> Hi
> Most scales use a device called a load cell which is like a Z shaped chunk
> of metal that gives an output proportional to load.  These use a strain
> gauge principal.  Try the South Africa branch of Maplin Electronics.  They
> should get you strain gauges or try RS Components.  Remember to use at
> least two sensors.  It gives the best readings.
>
> The CPR practice doll I was using during a heartstart course used a large
> spring and a slotted opto switch with a very fine scale that went through
> it vertically.  This gave amazing accuracy and was cheap and easy.  As below
>
> ___________Surface____________
> ///             -               ///
> ///             -               ///
> ///             -               ///
> ///             -               ///
> ///             -               ///
> ///             - _____ ///
> ///           |-|_____| ///Spring
> ///                     |       ///
> ///                     |       ///
> ///                     |       ///
> ///                     |       ///
> ///                     |       ///
> ///                     |       ///
> ///                     |Post   ///
> _____________________|_________
>
>
>    _____
> |-|_____| Represents slotted opto switch
>
> As pressure is put on spring, a pulse train is output.  The post limits
> maximum weight and should come to half total height.
>
>
> Tim
>
Hi there again
I checked through the Maplin catalogue and couldn't find anything - I
do remember that RS does have something like that in their catalogue
- I'll check.
Dare I say, I like the Idea from Chris Eddy - using a solenoid to
balance the load and then measure the current needed to achieve this.
The mechanics shouldn't be too difficult to implement, or the
electronics for that matter.

Jacques Vrey
Iscor Profile Products Newcastle
Tel:27-(0)3431-48759
jvrey@it.new.iscorltd.co.za