e-mail

Yeah. Jim Williams used an LM389. I suppose that worked OK, but I never saw any convincing evidence that it worked very well. Thanks for writing.—RAP

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

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

TAGS: Digi-Key
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