Summary of LOW-POWER BUTTON REPLACEMENT WITH FORCE-SENSITIVE RESISTOR
This article describes using a Force-Sensitive Resistor (FSR) with an SLG47004V to replace metal contact push-buttons. The FSR forms a divider with the IC’s internal rheostat, sampled by a low-power ACMP that detects presses. Built-in GreenPAK logic interprets single, double, and triple taps and outputs signals. An auto-trim routine adjusts rheostat resistance to maintain a stable no-press reference voltage despite FSR temperature variation and sensor/system voltage differences, enabling accurate low-power operation across varying conditions.
Parts used in the LOW-POWER BUTTON REPLACEMENT WITH FORCE-SENSITIVE RESISTOR:
- Force-Sensitive Resistor (FSR)
- SLG47004V (GreenPAK IC with internal rheostat and ACMP)
Introduction
A common requirement for devices that must be sealed from environmental exposure is to replace all metal contact style push-buttons with functional replacements that do not have those weather-sealing difficulties. This article describes one such way of creating a contact button replacement using a Force-Sensitive Resistor (FSR) in conjunction with an SLG47004V.
Design Description
Overview
The design highlighted above is intended for use as a functional replacement for a metal contact style push-button while featuring ultra-low power consumption. Using an external force-sensitive resistor in series with one of the IC’s internal rheostat modules, a resistor divider network is created which is subsequently sampled by one of the SLG47004V’s internal low-power ACMPs. This ACMP determines the state of the button based on the voltage present at the divider output. Once the internal low-power ACMP determines that the button has been pressed, internal GreenPAK logic processes the button press signal to determine whether a single, double, or triple-tap has occurred, and outputs these signals to dedicated pins.
Since the force-sensitive resistor present in this design is highly susceptible to variation in resistance due to temperature changes, often as severe as +/-15% at extreme high/low temps, a constant “no-press” reference voltage must be maintained throughout a wide range of temperatures. This constant reference voltage is achieved using the circuit’s auto-trim functionality, which periodically samples the divider output voltage and adjusts the SLG47004V’s internal rheostat resistance, allowing the divider output to return to the specified “no-press” reference voltage. This auto-trim functionality not only allows for accurate operation over a wide range of temperatures, but it also allows the design to accommodate for sensor-to-sensor variations as well as variations in the overall system voltage level.
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- What is the purpose of using an FSR with the SLG47004V?
To create a contactless button replacement where the FSR in series with an internal rheostat forms a resistor divider sampled by the SLG47004V ACMP to detect presses. - How does the design detect single, double, or triple taps?
Internal GreenPAK logic processes the button press signal from the ACMP to determine single, double, or triple taps and outputs dedicated signals. - Why is auto-trim functionality needed?
Because the FSR resistance varies with temperature and between sensors, auto-trim periodically samples the divider output and adjusts the internal rheostat to maintain a constant no-press reference voltage. - Does the design handle sensor-to-sensor and supply voltage variations?
Yes, the auto-trim feature compensates for sensor-to-sensor variations and overall system voltage level changes. - Which internal component samples the divider output voltage?
The SLG47004V’s internal low-power analog comparator (ACMP) samples the divider output voltage. - Can this approach provide ultra-low power consumption?
Yes, the design is intended to offer ultra-low power consumption by using the SLG47004V’s low-power ACMP and internal logic.
