Here is a picture of the faceplate that
holds the LED bargraphs for air/fuel and pulse width as
well as the pot to control the fuel mixture above 7psi
boost. The switch allows either the primary or secondary
injector pulse to be monitored. The faceplate is made of
polished aluminum. The air/fuel (O2) monitor, pulse
width monitor, and fuel controller are all on one
printed circuit board.
Shown below is the schematic for the
pulse width monitor. The heart of the circuit is the
transistor current source which charges cap C10 to form
an integrator. The theory is that the cap is charged and
the voltage across it increases linearly for the amount
of time that the injector is energized. As the injector
is switched off the voltage in C10 is stored in cap C9
and then C10 is reset to 0 volts to get ready for the
next cycle. The output of the circuit is the voltage
across cap C9, this voltage is fed directly into U3 the
bar graph chip to be displayed as a value between 1-10
on the LEDs. Potentiometer R31 sets the charge rate on
C10 and basically sets the max injector on time that can
be displayed.
The calibration on this circuit is
pretty tricky. You'll need an oscilloscope and a
waveform generator. Setup a 62Hz 0-12v 50% duty cycle
square wave at the input to the circuit. Use the scope
to monitor the voltage across C10 Adjust pot R31 such
that the ramping voltage across C10 just reaches the
peak (starts to flatten out). Now adjust the pot R22 in
the display circuit such that LED 10 just comes on.
With the calibration described above LED
1 corresponds to about 1.5ms and LED 10 corresponds to
about 8ms of injector pulse width (on time). Injector
duty cycle in dependent on engine speed according to the
following relationship:
% duty cycle = ( pulse width / period
)*100
period = 1/[engine speed (rpm) / 60]
Click
Here to View Circuit
|