Lego Sherlock Holmes figure focuses a light beam on a Lego criminal with a small magnifying glass.

Force Sensor

Interesting things can be built with a force sensor, such as a weight scale, pressure pad for an alarm, or a button that initiates different actions for different pressures applied. LEGO does not make a force sensor for EV3, but a custom one can be built using a force sensitive resistor. Force sensitive resistors come in various sizes and shapes depending on the application. The one used in the video above is 1.5-inch square made by Interlink Electronics, purchased from robotshop.com. The force sensitive resistor doesn’t produce a signal on its own--it’s a resistor that changes resistance value with force applied. The Interlink Electronics device decreases resistance with force applied: greater than 1-Mohm for no force, ~100-kohm for moderate force with a finger press, and less than 1-kohm for high force with a finger press. To create a signal that the EV3 Intelligent Brick can read, the variable resistance of the force sensitive resistor can be placed in a voltage divider circuit to create a voltage output proportional to force applied to the force sensitive resistor. Such a voltage divider circuit is shown in the schematic below. The voltage divider is fed with 5 volts from pin 4 of the EV3 connector. The blog entries on Breakout Board and Reading Analog Voltage give details on implementing the design. Further down on this page is the EV3 program to create the function shown in the video. Since the signal counts read by EV3 decrease with higher force on the resistor, I inverted the signal so the more intuitive relationship is created of higher frequency tone with higher force.

The schematic is shown for a voltage divider built around the force sensitive resistor.



The blocks are shown for the EV3 program.