Need help choosing resistors for a circuit

Thread Starter

boosted_v6

Joined Dec 3, 2017
2
Hey guys, new here and thank god I found this forum!
So!, I found an old thread from 2013 that has pretty much solved the same issue I've been having of late (am only a beginner when it comes to circuits and their components)
Thread located here:
https://forum.allaboutcircuits.com/threads/converting-negative-signal-into-positive-signal.8030/page-1



So I need to convert a ground output from my cars ECU to a 5v input to a coil igniter. The attached circuit works apparently, though it is for 12v, I need to basically know what resistors etc I need to make it work for 5v? The 5v source I will be running from 12v through a 5v regulator to get it to 5v (unless you can recommend a better way?

Any help would be greatly appreciated guys! An easy job for you gurus

Thanks in advance
 

DickCappels

Joined Aug 21, 2008
5,930
Making +5V from +12V with a 7805, 78L05 or similar sounds like a good idea.

That circuit will work just fine with a 5 volt power supply.

There remains two questions concerning whether that inverter will operate properly in your system:

What does the input signal look like? (Exactly what is making it?)

To what,exactly, does the output signal connect?
 

Thread Starter

boosted_v6

Joined Dec 3, 2017
2
Thanks for the reply.

So resistors don't need to change values when running 5v through the emitter as opposed to the 12v in the schematic?

Ok, so in a nutshell (hopefully makes sense)
-
ECU (Subaru OEM) to charge/fire the ignition coils on the factory wasted spark system, the ecu grounds the wire that runs to the ignition coil/ignitor, which begins the charge, then when ungrounded some milliseconds later, the coil fires.

However, because I am running a much higher horsepower engine setup, to cope with higher boost pressures and having the spark not 'blow out' at the spark plug electrode due to much higher cylinder pressures etc, I am converting to the extremely powerful GM LS2 coils. These coils are internally ignited, they need a 5v high signal (which starts dwell) and when signal goes low, the coil fires)

So in essence, the ECU ground signal gets converted to a 5v signal (same as diagram, but 5v instead of 12)
 

DickCappels

Joined Aug 21, 2008
5,930
Just to be safe, add a small signal diode like an 1N914/1n(16/1N4148 or even a larger one like 1N4001 through 1N4007 from the base to the emitter so that the cathode is on the emitter and the anode is on the base. This will protect the transistor from base-emitter reverse breakdown which might result in its gain dropping with time.

Do you know for sure that 1k to ground with 1k in series is a low enough resistance for the igniter's input?

That is the extent of my concerns though others with more experience might have some additional helpful insight.
 
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