Summary of LOW-POWER BUTTON REPLACEMENT WITH FORCE-SENSITIVE RESISTOR
This article details a low-power push-button replacement design using a Force-Sensitive Resistor (FSR) and the SLG47004V IC. The system creates a resistor divider to detect button presses via an internal ACMP, while GreenPAK logic distinguishes between single, double, or triple taps. To ensure accuracy across temperature variations and voltage fluctuations, the design employs an auto-trim feature that adjusts internal resistance. Ultra-low power consumption is achieved through a wake/sleep controller that powers down the ACMP and resistor divider when inactive, with periodic auto-trim cycles managed by a counter.
Parts used in the Low-Power Button Replacement:
- Force-Sensitive Resistor (FSR)
- SLG47004V Integrated Circuit
- Internal Rheostat Modules
- Low-Power ACMP (Analog Comparator)
- GreenPAK Logic
- Wake/Sleep Controller
- Internal Analog Switches
- Chopper ACMP
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.
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.
To attain ultra-low current consumption, the ACMP and external resistor divider network are controlled by the integrated circuit’s wake/sleep controller. This wake/sleep controller keeps the ACMP in sleep mode via matrix signal, and the resistor divider circuit in an open state via one of the IC’s internal analog switches, which is placed on the low side of the divider. The Chopper ACMP, which is responsible for the IC’s auto-trim functionality, is also indirectly controlled by the wake/sleep controller using an intermediate counter that triggers an auto-trim cycle once every 100 normal wake/sleep cycles.
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- How does the design determine if a button is pressed?
The internal low-power ACMP samples a resistor divider network created by the FSR and an internal rheostat to determine the button state based on voltage. - What functions does the GreenPAK logic perform?
The GreenPAK logic processes the button press signal to identify single, double, or triple-tap events and outputs these signals to dedicated pins. - Why is auto-trim functionality necessary in this circuit?
Auto-trim maintains a constant no-press reference voltage to compensate for FSR resistance variations caused by extreme temperature changes and sensor-to-sensor differences. - Can the design handle variations in system voltage levels?
Yes, the auto-trim functionality allows the design to accommodate variations in the overall system voltage level alongside temperature changes. - How is ultra-low current consumption achieved?
The wake/sleep controller keeps the ACMP in sleep mode and places the resistor divider circuit in an open state when not actively monitoring. - How often does the Chopper ACMP perform an auto-trim cycle?
An intermediate counter triggers an auto-trim cycle once every 100 normal wake/sleep cycles. - Where is the internal analog switch placed in the circuit?
The internal analog switch is placed on the low side of the resistor divider to control the circuit state. - What specific type of resistor is used as the external sensor?
A Force-Sensitive Resistor (FSR) is used in series with one of the IC’s internal rheostat modules.
