A PIC can very easily measure things like frequency and period.
So, what you need is a way to convert mass to frequency or period.

One way to do this is to suspend the mass to be measured from
a spring and make it vibrate. The "spring" can be a conventional
coiled spring with the mass hung below it, or a horizontal flat
spring, or a rocker assembly in which the mass is on one side of
the fulcrum, and the spring is on the other side of the fulcrum.

There are many different ways that you can set up the "spring"
type assembly. The important thing is that the greater the mass,
the LONGER the period. A way needs to be set up to put the
mass in motion. For the cheapest method you could have the
user push a mechanical button that then transferred that motion
to the spring. Once the mass is in motion, it will oscillate
with a period that is proportional to the mass. Now, realize
that the system itself has mass, and therefore you are measuring
more than just the mass you WANT to measure. Such a device
would obviously have to be calibrated. Temperature will have an effect
on the springiness of the spring, so the BEST type of calibration
is one which is done just prior to the actual measurement.

If a small bar magnet is caused to move back and forth through a
coil of wire, it will generate a sine wave of diminishing
amplitude. Use a comparator to convert this to a square
wave, and measure the period using a PIC. You may have to use
an algorithm or a lookup table to convert the period to the
measured mass.

Disgusting Ascii diagram follows:

     S   where S=Spring
     S
     S
     B   B=Bar magnet and coil assembly
     |
     M   M=Mass to be measured



Fr. Thomas McGahee


> ----- Original Message -----
> From: "Dave King" <KingDWS@HOME.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Tuesday, September 11, 2001 12:18 AM
> Subject: [OT]: Force Sensors or load cells
>
>
> > Hi
> >
> > Just received an email asking if I can build some snow pack sensors. In
> > other words a digital scale.
> > It needs to measure 0-1000grams with 0.1g resolution. They of course
want
> > me to build 20 of these for
> > about $20 each. ( I assume they are joking but you never know). Anyway
I'm
> > wondering if anyone has a few
> > urls for some cheapy load cells or sensors. I'm still waiting to hear
back
> > about the actual requirements so
> > I can see just how cheap I can go.
> >
> > Any hints help  would be appreciated.
> >
> > Cheers
> >
> > Dave
> >

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.