Digital tachometer

[steve ravet]

This is a pretty straightforward project.  I don't completely understand
the operation of the LM2907 frequency to voltage chip, but basically the
coil signal is the frequency input, and this chip produces a voltage
output that is proportional to the frequency.  The other 2 (LM3914)
chips are LED display drivers.  They take an analog voltage in and drive
an array of LEDs.  The higher the voltage the more LEDs are lit up. 
These chips also have a moving dot mode where only 1 LED is lit, it
moves around according to the input voltage.  These chips can be
cascaded (like in this circuit) to double the number of LEDs that are
driven from the input voltage.  I also don't understand why the LED
colors are all mixed up but there must be a good reason for that also.

R4, connected to the output of the F/V chip, I think is the one that
determines what voltage is generated based on the input frequency.  This
is the one that you'd have to change to match the number of cylinders
you have.

If you go to www.nsc.com (national semiconductor) and enter those 2 part
numbers you can get datasheets and application notes for them.  The app
notes will probably do a good job of explaining their operation.

Someone else mentioned www.velleman.be, if you want a complete project
including board that you can just assemble by following directions, then
that would be a good choice.  You'd learn more by understanding and
building this circuit.

--steve

Santi Udomkesmalee wrote:
> 
> I found this schematic:
> 
> http://www.3riversjunction.com/Auto/Tach/tachometer.htm

-- 
Steve Ravet
steve.ravet at arm.com
ARM,Inc.
www.arm.com
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