[Diy_efi] schematic critique

[Mark Bowers & Family]

J,

I have finally gotten around to the schematic, drew it by hand, and have
scanned it for you, into a 750k .jpeg file; it a bit fuzzy, but readable.
Let me know where I can email this, and you can put it in your schematic SW
and post.

Items to note about this circuit:
1) Includes an input buffer resistor on the 12v supply to the 7808; this
prevents the input filter cap from trying to filter out all the noise on the
car's 12v system, which will dry out the cap in short order and make it
useless
2) Adds filtering on the line from the fuel level sender, plus a slow time
constant (several seconds) to smooth out any "wiggling" of the sender
output. The noise filter is important as you will ground the sender near the
tank, and the gound for the control circuit may be in a different location
in the car. If so, you will need a noise filter.
3) Feeds the fuel sender from the 8v bus; this will be a MUCH more stable
circuit, rather than feeding the sender form the car's 12v syste, If you use
the 12v system to feed the sender, without running cvia the dash units
regulator, the switching point of the circuit will vary widely as the car's
12v system voltage varies. (Hence why the put soem soret of regualtor in the
dash!)
4) Implements the hystersis you wanted of approx 0.1v; you can increase this
by DECREASING the 220k feedback resistor, within reason
5) Uses a simple NPN output driver

Things to note:
a) Assumes that the fuel sender will present about 55 k ohms resistance at
the point that you want the pump switched off; this can be changed by
varying the pot connected to pin 5.
b) The output drive transistor can support about 100mA max current, and
still be saturated. (Which means it will turned on hard, and there will not
be much heating in the transistor.) This means your relay coil resistance
needs to be about 1200 to 1500 ohms. It is important that you measure this.
If it is much lower, then the FET driver circuit that was espoused by one
fellow should be used.
c) Assumes that the fuel guage will NOT be connected to the sender; this is
important.

I took a quick look at the schematic 7 and I can't see it working at first
glance; I lost the link so I can't look at it more..

 And, my apologies for the long time to work on this; gotta work for a
livin' !

Regards,
Mark B.




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