If you are wiring something and you need to fix something
it helps to know how it works.

I am  wiring up an RX8 test engine and EC2
and I found out I had a lot of EC2 break out PCB board
space left over. I then had the bright idea of using that
space to implement OBD II for aircraft EFI systems using
a BASIC stamp. One of the first problems I ran into was
how to implement monitoring the injectors.
Not as simple as it seems.

Here is how the computer controls the injectors.
Injectors are simple solenoids. The top of the solenoid
is connected to 12 Volts. When the solenoid is
off there is no current flow. That means the bottom
of the solenoid is also 12 volts believe it or
not. One of the strange things about OHM's law.

Tracy uses a transistor connected to the bottom of
solenoid. Transistor are merely switches.  The mechanical
lever is replaced by a wire with a voltage on it fed into
the transistor to turn it on. When the transistor is off
it is seeing 12 volts from the injector. Now if I feed
12 volts into a computer chip on the BASIC stamp it would
blow up lickity split.

How then do I use another computer (The BASIC stamp) to monitor
the injector pulse width with out taping into Tracy's computer?

The trick is to use a device called an opto isolator
and connect that to the bottom of the injector.
The 12 volt lights the LED in the device and a photo
transistor turns on when it sees the light. When the injector
turns on the LED goes out and the BASIC Stamp sees a
negative going injector pulse.

Here is the schematic. I added four of these to
the breakout board. One for each injector.

Questions? Did I explain it well?

Paul Lamar


Good explanation Paul. 12V to the sillynoids. Transistor in the ECM grounds
the circuit to turn on. Safety thing to ground it. As you said, ohms law;
supply volts drop across the resistor (sillynoid), so virtually no volts
current flowing.

Dale Davies