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

[Saxon, Howard (AFS-Warren)]

Thanx a lot Brian for the basic walk thru.  That's the type of stuff that I
need to read.  Just looking at the circuit diagrams doesn't tell me the
whole story so your explanation really helps out.
Of course, I want a working electronic device but I also want to know how it
works and why.  This is probably a bit too involved for a super beginner,
such as myself, but it's the reason I'm learning about electronics to begin
with.
I'm going to try and get some samples of the LM2907 to work with but if I
want 2 different switches to turn on at 2 different speeds, would I just
need 2 LM2907's.?

Howard 


	-----Original Message-----
	From:	Brian Dessent [SMTP:brian at dessent.net]
	Sent:	Friday, June 21, 2002 10:52 PM
	To:	diy_efi at diy-efi.org
	Subject:	Re: [Diy_efi] Looking for some help on a speed
sensor project - added

	"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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