I would substitute a transistor for the MOSFET and use it to switch the relay. I will modify it tomorrow to suit.

 

I would substitute a transistor for the MOSFET and use it to switch the relay. I will modify it tomorrow to suit.

Okay. Thank you very much for all of the work that you are putting into this!

 

OHHHHH didn't I send the bill already????????

 

Oh, so that's what that was! I just shredded a request for a donation of a couple thousand dollars to a Swiss bank account! Sorry about that I didn't realize that was from you. Would you mind resending it?

 

Try this one. Should do the job.

 

Ok I think I can do that, but I do have 3 questions:

1. What size Zener diode do I use? I have a 13v one but now I am not sure if that is the right one.
2. What type number of transistor do I want to use? Will a PN2222A work (link to datasheet)
3. I'm not sure what the voltage regulator is there for but I assume it is to protect both the lm393 and the relay. I have a couple of lm 317T voltage regulators (link to datasheet). Will one of them work set at 9 volts? Neither circuits draw much current so the lm317 shouldn't even heat up.

Thanks again for all the help.

 

Here we go.

1. No. For the circuit shown you should use a 6.2V zener.

2. The one I have shown is a darlington. I like to use those because they can handle higher current. A 2N2222 would probably be ok.

3. Any regulator that can be set to 9V and can handle 1 or 2 amps will do it.

 

Great! I can go buy a 6.2v Zener diode and a TIP142 (Link to datasheet). The TIP142 is a darlington that can handle up to 2 amps without a heat sink according to this site. I will try to use the lm317 (max 1.5 amps) but if it heats up to much I will use the one that you suggested (LM78L09ACZ). Once I get the parts I will put the circuit together.

Thank you very much for all of you help and patience with all my questions! I would never have been able to get this circuit together without you all. I would also like to give special thanks to windoze killa for all the work that he did drawing the schematics.

 

I would use a standard LM7809 in a TO220 case if the 317 doesn't work.

 

Hi, I got the circuit to work! I used the lm7809 right away and didn't bother with the lm317. I had a lot of trouble getting the whole circuit to work because the relay would trigger at around 8 volts but then only "untrigger" at around 5 volts. I figured that I had messed something up building the circuit so I did the whole thing again from the beginning, but the results were the same. To make a long story short I did a lot of voltage testing, connectivity testing, and double checking. I used leds to give me a visual signal for when what was triggered. After noticing that the led inserted after the darlington faded in and out as the power went up and down, I decided to just remove R5 from the schematic. After that everything worked perfectly. The relay triggered at exactly 12 volts so I added 2k to R2 and that raised it so that the relay triggers at 13.15 volts. I decided that 13.15 would be good because that way there is no accidental triggering but it would still definitively be triggered when the car turns on.

Is it ok to leave out R5?

 

Relays always switch off at a lower voltage than when they switch on. I am not an expert with comparators so I am sure others could help better but I would have thought that when the input came back below the 6.2V the comparator would switch and turn the transistor off.

As far as removing R5 that would probably fix the problem. I am assuming you are asking because you have tried it.

 

Yes, once I removed R5 the relay acted exactly as you had predicted before, it triggered around 12.01v and then untriggered around 11.99v. As far as I can tell the LM393 always worked correctly and was triggering at 12 volts but the darlington was being slowly triggered by the voltage across R5 and the voltage faded in enough to trigger the relay at 8 but then didn't fade a way enough until I lowered the voltage down to 5 volts. All of that was happening without the lm393 doing anything. Once I removed R5 and so removing the voltage to the base of the darlington everything worked as planned. My question was to make sure that I wouldn't eventually burn anything out because I removed R5.