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New (ignorant) guy and have my first question...

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qikgts

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Hi. I have an odd situation and I hope you guys can help me out of little jam. I'm working on a car that has a door trigger circuit that needs to turn on some lights, but each door circuit rests at ground and switches to open when the door opens up. This would not really be an issue if I wanted to keep a spst relay energized all the time, but I don't.

So, my question is: Is there a simple (easy for the novice) circuit I could build that could look at 4 ground inputs and give me an output (either pos or neg) when one goes away?

I'm pretty sure I can do a simple project on a breadboard as I do have some soldering skills. I also just took a look at the 5 zone alarm on the free electronics circuits website linked at the bottom of each page here.

I am really clueless, but it seems that if I use a single CMOS 4072 I can get the job done. Now I'll let my ignorance shine... Can I just put that chip on a breadboard and solder the door trigger wires, power and ground and the output wire right to it?

Please help....


Thanks all... Cool forum here....
 
You could use a pull up resistor on each input. the resistor would just connect to +12V and then to your door trigger.
**broken link removed**
R1 is the pull up resistor, s1 is the door switch and the inverter (triangle thingy) would be your circuit.

When S1 is closed, the path of least resistance is to ground, so you would see 0V at your circuit. When S1 is open, there is no path to ground, so the next best path is through R1, which pulls the output up to 12V.

Hope this helps, and welcome to the forum.
 
Thanks for the response Mr. Zimmerman. Your idea looks simple enough, I think I'll give it a try. I do have a question though...

After I make the connections and have each circuit switching to 12v when the door opens, would I need to diode isolate each door at the point where all the outputs get paralled together, for example on the coil input of the relay which switches lamps on?
 
His real problem is he needs to OR the four sensors.

A CD4072 OR gate (high output) or CD4001 NOR gate (low output) should work. You might need the pullup resistors, depending on what is connected to the switches. You should probably also add input protection and power supply filtering, since automotive power systems can have large spikes on them. We can provide better advice if we know what you are going to drive with the output of your OR gate. Are you driving a relay? If so, how much current does the coil draw?
If you use CMOS circuits (4072, etc.), you will need to connect the inputs of all unused gates to GND.
 
so you want to turn something on/off when any of the 4 doors are opened?

Wouldnt the 4 diodes act like an OR gate?
 
Another question: Are the switches already connected to individual loads (e.g., lamps)?
Are you just wanting to turn on a light when any door is open? I thought all cars had this function built in.
 
Thanks for the input everyone.

I'll try to give all the information. Sorry for being incomplete in my previous post.

All 4 doors see 12v- when they are closed, when they open they go to nothing. None of the switches drive anything right now, there is no load on them.

I need to trigger a relay when they open. I believe a standard Bosch automotive style relay draws about 110 milliamps on the coil.

Ron H, you are correct that every single car on the road turns on the dome light when a door opens, however the correct fix for this vehicle is to replace 2 modules which failed and it will cost about $1200.00. I can duplicate the lost functions of the failed modules, but the car is not set up where I can easily grab a ground or 12v when the door opens. Hence, the work around.

I did try the circuit jrz126 provided and added diodes to each door trigger line to block the ground from going back up the line and seeing the 12v going through the 10k ohm resistor. It does add voltage to the circuit, but its only .100 volts.

What size resistor will I need to get 12v? 100 ohm?

Thanks again gents....
 
Update....

I went down in resistance until I could trip the relay and wound up settling on 68 ohm 1/2 watt resistors. It seems to be working, but it's generating a little too much heat for me to feel comfortable stuffing it up under the dash. The temp was fine until I had all 4 doors open for about 5 minutes. It's been sitting with one door open for around 25 minutes now and it's still hot but not burning down :lol: , yet.

Should I stick with this circuit or switch to a or, nor or cmos?

Will the or, nor or cmos generate less heat?
 
Here's a couple if ideas. There may be a simpler way, but these were my first ideas. You could drive the lamp directly (not with the 2N2907), but I don't know if the other end of your lamp is tied to GND or +12V, so I put in the relay.
The NAND gate has lots of resistors around it to protect it from voltage spikes.
 

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