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Old 06-07-2011, 04:14 PM   #20 (permalink)
glug
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I finally have an LM2907 in hand, and I'm ready to make this thing work... I'd like to cut down on the experimenting, so suggestions are appreciated.

The application note shows two circuits which seem to be appropriate (reproduced below, figure 23, from the top of page 14):

http://www.national.com/an/AN/AN-162.pdf

Any thoughts on which of those would be the best match for the MPG's VSS input?

The first circuit outputs pulses at 5v, and is also a frequency doubler (two-shot). The other other is a one-shot, and outputs a square wave of about 2.5v. Another gotcha - digi-key did not have the through hole 500nF cap needed for the first circuit. I have a 1000nF.

My VSS is about 10 pulse/mph, for 36K pulse/mile. So 70 mph is 700 Hz. Would 1400 hz be too high for the MPG input?

Since my proto mpg perf board is now a bit cluttered, I am planning on building this on a small sub-board.

Thanks!



From the AN:

Digital Interface
A growing proportion of the complex control systems today
are being controlled by microprocessors and other digital
devices Frequently they require inputs to indicate position
or time from some mechanical input The LM2907 can be
used to provide zero crossing datum to a digital system us-
ng the circuits illustrated in Figure 23 At each zero crossing
of the input signal the charge pump changes the state of
capacitor C1 and provides a one-shot pulse into the zener
diode at pin 3 The width of this pulse is controlled by the
nternal current of pin 2 and the size of capacitor C1 as well
as by the supply voltage Since a pulse is generated by each
zero crossing of the input signal we call this a ``two-shot''
nstead of a ``one-shot'' device and this can be used for
doubling the frequency that is presented to the microproc-
essor control system If frequency doubling is not required
and a square wave output is preferred the circuit of Figure
4 can be employed In this case the output swing is the
same as the swing on pin 2 which is a swing of half supply
voltage starting at 1 VBE below one quarter of supply and
going to 1 VBE below three-quarters of supply This can be
ncreased up to the full output swing capability by reducing
or removing the negative feedback around the op amp
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