I'm not entirely sure about this, I honestly don't know enough about the practical circuits involved, but you may be able to get away with using a single NPN transistor. Feed the 4.6 volt signal into the base. Connect the collector to +12V and connect the emitter to the ECU input. Pretty much any NPN transistor will do.
That's what I think the single transistor should do, the problem is I don't know what type of output your AC switch actually is, or what type of input your ECU actually takes, the circuit components on the other side of those wires makes all the difference, and I'm just not that good at the practical side =) I'm surprised other users aren't chiming in here. I'd try the single transistor, it's something you can pick up at radio shack for less than a dollar.
If you can get a discrete transistor out of an old device the website I'm linking bellow tells you how to determine what the base/collector/emitter leads are. If you don't get results consistent with what it says try another transistor as the one you're checking could be PNP
The side labeled bc182 is what a typical small signal transistor looks like. BUT the collector emitter base pins may be different, that's what the above link is for determining. **broken link removed**
I'm not entirely sure about this, I honestly don't know enough about the practical circuits involved, but you may be able to get away with using a single NPN transistor. Feed the 4.6 volt signal into the base. Connect the collector to +12V and connect the emitter to the ECU input. Pretty much any NPN transistor will do.
That won't work. What you are describing is an emitter follower. The emitter voltage will never get higher than 0.7V below the base, or about 3.9V, in this case.
The resistor divider on the NPN's gate lowers the 4.6 volts bellow 500mv, the base junction never becomes forward biased. If you skip the 10k and 1k resistor divider on the base and just use a 1k resistor to the base it works fine. What's the resistor divider for?
I was hoping the single NPN would provide a low enough impedance compared to the existing AC switch signal that it might trigger it, I'm shooting in the dark here as I don't know how to determine what type of input/outputs those are though. As I said I'm not so good at the practical side of things. If Sketch can find an NPN and PNP transistor and a few resistors he should be able to replicate your circuit, with a single 1k to the base of the NPN instead of the 10k + 1k to ground.
I played around with it a little more, for simplicity sake the 3.9k resistor can be replaced with a 1k doesn't alter anything in the simulation I ran
With 4.6 volts in and using a 600 ohm resistor as a load (which matches the current draw he's getting directly shorting the 12V to the ECU) I'm getting a nice clean 12V signal.
Hopefully Sketchy can put that together. You can probably get all the parts off of old boards, for assembly though I'd buy a small piece of perfboard to solder it to.
I'm not entirely sure about this, I honestly don't know enough about the practical circuits involved, but you may be able to get away with using a single NPN transistor. Feed the 4.6 volt signal into the base. Connect the collector to +12V and connect the emitter to the ECU input. Pretty much any NPN transistor will do.
an emitter follower will only output the base voltage minus 0.6 or 0.7V, i.e. 4V
try something like this, no relays required..... just about any NPN and PNP transistor will do. the input resistor can be anything from 100 to 470 ohms, and the other two resistors can be anything from 1k to 10k.....
Uncle's last schematic should work just fine, given what you've stated so far I don't think the 470 ohm gate resistor is NOT required though, use it at first remove it if it doesn't work. The voltage isn't incredibly sensitive, generic NPN/PNPs will be well within a car voltages range.
an emitter follower will only output the base voltage minus 0.6 or 0.7V, i.e. 4V
try something like this, no relays required..... just about any NPN and PNP transistor will do. the input resistor can be anything from 100 to 470 ohms, and the other two resistors can be anything from 1k to 10k.....