Measuring weight on a gym machine
What not to do

One obvious solution would be to put a load cell in-line with the wire, above the weights.  That would probably work well, albeit with some extra peaks and troughs in the signal as the weights move up and down. But: the wire and it's connection to the weights may be safety critical.  If the wire parts, it will lash around, and the weights would drop with quite a bang.  So don't interfere with the wire unless you're very sure it's remaining strong.

Some Ideas


Put a microswitch in each hole into which the weight selector can be placed. Use a microcontroller to monitor the switches.
Put a load cell under the unused weights, and subtract that from the total.
Put an ultrasonic sensor near each weight, and watch for them dissappearing. Watch out that only the lowest one which is being lifted will definitely dissapear from the sensor, as lower weights may still block higher sensors.
As above with a LDR and LED for each weight.
As above with a microswitch on a stick for each weight.