SPST pushbutton switch combines power-control, user-input functions

摘要

This Design Idea describes an enhancement to a previous one (Reference 1). The circuit in Figure 1 uses a normally open SPST pushbutton switch, S{sub}1, instead of the SPDT switch that the original design required. You can use a membrane switch to significantly simplify the industrial design of the device and enhance its ergonomics. In addition, this circuit slightly reduces the current drain in active mode by eliminating current flow through the unactuated switch. In standby mode, MOSFET Q{sub}1 remains off and consumes less than 1 μA of leakage current from the battery. Pressing switch S{sub}1 turns on Q{sub}1 by pulling its gate to ground through diode D{sub}1. Voltage regulator IC{sub}1 turns on and supplies power to microcontroller IC{sub}2. The microcontroller boots up and asserts its P 1.1 output high, turning on transistor Q{sub}2 and latching on the system's power to allow release of S{sub}1. Meanwhile, resistor R{sub}3 pulls the microcontroller's input, P1.2, to V{sub}(CC). Pressing the switch a second time pulls the microcontroller's P1.2 input low through diode D{sub}2 and signals the button-pressed event to the firmware. After completing its program, the microcontroller asserts its output P{sub}(1.1) low to turn off Q{sub}2 and, consequently, Q{sub}1, removing power from the system until the user presses S{sub}1 and restarts the process.