Welcome to part 17 of our series.
We've been talking about DC Power, and I ran across a challenge recently that I thought I'd share with you, along with one possible solution. If nothing else, it may get you thinking about a handy gadget you can build to enhance your own shack.
My young assistant Gary recently bought a car that does not have a "you left the headlights on" alert. Since he likes to drive with the lights on during the day -- that's good -- he's been getting a lot of practice with jumper cables -- that's bad. He asked me if I could think of a way to add a warning circuit to remind him to shut off the lights. After doodling through some rather complex ideas involving voltage comparators and the like, I hit on this simple scheme. Even better, all the parts were available at my local Radio Shack. They might even be in your "collection of stuff I can't throw away because I might use it someday". More commonly called the junk box.
You'll need a normally closed 12V relay (or a double throw relay), a small piezo buzzer, and a diode. Pretty much any diode will do, like the popular 1n4xxx series, for example.
Here's how to build it, and how it works:
Take 12V from the headlight circuit -- either the wire that feeds the lights, or if you can get at it, the wire from the headlight switch to ground. It needs to be a wire that's energized only when the lights are on. Connect that to the positive (red) wire on the buzzer. Connect the buzzer's black wire to one of the relay's normally closed terminals. Connect the other relay terminal to ground. Verify that when the headlights are turned on, the buzzer sounds; and of course, does not sound when they are off!
Next, locate and tap a wire that is only energized when the ignition key is in the on or "run" position. Connect this to one terminal of the relay coil, and connect the other coil terminal to ground. Finally, connect the diode across the coil terminals, being careful to observe polarity as shown -- the band on one end of the diode should connect to the positive side of the coil. You could enclose the assembly in a small plastic container, like a pill bottle; or otherwise make sure that there are no exposed terminals. For added safety, you should add fuses to both wires, especially if you run them through the firewall.
When the ignition is on, the relay coil is energized and the relay is held open, silencing the buzzer. When the ignition is off, the relay closes and -- if the lights are on -- the buzzer will sound. The diode protects the rest of the system from the inductive "kickback" generated when the relay coil's magnetic field collapses after power is removed.
It's not exactly cutting edge "rocket surgery," but I hope it gets you thinking about simple things you can build for your mobile, portable, or home station.
That's it for this month. Next time, we'll continue looking at DC Power in the shack.
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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