Electronic Design

What's All This circuits-in-your-car stuff, anyhow?

Hip, hip, hooray. Pease is finally going to talk about electronic circuits. So many of these recent columns have been about things that have nothing to do with electronics. True...Sorry about that. Well, I have been designing lots of circuits, but a lot of them I can't talk about. I can't talk about a circuit we are developing and planning to sell. I can't talk about circuits that we are patenting. There's lots of things that I can't very well talk about. But these circuits here are NOT confidential. You might call them chicken-manure circuits, but they do work OK. They are useful, and not completely obvious. I have never seen them in print. Even though these circuits are not very sophisticated, if applied in a useful way, they can be very valuable.

Would you like me to write about all the U.S. patents I have? I could do that. Many of them are circuits. Circle 550 on the Reader Response Card if you think I should do that. I was really sure I had 15 U.S. patents, but the last time I checked, I had 16. I have no idea how that extra one crept up on me. I'm working on two others that might be out in a year or so.

Just one caution: if you are trying to search at the U.S. Patent Office to find out the patents of an inventor, and type in: IN/"Jones Jr.; John."—you can't find anything, even if "John Jones Jr." has a patent. Their search engine can drive you wild. You have to type in a comma after Jones, and then they will lead you to the patent.

But, today I will talk about the circuits I am putting in my "new" car. These days, there are microprocessors and circuits everywhere, in most new cars. That is probably half the reason I prefer to buy a not-so-new car. Some guys have asked me, "How can I protect my car so it will not be disabled by an ElectroMagnetic Pulse?" I tell them it's easy—just buy an old car.

Chimes: I am always amused when I rent a car with chimes. I don't want to BUY one, or to OWN one, but it's fun to drive one, for a change. Some cars have chimes to warn if you are leaving your keys in the car when you open the door. Or if you leave your directional signals blinking absent-mindedly for too long. I am always reminded of the story of the man who gave out an awful belch. A very proper lady nearby was horrified, and she said so. He responded, "Exactly what were you expecting? Chimes?" Modern cars have circuits to turn chimes and lights on and off, circuits to run your ignition and your fuel injection, and 999 other things. Some high-end cars have dozens and dozens of 'processors'... but not the one I just bought (a relatively new, 1970 VW Beetle).

I got a radio. I think it has eight transistors. It doesn't run very well. Flakey. I'm gonna rip it out and put in one that has 10 transistors. I got ignition points that open every time I need to fire a spark plug. I got a coil. I got a horn. I got lights. I got electricity.

So, what electronic circuits am I going to add to my latest car? Look for a horn-rattler, alarms, reminders, timers, radio turn-off, etc.

Radio turn-off: Sometimes I like to sit in my car and listen to the radio, with the ignition off. But I don't like to just leave the +12-V dc power on permanently for the radio because it could easily run the battery down, if I forget to turn it off. So, I put in a timer that turns the radio off a couple minutes after the key is turned off.

Refer to the circuit in Figure 1. I have built about four versions of this circuit, for various cars. It works OK for me. So, I just built another one for my new red Beetle. Hey, I never had a red Beetle before.

How does it work? When the ignition is ON, the transistor at the upper right acts as a diode and keeps the 0.1- µF cap charged up. All the transistors are turned ON. When the ignition is turned OFF, that capacitor is discharged down in a couple minutes, by the base current of the transistors—perhaps 5 or 10 nA. When the voltage on the cap gets low enough, all the transistors turn off, and the radio turns off. The positive-feedback capacitor (0.01 µF) is optional.

Maybe this time I'll build it without that, to minimize the click. No, I will solder in the capacitor on one end, but I won't connect it—so I can easily change my mind.

When I built this circuit in June, the shut-down time was more than 10 minutes, so I had to put a 27-k‡ resistor from the emitter of the Darlington, at "x," to ground, to get the delay near three minutes. The NPN betas were too high.

I've built this basic circuit with a PNP Darlington output driver, and a PNP-NPN, and a big Germanium PNP that came out of a Minuteman I nose-cone. Most of these schemes have an ON voltage of about 0.9 V at 1 A, and that's OK with me. If I needed a better switch, with lower ON voltage, I could drive the gate of an N-channel MOSFET to +23 V. But, since that is not a big deal, I've never done it.

The "3-M resistor" is just a place where I solder up a single strand from a stranded wire, to act as a fuse. If the fuse blows, I'd swap in another 3-M resistor—with a new strand.

Of course, in all these circuits, unless otherwise noted, NPNs are 2N3904 or similar; PNPs are 2N3906 or similar; resistors are ±10%, and diodes are 1N914/1N4148.

Headlight reminder buzzer: See Figure 2. It is so easy to turn on your headlights or parking lights and forget to turn them off, that I really need this buzzer. On a rainy day, it is a VERY good idea to turn your headlights ON. But it is also a good idea to turn them OFF. I just connect this circuit between a parking-light fuse and the ignition switch. So, if the lights are left on when the ignition is turned off, I will be reminded.

I like to make this a 1.5-kHz triangle wave, so if I have to leave my lights on, the buzzer will not annoy me to death. I typically feed this signal into one of my car's speakers, through 39‡ in series with 10 µF.

Burglar alarm, Mode I: Type this up and print it out, and tape it to each rear window: "IF ALARM SOUNDS, call 911, or call (Your home phone number), or call (your work phone number)." That's what I do. If a car thief or robber wants a reason to pass over your car, this should give him a good reason to look elsewhere.

Burglar Alarm, Mode II: I connect a detector to my indoor light switch, so when a door is opened, a sequence begins: there is a wait of four seconds. Then a LOUD beeper starts. After four more seconds, it starts blowing the horn. This is enough to chase most car thieves away. Schematic not shown, but you could easily figure it out yourself.

Burglar Alarm, Mode III: I built up a timer that is very useful. When I leave the car, I just set it to run. It blinks for a while and then turns itself OFF. I don't know very many people who are stupid enough to break into a car where all sorts of winken-stinken-blinken is going on. In my next column, I'll write about some more circuits.

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

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