[Diy_efi] Looking for some help on a speed sensor project - added

[Brian Dessent]

"Saxon, Howard (AFS-Warren)" wrote:

> I really appreciate the help you guys have given me so far, it seems I'm in
> the right direction now.

Okay, so I would start with the circuit on p7 of the datasheet, which
has the caption "switch energized when f_in >= 1/(2*R*C)"  In the
diagram, the block marked "Load" represents the coil of the relay that
you want to energize.  If you're using a relay that requires more than
50mA coil current, you may have to use an external driver transistor,
otherwise the chip can drive it directly.  'f' here means a frequency in
Hertz (pulses per second), R is the resistance in Ohms, and C is the
capacitance in Farads.  On the diagram, the large block represents the
chip itself and the small numbers refer to pin numbers.  The symbol
shown connected to pin 1 represents the input source of pulses.  So you
just need to choose R and C so that the relay closes at the right
speed.  Typically there are fewer choices for capacitance (and generally
the listed component values are only accurate to 10% or 20% or so) so
what you want is to make R variable -- a potentiometer (pot) is a knob
that varies resistance as you turn it.  To take your example of 25 mph
(55.6 Hz):  let's say you choose C = 0.1uF (the u prefix means micro or
10^-6) which is a common value.  That means that R should be about
90000, or 90k ohms.  If you used a 250k pot this would allow for easy
tuning, and your specified speed would be kind of near the middle of its
range.  Or you could use a 100k pot and have less range but finer
control.  For 60 mph (133 Hz), try 0.047uF and 80K (or a 100k pot.) 
Note that you should try to find ceramic capacitors for this component,
do not try to use the electrolytic type.  Also note that you'll probably
have to play the units game when buying caps: pico = 10^-12, and 0.047uF
equals 47000pF.

Note also that there is a second capacitor in the circuit (the one on
pin 3) but its value doesn't affect the output function.  1uF is a
pretty good default value for this one.

You can probably connect the input pin 1 of the 2907 directly to the
speed sensor (with it connected to the computer as well at the same
time.)  The input is a high impedance input which means it won't draw
much current, i.e. it won't "load" or disrupt the signal much.

For quick prototyping, try a small breadboard and some 22AWG solid
wire.  Once you're happy with the results, there are a number of ways to
make it more permanent.

As a final note, you can get free samples from National Semi on their
website (www.national.com) which is a great way to get a small quantity
of the part.  You'll probably want the "-8" version of the chip, and
avoid stuff with "SOIC" in the part number, this means surface mount. 
You want the part called "through hole" that has DIP in its name.  For
other stuff you can use Radio Shack, but if you find yourself in need of
more selection, try Digikey (www.digikey.com) or Mouser
(www.mouser.com.)  Mouser doesn't have a minimum order amount, I think
Digikey has a $20 minimum.

 
Brian

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