Multiplexed FSRs

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The concept
To construct a grid of dynamic and discrete FSRs, without using independent analogue read pins for each FSR. The concept is borrowed from Oxer and Blemmings (Apress 2009), chapter 8, which describes and demonstrates how to hack a resistive touch panel overlay. The concept requires alternation between 5v (HIGH) and ground (LOW) using digitalWrite on the analogue pins. Separate analogue pins are used in the present example to read discrete touch and force, with the novel implementation that the number of discrete points = N 2 , where N = number of analogue read pins used. For example, 5 analogue input pins can be used to read 25 discrete and dynamic contact points. A delay of 5ms is applied between the alternating functions, to allow the pin voltage to settle.

Proof of concept
A proof-of-concept circuit is constructed using copper tape and Velostat, common materials used to construct FSRs. However, we use a column and row arrangement of power/ground electrodes separated by Velostat. The lower two electrodes connect to a grounding resistor (10k Ohms) and the top two connect to analogue inputs A0 and A1. These pins are used to digitalWrite HIGH or LOW, to read one column at a time.By powering the leftmost column HIGH and the rightmost LOW, we can read the variable current on either of the two lower electrodes; this represents top left and bottom left FSRs (analogue read pins A2 and A3). Setting the leftmost column LOW and the rightmost HIGH, we can again read the variable current on the same pins, this time representing top right and bottom right FSRs.

The only unknown variable in this circuit is how safe it is to pull an analogue output pin LOW, without the use of a grounding resistor – there is a grounding resistor in the circuit, on the lower electrodes, so it is presumed to be okay.

Brendan McCloskey March 31st 2015

 

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