Electronic Design

What's All This Cold Toes Stuff, Anyhow?

I sure walked into it. I've always known that I can stuff my warm feet (with warm socks) into frozen boots and just start walking, and they would warm up nicely. These are Vasque trekking boots, weighing about 2.1 lb each, well insulated down to about –20°F and extremely comfortable. So I put them on and hiked up the trail, all very cozy, on Jan. 17. The air temperature was around 10°F to 20°F—not bad.

But after several hours, I got tired and needed some rest. My metabolism cooled down, my body and limbs cooled down, and my feet cooled down—and they didn't complain. It's like throwing a frog into warm water and adding heat gradually to boil the frog. So my feet got quite cold, and I neglected to dive into my bivouac sack.

Four toes got frozen badly, and six just frosted. But the next day, I walked three miles down the hill, very comfortably. After I came off the hill, I took a shower in my motel room, and my feet didn't hurt. I washed them, dried them, and didn't even look at them, as they didn't hurt, and I had no clue they had been damaged. Four days later, I looked at my toes, and I figured it out. They looked lousy, and I went to a podiatrist. I'll save 92% of my toes.

I've learned that I can't trust my nerves to tell me that my feet are cold. Dr. Bolognini calls this "neuropathy." People with diabetes have to watch out for this. I learned the hard way.

How can I go trekking or hiking again next year after I get my feet repaired without fear of chilling my toes? I suddenly began planning (at midnight in my bed) some sensors to keep an eye on my feet and toes.

I could go visit my Friendly Neighborhood Applications Engineer for Temp Sensors. And who is that? R.A. Pease, plus Emmy Denton. So I'm going to write down my plans, show them to Emmy for a sanity check, and build up some toe temp sensors. I don't think it's going to be that hard.

Should I use the LM35Z, or the LM45M, or the LM62M? I'll debate this—any of them would work well. But I'll bring three thin wires up my leg (secured using paper tape) and right past my tummy to plug in to a control panel on my chest, hooked on a lanyard around my neck.

When I push a button, I'll supply +5 V to the +VS pin of the analog temp sensor, which will indicate how cold my toes are on a tiny analog voltmeter. I can monitor my toe temperatures—and fingers, too, if I want. Will I add an automatic timer to take a reading every 10 minutes? And beep three times if the toes are okay but only once if they're too cold? Oh, probably.

Battery life should be several days. I'll start with four AAA cells, but later I'll probably go to a lithium cell. Sensor weight plus 5 ft of wires should be less than half an ounce, so I'll be able to bring spares.

It will be easy to build this sensor, but I can't very well test it out for a while. (Oh, I can test it on my wife's feet!) Will it work if my boot gets full of cold water? Steam? To be seen. What would I do if my feet are too cold? Add some foot-warmers. Get in my sleeping bag. Take heroic measures, as after your toes have been frostbitten, they are more susceptible to the cold.

The adjacent block diagram shows a simple scheme to apply power to the LM35 sensors, automatically, alternately, every few minutes. Then, the two sensors have their outputs paralleled (the one that's not powered has no effect on the other sensor) and fed to a voltage-controlled oscillator, so we can hear who's cold. The resistor-diode path tells us the power supply is alive. For the complete schematic, see www.national.com/rap/coldtoes.html

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

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