Force Sensitive Resistor sensitivity

Other Hardware Sensors
1

I want to use a FSR to sort/identify objects. I plan to have three different boxes of different weight. We'd do a process based on what we get from the FSR (lets say for now, just turning on one of three LEDs).

I have never handled an FSR before. I've seen them in videos where they are squeezed but I'm wondering if this is a practical solution for my application? Will it be sensitive enough for weights of, say, 20g, 50g, 70g? The chart on this page says that the resistance will change at least after 20g: Using an FSR | Force Sensitive Resistor (FSR) | Adafruit Learning System I'm just wondering how accurate it is and the resolution.

Thanks!

2

The force vs resistance chart for that FSR says it would provide resistance from 30K (20g) to 8K (70g). Using this in a voltage divider as described in the tutorial with the second resistor at 10K and an input voltage of 5V then your resistance is:

10K / (10K + 30K) * 5V = 1.25V (at 20g)
10K / (10K + 8K) * 5V = 2.78V (at 70g)

The Arduino gives 10 bits of precision (1024 values) with its analog readings from 0V to 5V. Between 1.25V and 2.78V (a 1.53V range) that gives you 1.53/x = 5/1024, and solving for x that's 313 values. So, yes, you should be able to adequately identify 20g, 50g, and 70g blocks.