I wish to monitor the rotational speed of a 2" shaft in the 10 to 400
RPM range. I could attach a magnet to the shaft and position a pickup
coil to generate pulses. What I am looking for is a simple, very basic
circuit that will process these pulses and drive a 3 digit LED display
showing RPM. The circuit should be able to be built on a Radio Shack
project board out of easily obtainable parts.

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Clarification of Question by screener-ga on 21 Feb 2003 13:03 PST

I am looking for a tachometer that I can build, for under say $45.00.
Preferably as cheap as possible, something thrown together with a few
ic's from radio shack. Any type sensor would be fine, if it's cheap
and I am able to build the circuit to utilize it.

do u want to Build it ? or  a ready made tachometer is also an option.
In either case . what price range do u think it should be ?.
if ready built is an acceptable then there area number of options 
http://www.hobbypeople.net/gallery/360880.asp
is a choice u might alos find a number of these avaiable on ebay .
also IF u put a magnet on the shaft u would have a bit of problem
holding it. a simpler solution would beto drill a hole on it(or find
one already on it.. such as a keyhole and use a halleffect sensor to
pick up pulses. )
or use reflective tape and pick up the reflections useing a photo
sensor
.

A far better solution than a magnetic pickup is a reflective
photosensor.  You just have to
print yourself a barcode-like strip of paper and tape it to the shaft.
 Then take a look
at the Digikey part QRB1114-ND (Fairchild Infrared Photosensor).  
(http://digikey.com)  The photosensor will provide a transistor output
that you can connect to a SCBR-200/A  (SUB CUB R 6-digit rate
indicator).  The SUB CUB R is an integrated tach/display.

The result will be much more accurate than a magnetic pickup design. 
This will be important
if you want to maintain precision at low revs.   You need to define
the precision at the lowest
revs, as well as the highest revs in order not to saturate the
photosensor.

The precision at the lowest revs will determine the minimum number of
stripes on your shaft,
while the tach display will determine an upper bound on the number of
stripes.

It's easier than you think!

Good luck!

andy22

http://www.electronics-tutorials.com/devices/555-tachometer.htm
that is  one  the simplest tachometers.. unfortunately its analogue.
but it is also the only one i could find that would work with
radioshacks online list of avaiable componets (aside from useing the
BASIC Stamp module. which would be an over kill)
however thre are several options aside from Radioshack . but because i
do not know what locality ure in the online ordering of kits is what i
propose . one of these is
http://www.oatleyelectronics.com/kits/k117.html
u can also look for local hobby stored and they would have a
tachomeetr kit for sure.  also u can look at stores which sell surplus
electronic items   (Industrial ) they would also have something along
these lines