On my 93 f150, i made the over-cab light (one outside that lights when you open the door), Well i made it light up when i put the truck in reverse. i ran the hot wire from my left reverse light to it. what i want though is, to make it light up in both cases: When in reverse and when the door is open. How will i go about to make it light without the reverse light turning on when the door is open? is there a way?

 

Look in my gallery. I'm posting you a diagram called "diodes" that will show how it is done. The diodes are available at Radio Shack or any electronics supply house for less than a couple of bucks. They act like a "check valve" for DC current. It can pass freely one direction, but not the other. When power flows from the backup light circuit it can go through the diode to power the light, but when power is applied to the light by the normal circuit, the diode blocks it from entering the backup light circuit. It has to be installed in the direction shown in the diagram. Get one rated for about one amp of current and crimp it inline. Cover with electical tape and a wire tie, or preferably pick up a piece of heat shrink tubing while you are at the store. Slip it on the wire before crimping in the diode, slip it over it when done and heat with a heat gun or cigarette lighter. It will shrink down over it and insulate it and never come unravelled like tape will. Hope this helps you.
I just added a second drawing with an additional diode. This may be better for you, as it will keep voltage from feeding back and keeping on your interior lights.

 

Great Diagram: thanks for helping out, i'll get those diodes tomorrow

 

Ok, so i got 2 diodes, but they werent exactly rated a 1 Amp. The radio shack guy couldn find any at 1 amp 12volts, but he gave me a 12v,21mA. I was skeptical of that working, but it did.

 

Glad it worked, but that rating sounds a bit strange. He didn't give you Zener diodes, did he? I just reached in the drawer and pulled out a package from there. They are 1N4003's rated at 200v / 1amp. RS part# 276-1102. That's what I figured you'd end up with. The voltage rating isn't important as it's a maximum rating, and most are at least a hundred. The one amp rating was probably overkill, but better too big than not big enough. Anyway, post back or message me if you run into any problems.

 

No, 21 milliamps is way too low. You need the bigger rectifier type diodes, something on the order of at least 1A, and a 50V/6A (RS #276-1661) is even better. Go to another RS or order some online but don't keep that 21ma diode in there.

Stan

 

Yea i wanted the 200v(or 100v) 1amp rated ones, but the guy insisted on the 12v. Then this other sales guy came and said, "21 milliamps should be enough for the 1 amp". I was like wtf, but they're the "experts" . Anyway, no problems yet, its been working fine for now. What will happen to the diodes for being underrated? Will they melt/blow or something? As for the Zener diodes, I cant recall the kind. I got some long skinny ones, or do they all look like that?

 

Most of the Radio Shack guys I've seen "these days" are experts at sales, but not electronics. Well, not hard electronics. They know cell phones, A/V, computers, but find me one who actually knows Ohm's Law and I'll Fed-Ex you a beer. Most of my trips there now are weekend emergencys when the "real" electronics parts stores are closed, or when I need one little part and don't want to drive 45 mins round trip. I usually muster something more polite to their "may I help you?" 's than what I'm thinking, which is something like "Yes, you can help me. You can shut up and step out of my way, so I can get to the parts bins and finish the wattage calculation I'm trying to do in my head." I really miss the days of my youth when they had a paper catalog that showed ALL of those parts(which were more numerous then), and the counter guys would say "Yeah, this will work good. I used this in a project that I built last month." Guess I'm just getting old...

 

BTW, yes, if they are underrated they'll blow/meltdown. Probably not Zeners(which do look essentially the same, but are an entirely different animal). They are used as voltage regulators and probably wouldn't work at all in your application. I'm just not accustomed to seeing that low of a voltage rating on a diode unless it's a Zener, and they are usually accompanied by a wattage rating as well.

 

I'm betting the diodes are Zener, I too have never seen a diode rating that low for peak voltage. Are they kinda orange/redish? There should be a number stamped some where on the diode itself, that will tell you what you acutally have, anything 1N100* will work, the last digit denotes the voltage rating.

 

You said they might be Zener if they had a wattage rating. Im pretty sure they did. As for the number of watts, I cant remember, but it was a pretty low number.

 

Well, i searched on the radioshack site, and found the ones that i bought:
1N4742A 1-Watt Zener Diode
$1.29
Type 1N4742A, has 12.0V. Case: DO-41. There are 2 per package.

So it turns out, they are Zener. But 1 watt?? Anyways, one of the diodes did mess up (the one that keeps my interior lights off). The other one is still good. What a waste of $1.29 What rated diodes should i get?

 

The diode voltage rating isn't very important as long as it is at least twice the voltage you want to block. To be on the safe side, I'd look for diodes rated for at least 50 Volts. The current rating is more important. They sold you 21 miliamp diodes. That is 21 thousands of an amp. A 12 Watt lamp will draw 1 AMP of current. The diodes they sold you are under-rated by 979 miliamps. Take 65CJ5's advice and get a pair of RS#276-1661 diodes.

By the way, a zener diode is not ment to block current like a "regular" diode. It is designed to flow current in the reverse direction but develope a specific voltage across itself as that reverse current flows through it. A 1 Watt, 12V zener diode is designed to develope 12V arcoss itself as up to .8A flows through it. I'd look for another Radio Shack store.

 

Originally Posted by 200000+F150:
Most of the Radio Shack guys I've seen "these days" are experts at sales, but not electronics. Well, not hard electronics. They know cell phones, A/V, computers, but find me one who actually knows Ohm's Law and I'll Fed-Ex you a beer . . .

I really miss the days of my youth when they had a paper catalog that showed ALL of those parts(which were more numerous then), and the counter guys would say "Yeah, this will work good. I used this in a project that I built last month."

I second that emotion, 200000. Some years back, my local Radio Shack dude helped me with some of the tuff stuff on a project I was working on.

He knew his business from electrolytic capacitors to choke coils to diodes AND knew where everything was in the store.

Guys like that are a great resource. Unfortunately, they're a dyin' breed, methinks.

 

Well, now the other diode crapped out...So much for those idio...i mean "experts" at radio shack....