converting polarity issue

Kevin, Thanks a lot for that diagram. I have never used a transistor before, but I have been reading about them and they do not seem to be too difficult. I imagine in this case I will not be amplifying any signal but instead just using the transistor as a switch. On your diagram at the bottom you show the output as a negative out, but you have it labeled as 12 VDC, are you just saying that it is a ground output? Also, on my install I diode isolated all four of the door triggers, but I do not see any diodes on your diagram, that will not make a difference will it?

The diodes are there - they are the black triangles before all four switches come together.

The transistor collector should NOT be connected to 12vdc.  I'm not sure why I did that.  That (-) output should be connected directly to the (-) input of the alarm and voice module.

Make sure you connect the emitter of the transistor to ground, the base to the output of all 4 doors, and the collector to the inputs of the devices.  Should be pretty easy.

Kevin Pierson

Scam, the diodes are represented by the arrows at bottom going to the transistor. The bands would be towards the transistor. I agree with your polarity comment, what's the answer KP?

Sorry, KP, bad timing.

thanks a lot guys, this forum is awesome. I am actually getting to use some of the crap I am learning about in my electrical engineering classes.

I presume the output's input has a pull-up resistor etc? (IE - In KP's tranny diagram. That's a classic "open collector" output - great stuff - the "universal interface solution".) I also note a lack of base resistor... Base current will vary 4-fold depending on how many switches are open.... OH NO IT WON'T! Geez there are some idiots on this forum!   Base current will be via 2.5k else nothing. (~12V/2k5 ~5mA) See - I knew KP was right! ... Though I was going to suggest a FET to overcome that - ie, "base" = gate current irrelevant (nA or uA), merely its gate voltage higher than about 5V to turn on, but must be pulled low for off (normally a resistor as with a transistor, but 1M etc - but not needed in this case because either a switch pulls it to ground else dosn't. Of course one wonders how those diodes will pull the base or gate down to 0V/GND with that orientation - but maybe I am the "one" that wonders that? I'd have them the other way with a single base/gate resistor to +12V (eg, 2.5k => 2.2k or 2.7k etc, or 1M (or 1k to 10M) for the FET).... But it is before 8AM here.... (and I skipped the switch resistance detail).    And of course, only "the first" door triggers, and last door "resets" - ie, the change occurs between all closed and [any or all] open. Coffee time.... Then maybe a second laugh at some nameless dork, else maybe a chance to redeem mys... him/herself. (I'll bet the former...) (More) Trivial FYI only:

scam404 wrote: I am actually getting to use some of the crap I am learning about in my electrical engineering classes.

WHAT? It wasn't until 3rd year that happened to me - and that was merely finding out that by moving the fast brush closer to the ground brush in my wiper motor, I would have a reasonable high speed instead of "slow plus stuff all". Mind you - I was totally rapt. It took 3 years, but I finally learned something practical. Compare that to second year where I had a lecturer removed when he suggested the LM317 adjustable 3 terminal voltage regulator was "probably" adjusted with a screw driver.... and that was for a practical "Design" class lab-prac! (Actually I let that one go. I had him removed later when - in his other "theoretical" Electronics class - he scoffed at those that couldn't "design" as simple 2 stage tranny amp (but with DC coupling!). Yet neither could he as we later confirmed. That was an other implementation of what some knew or now know as "DFWAA". I won't mention the other times, but I strongly suggest you "go with the masses" - never EVER correct long erred beliefs or prac class results - let others do that. But it was good training for real life LOL!)    Sorry - I love formal education and qualifications as you may well be aware. (Experience - just get that from a book or school. My old uni still uses the latest in SSL2 protocols and new yet disparit(??) IT systems with manual transfers - of REPEAT info! But my later uni now allows working in groups and does not consider it "cheating"! Masters of F-wits those Degrees!)

I run much higher resistances through a diode though the base of a 3904 with no issues (as high as 1.5 megaohm).  Because the transistor is only a switch the changing of the base resistance won't matter. 

The alarm inputs should have pull ups on them as most car alarms are NO and go to ground when door is closed.  This, is theory, of course, and a 10K resistor can be placed between the (-) output and 12vdc if there are any issues.

Kevin Pierson

Ok I just got done installing the transistor, and everything works perfectly now. That thing was a real pain to solder to the individual wires though. Is there some way I dont know about to solder those things? It looks like it is meant for circuit board use only... I had to CAREFULLY put tiny pieces of electrical tape on each one to keep them from touching.

I would have bought a little prototype board and soldered it ot that, then soldered the wires to the bottom of the board.  If you solder directly to the pins and make sure all the wires are taped together so no individual leg can be pulled off you'll be ok.  Alternatively, you could have went with a much larger NPN transistor that is available in a TO220 package - a little friendlier then the TO92 package.Kevin Pierson