Electronic Design

What's All This Cruise Control Stuff, Anyhow?

Okay, Pease. You brag that you can slap together a good breadboard in an hour or two, all hay-wired over a copper-clad ground plane. How come it took you two years to build this circuit? Well, most breadboards are quick to build, and easiest to work on, if they are openly spaced and laid out broadly.

This circuit has to go in my car, be reasonably compact and neat, and have trim pots that are usable when driving—not by a screwdriver. So it took a while for me to find the time to lay it out right and make it easy to control.

Yes, one can buy a car with cruise control. I rented such a car recently. It was boring past tears. But I must admit that it kept my speed down so I didn't get a ticket. Have you ever seen a "High Traffic Enforcement Zone" highway sign? Several of these signs are on Route 6 between Orleans and Wellfleet, Mass.—and they mean it. So I set my speed at 43 mph in the 40-mph zone.

I would not want to fit a "conventional" cruise control on my 1970 Beetle. The car slows down a little on long upgrades anyway, and I like that. But I don't want a "speed limiter" as I featured in Parts 4 and 5 of "What's All This Fuzzy Logic Stuff, Anyhow?" (Electronic Design, Nov. 6, 2000, p. 146 and Nov. 20, 2000, p. 159). I just wanted a little reminder, so I cooked up the circuit shown in the figure.

A1 rattles back and forth at a low (~4%) duty cycle, at about 12 seconds per cycle. The high and low parts of the duty cycle are set by P1 and P2, to suit your taste. A1 turns A2 ON and OFF—mostly OFF.

A2 is an adjustable-frequency oscillator. I set the pitch, using P3, to match the speed of my engine at the speed I want. The exact pitch is not important. I just set it to the speed I like, perhaps 68 mph in a 65-mph zone. About 3700 rpm?

A3 and A4 are amplifier/drivers. Their output amplitude is adjusted by P4 on the way to the speaker. The whole thing runs on a 9-V battery, or a 9-V regulated supply powered by the car's 12 V.

The theory is that if the brief "beep" of the speaker matches the pitch of the engine, I'm happy. If the engine's pitch is higher, that's supposed to remind me that I'm going faster than my set speed, and I should perhaps slow down. If the engine's pitch is too low, I am either slowed down on a hill, or I am dawdling. Of course, I have to be able to hear the engine's exhaust pitch over the whining of the cooling fan, the gears, and the passengers. (Of course, this will work only in a stick-shift car, not on any automatic transmission. But it will work in every gear.)

Will this work? Can I train myself to respond to the match or mismatch of the pitches? Will it work naturally, almost as second nature? Tune in next issue! Actually, it does seem to work pretty well. I wouldn't, in my right mind, recommend that you, in your right mind, should build one. But if you did, it would work.

Comments invited!
[email protected] —or:

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

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