Re: CNC band saw, FPM LED display

All the tach suggestions posted to date are digital, and at low
frequencies they can have problems with accuracy.

There is a very cheap analog solution. Inexpensive DC hobby motors
will also act as generators, and their voltage output is proportional
to speed. An inexpensive ($10.00 or less) 3.5 digit LED panel meter
can read the output, although the maximum (stock) input is 1.999
volts. This can be scaled upwards with simple voltage dividers.

The problem of scaling the output voltage to correspond to FPM can be
solved with voltage dividers (to scale down), or inexpensive op-amps
(to amplify to higher voltages).

Another cheap analog solution is a frequency to voltage IC (such as the
LM2907, available for $2.15 from DigiKey). Voltage output is linear
within 1%, again the scaling problem also needing a solution. Forrest
Mimms 'Engineer's Notebook' (from Radio Shack or Amazon) has clear,
simple, tested, and easily buildable examples of circuits for the
LM2907 or op amp gain circuits. Simple frequency divider circuits with
the cmos 4017 IC are also included.

For inexpensive digital, a cheap surplus counter - six digits, yet
-good to 500KHz, is available from Electronics Goldmine for $5.95.

http://sales.goldmine-elec.com/prodinfo.asp?prodid=3967

But it is small (.2" high digits) and the display is LCD (maybe harder
to see than a LED display, and definitely needing illumination.)

You would also need a timebase for this counter. For less than $5.00
Elm Electronics sells an 8-pin dip PIC which only needs a 3.57Mhz TV
crystal (cheap and readily available) and two caps to output both 60Hz
and 1 second pulses.

Hope these help, and you have fun.

Grant

http://www.elmelectronics.com/index.html

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "dan" <dk1machine@y...> wrote:
>
>
> I am rebuilding a band saw, and will add an inverter drive
> for the blade motor. One thing I want on it, is an LED display, most
> likely 3 digit, to show Feet Per Minute blade speed (50-350FPM).
> The gearbox input shaft turns 10 times per foot of blade travel.
> Anyone know of a source for a low-cost display and appropriate
> electronics (or a schematic to build same) that will count the input
> shaft turns (divided by 10) using a prox sensor or similar?

> dan k