2.1.8. Exercise: Pressure pad with bias resistor

2.1.8.1. Objective

Measure mechanical pressure as a varying voltage.

A pressure pad (i.e. force-sensing resistor, or FSR) is a contact sensor in which the resistance changes in response to mechanical pressure. The resistance decreases with increased force. A voltage divider circuit can be used for forming a voltage which depends on the pressure.

2.1.8.2. Steps and observations

  1. The circuit diagram shows a homebrew conductive fabric FSR but you may use a commercial FSR at first. These are labeled “pressure sensor” and can be found in the Sensors cabinet.
  2. Measure the resistance of the pressure pad by itself over different pressure conditions.
  3. The exact value of the bias resistor depends on the properties of your sensor. A good starting point is a resistance equal to the mid-range resistance of the sensor.
  4. Measure the pressure pad voltage in the given circuit over different pressure conditions.
  5. Using the oscilloscope to observe the pressure pad voltage, try creating rapidly changing contact to estimate the response speed.

2.1.8.3. Comments

Commercial FSRs can be damaged by excessive force. In practice, it can take care to design them within a mechanical assembly so they are not directly exposed to excessive force or forces which pull them apart. But they are a relatively cheap and robust way to make an analog human interface.

As a challenge, rather than use a packaged FSR device, make your own using tape and conductive fabric. Please see the following as a guide.

  1. Kobakant DIY: extensive guide to homebrew fabric sensors.
  2. Stickytape Sensors: quick and simple pressure and bend sensors.
  3. Handcrafting Sensors: long guide to variety of fabric sensors.

Conductive fabric can also be damaged by excessive force and can some types can wear out from washing. But these sensors can be incorporated into clothing, made with multiple sensitive points, turned into tilt sensors and linear controls, etc.

../../../_images/pressure-pad-with-bias-resistor_bb.png ../../../_images/pressure-pad-with-bias-resistor_schem.png

2.1.8.4. Other Files

  1. Fritzing file: pressure-pad-with-bias-resistor.fzz