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Old 10-06-2008, 06:50 AM   #1
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Anyone know how NPN transistors work?

Ok, there's a lot of smart electronics people on here I've notice. I only have a rudimentary knowledge and I'm having an issue.

The Festiva I got, the first day I drove it I noticed that the headlights are wired directly and ignore key input and do not buzz or anything, and thought "Man I'm going to forget about these lights sometime, I should wire this to go off with key out".

Then 3 days later I forgot about the lights and killed the battery and got stranded on the side of the road and lots of fun stuff.

So I ripped apart the dash, cursing my laziness, glad I'm finally doing this and find out that the parking lights and headlights are wired through the relay already to main power and the headlight switch is just grounding them. Not as simple as just supplying a different power source to the relay as I had hoped.

Then I thought, hey, I know how transistors work, if I wire the ground wire coming from the switch to the collector, the emiter to ground and the base to accessory power, that should work great!

So I wired it up everyway possible with this transistor. With the ground wire from switch cut the headlights don't come on. WIth leads hooked to emitter and collector or vice-versa, either one, I can diode test and connection test and verify there's no connection between the leads, but when I turn the headlight switch there's a connection between the emitter and collector. This is with NOTHING hooked to the base. So is there a certain voltage or current that just 'jumps' the base of a transistor? Everything I've read says 'no' and that anything less than 15V will work fine as expected.

Also am I going about this the wrong way anyway? Even if it works as I expect and I need to put power at the base to complete the NPN connection, removing power from the base, will that 'turn off' the NPN connection? I've seen a lot of applications where once transistors are 'switched' they stay that way until main power is removed.

Any thoughts on how I can accomplish my goal? I guess when in doubt I can just put a buzzer inline connected to accessory power too.
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Old 10-06-2008, 07:22 AM   #2
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The easiest way to solve your problem is to use 2 12 volt relays in a series and a 12 volt buzzer. Wire the relays in a series so that the buzzer will not sound unless the lights are on, and the key is off.

Also, if you're into a little hunting in a scrapyard pull the chime assembly from an early 80's Buick. Its a big white plastic box plugged into the fuse panel under the dash. Simple experimentation will tell you what power neeeds to go where. The only thing I can remember for sure is that the grey wire coming out of the top goes to the dash lights so it knows when the lights are on. I did this on Rusty. Left the lights on once, decided on installing a warning circuit. I took the stock seatbelt/door open/key in/headlight warning buzzer out of the fuseblock and replaced it with the chime from a 1980 Buick Century, and then hooked up the loose wire to the dash lights. I then had a chime, not a buzzer and I had a headlight warning just like the one in my 1981 Regal.

-Jay
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Old 10-06-2008, 08:52 AM   #3
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I must admit, I didn't read the entire post but why not just use a relay that is energized with the ignition switch and splice that inline with the headlight wire.

that way your headlights will NEVER be on unless the ignition is on.
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Old 10-06-2008, 09:26 AM   #4
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I like the way you think BEEF.

-Jay
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Old 10-06-2008, 09:32 AM   #5
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you could also use a solid state relay so it will last a LONG time. they are expensive though.

I am not a ford guy but a friend of mine just read throught this thread and told me that a festiva should have a chime on it when the lights are left on. he figures someone has previously ripped it out. junk yard and some time. there again, this info is second hand.

*edit* I guess you could use a transistor for this application but why not go simple. if you do need to use the headlights without the car on, you could put it on the accessories so that the key has to be in but not on. though I like doing it the semi-complicated way just to say "I did it" sometimes you have to revert to the KISS method. Keep It Simple Stupid (I think it is from the military)
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Old 10-06-2008, 10:35 AM   #6
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Personally this is how I'd wire it.

Have the input to the relay coil come from something, anything that is only on when IGN is 'on'. Then, leave the ground working though the headlight switch.

If the switch is off and the ignition is on, lights are off. If the switch is on and the ignition is off the lights are off. If they are both on then the lights are on. Simple, easy, and doesn't require anything more than a wire moved.
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Old 10-06-2008, 11:09 AM   #7
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It's fine to wire the existing relay to acc on, but I traced for a couple hours and couldn't find the relay, I think I need to dig the mess out from behind the headlight and it's ziptied pretty tight and I'd prefer to avoid it if I can. Especially since I need to do this twice, once for headlight and once for parking lights.

As for putting another relay inline, of course this can be done, but I don't see how that's the easy way. It's bulky and doesn't make a lot of sense since a transistor should be able to be used as a switch and do the exact same thing.

If it's simply just a matter of hooking this up more correctly then why would I want to trouble myself to fabricate something from a junk yard?
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Old 10-06-2008, 11:27 AM   #8
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Quote:
Originally Posted by itjstagame View Post

If it's simply just a matter of hooking this up more correctly then why would I want to trouble myself to fabricate something from a junk yard?
Just giving you as many options as possible. You can choose what's best for you. Pulling the chime unit out of a 1980 Century was almost a plug in replacement on my 86 Chevy p/u. All I did was take the old buzzer out, plug in the replacement, then hook up the extra wire to the dash lights. Literally took 2 minutes to do. YMMV.

-Jay
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Old 10-06-2008, 01:11 PM   #9
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there are a thousand ways to do the same job. what is easy for me isn't easy for someone else.

it is good to have options. I would consider what everyone has said and take what you think will work the fastest and easiest for you (and still do the job) and run with it.
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Old 10-07-2008, 05:51 AM   #10
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Ok, so everything I read yesterday said that the way I was hooked up was correct and that power should never jumper between collector and emitter with no base connection. My transistor tested out ok on a multimeter diode test but I guess the 12v of the battery was enough to cause some issue, don't know. My guess was that I had shorted or broke it in some way.

So this morning I got out a new one, wired it in the same way (leaving base empty) and the headlights didn't work! Woo! Also reading yesterday, transistors work on current, so wiring Acc On to the 'base' directly is probably what broke it, so I put a 10k resistor inline and voila! Works perfect! Both headlight switch needs to be on and key on 'Acc On'. Turning off either turns off the headlights. In fact I pull 'Acc On' from a spot that goes out during cranking, which I think is even better, less amp load while starting.

There is one slight issue that if I turn the headlights off and back on quickly (< 1sec or so) that they don't come back on, I have to turn off and wait at least 3-4 secs. I'm not sure why that is but it's fine.

So a 5 cent transistor, 5 cent resistor and 10 mins of soldering. For me that's much easier than finding a junkyard (they all suck here) or tracing and pulling apart the harness looking for a relay somewhere. It would have been done yesterday if I hadn't shorted out my transistor.
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