Variable Resistors

Variable Resistor Types:

potentiometers (turn, slide, ...)
thermistors (Thermometrics, etc.)
photoresistors (PerkinElmer, etc.)
force sensing resistors (Interlink)
bending resistors (flex sensors)

Force Sensing Resistors:

Made up of a resistive material applied to a film, a spacer adhesive, and another film printed with interdigitating contacts.
The resistive material makes an electrical path between the two sets of conductors on the other film.
When a force is applied, a better connection is made, resulting in a lower resistance.
Three basic regions of operation:
1. initial abrupt change (around 10 grams of force) in resistance
2. a large region where force is basically proportional to 1/R
3. a saturation region where additional force produces little additional decrease in resistance
Not appropriate for accurate measurement of force.

Interfacing Variable Resistances to the BASIC Stamp:

Use the resistor in an RC-circuit as shown below. It might help to place a potentiometer in parallel with the variable resistor to help in calibrating the system.
Use the RCTIME routine to obtain a value proportional to the resistance (or capacitance).

RCTIME Pin, State, Variable

where:

Pin is I/O pin the circuit is connected to.
State specifies which state to measure (0 or 1).
Variable is a variable (usually a word) in which the time measurement is returned.

RCTIME works by measuring the time it takes for the RC circuit to charge or discharge. Since the BASIC Stamp's logic threshold is about 1.5 Volts, a higher resolution result is achieved by measuring the voltage drop from 5 Volts (rather than the voltage rise from 0 Volts).

'{ $STAMP BS2 } fsr VAR WORD loop: HIGH 8 'discharge the capacitor on pin 8 PAUSE 1 'for 1 millisecond RCTIME 8, 1, fsr 'measure RC charge time DEBUG CLS, DEC fsr 'show value on screen PAUSE 200 'slow down repetitions GOTO loop