555 EFI

[Robert W. Hughes]

> I've been thinking about a simple EFI using a string of 555s.
> Just like the "Normal" math would be, ie first 555, base pulse,
> second Coolant temp correction, third MAP, and so on for like
> 6+ 555 all in series.  Trouble is this gets me, an addition of
> correction factors, instead of multipling them, is this a major
> problem?.  I realize this ain't gonna be as good as a oem,
> ecm, but would like to get fairly close.

Another approach - the switching point (pulse width) on the 555 is
controlled by the voltage across the timing capacitor. When the voltage
rises to a certain percentage of the supply votage, the 555 switches. If
the capacitor value is constant and the resistor value is constant, the
only thing controlling this timing is the voltage applied to the
resistor. Obviously, this voltage must be high enough to trigger the 555
but it can be noticeably higher than the supply voltage. So, build an
op-amp circuit to add together the MAP voltage, the coolant temp
correction, perhaps even a function of engine speed vol eff curve) and
produce an output voltage of 5 to n volts. Run the 555 on 5 volts and
apply this voltage to the timing resistor. The timing resistor and
capacitor would be chosen for a pulse width slightly shorter than that
required for idle at 5 volts and the high voltage (n) would then be
chosen to give the maximum pulse width desired. You would probably need
a limiting circuit across the cap (like a 5v zener) to protect the 555
from the high voltage possible at high pulse widths and low speeds. The
major problem here is dynamic range, the wider the ratio of min to max
pulse width, the higher the voltage the op-amp circuit must produce.
-- 
Robert W. Hughes (Bob)
BackYard Engineering
Houston, Texas
rwhughe at hal-pc.org