[Diy_efi] Re: Photocoupler?

[Marcell Gal]

Hello, 

Tony Petroski writes:
> Could someone clear this up ? Or provide alternate ways in generating
> TTL signals from existing circuits ? i.e. like to log. Say we have to
> microchips communicating between TX/RX 0-5 Volts, & i want to log the
> traffic over teh data lines via a third microchip. What can i use to
> tap into the circuit without affecting (minially) the circuit?

If your logger is on vehicle ground, you do not need a
photocoupler. You need a resistor (10..20kOhm maybe) from the
injector to the logger input and better use protection diodes
to supply and ground (although most inputs have such).
Your injector might be connected to 12V and the other pin
pulled to ground when injecting. In that case you'll have to
invert the signal - preferrably at analysis-time. 

If you want to use the photocoupler:
2..4.7k Ohm  in series with photocoupler input LED
This connected parallel with injector (polarity counts).
Photocoupler output pulled up to +supplyV with
4..10 kOhm, and connect this to input of logger.
Take this with a grain of salt, it might not work
for your setup, or even blow up your dog. 

However this is very basic electronics, you'd better
get a local friend to help (tell them you have
inductive an load driven with quasi-squarewave 12V signal)
and read some more about it elsewhere, otherwise you can
make a lot of damage and headaches. 

  Marcell 

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