[Diy_efi] Re: Looking for circuit to turn transfer pump off at

[Brian Dessent]

Toyota Supra wrote:
> 
> it will be torn off if anything ever goes underneath the car (small animal,
> or any type of road debris), the fittings required would be positioned under
> the frame rail and under the driveshaft, it would also put it dangerously
> close to the driveshaft. i've contemplated the gravity thing, but just can't
> have that hanging over my head.
> 
> i was looking at using the 555 circuit, but i would need a little help in
> values and schematics to do so. if you have any suggestions or thoughts, i'm
> all ears :) . thank you for taking the time to reply

What you want is called a window detector.  It's basically two
comparators, and two reference voltages.  Check out the datasheet for a
basic comparator, such as:

http://www.national.com/ds/LM/LM119.pdf

Usually the two threshold voltages are generated from voltage dividers
(i.e. resistors) -- you can use potentiometers if you want to be able to
adjust the thresholds.

The tricky part is what happens right along the thresholds?  If you
don't use any sort of hysteresis, the output signal could oscillate or
"dither" on and off quickly as the input signal always has some amount
of noise to it.  Hysteresis solves this by making the circuit nonlinear,
for example: say you want a 2V threshold, so that something turns on at
2V.  With 100mV of hysteresis, the input would have to rise past 2.050V
to turn ON and would have to drop past 1.950V to turn off.  There's some
discussion of this in this application note:

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi