Let's continue with some more of the circuits I am building to put in my newest car:
Burglar alarm, Mode IV: When the ignition is OFF, this circuit drives some brief, low-duty-cycle pulses through a couple of LEDs (Fig. 3). You can use any low-power op amp you want. The overall battery drain is probably not enough to drain your battery in a month of disuse. I mount the LEDs near the lower corners of the windshield. These LEDs can also help you find your car in a dark parking lot. They stop blinking when you turn on the key.
Hand brake alarm: I am not going to put this on my new VW Beetle, because I never forget (or fail to notice) that I left my hand brake on. But, if I was setting up a VW Bus, I'd add that alarm. I would build a 400-Hz oscillator, and turn it on and off at about a 0.5-Hz rate (Fig. 4).
I'd couple this signal into my radio's speakers, perhaps through 150 , so it would not be too loud or annoying—just in case I want to leave my hand brake on. This can prevent you from burning out your hand brake.
Of course, if you already have a warning light, you might not have to build it. But, if the warning light is not sufficient, an additional beeper might be helpful.
Horn modulator: I received a nice letter from a guy who had read my column on Reflex Response, where I was trying to warn a dog to get out of the high-speed lane. He explained that if you REALLY want your horn to work well, and get the warning across to a dog or a person—chop it at 4 Hz. I asked, "Won't it work better at 2 Hz?" He said, "No, 4 Hz." I will insert a horn chopper, so I can modulate the horn at 4 or 2 Hz. Of course, I will leave the wires set up so I can disable it, and leave the horn in a dc mode. I'll share this circuit with you in a future column.
Oil-temperature gauge: Am I going to wire up an oil-temperature gauge? Probably. It is pretty easy to tack-solder an LM35H to a piece of copper-clad, and slide that down the dip-stick hole. I did this once, 20 years ago. Not a big deal. I know my oil temperature gets up to 85° C, in a while. I don't really have to do this, but I guess I will. And, if the LM35 falls off because the solder melts, I'll know the oil temperature got too hot.
Tachometer: Am I going to put in a tachometer? No. I did that 30 years ago. It used a 2.6-V mercury cell, and every time the engine turned faster, the tachometer indicated that it was happening. When the engine was off, there was no drain from the battery. It ran fine for years, and then I quit running it.
I used to have an old Heathkit CD ignition module. It probably did give slightly longer life on accurate ignition timing. One of these days, I might get around to trouble-shooting it, but it quit working a few years ago, and I've done just fine without it.
Those are all the circuits in my car, until next month. I've been trying to figure out how to get my GPS receiver linked up, because you can't just leave it on, and you can't just turn off the power. But I think I have a solution. Now, as I said at the start of all this, these are kind of chicken-manure circuits. And we will all agree these circuits are not very sophisticated. But it is what you do with them—how you interface them to the world—that makes them effective.
All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
Mail Stop D2597A
P.O. Box 58090
Santa Clara, CA 95052-8090