Single switch serves dual duty in small, microprocessor-based system

Traditional control-system designs use separate switches to control power and various system functions, but adding a few components to a small, microprocessor-based system can combine a control function with the system's on/off switch. For example, you can design a system to display relative humidity and temperature (Reference 1). This small, battery-powered system requires a microprocessor-controlled on/off power switch, which you implement with a pushbutton, and a function switch to change the display from degrees Celsius to degrees Fahrenheit, which you implement as a toggle switch. From ease-of-use and total- cost perspectives, combining these two functions in a single switch makes sense.

Figure 1 shows a circuit for this application. Initially, Q₁, a P-channel MOSFET, is off because R₁ holds Q₁'s gate-to-source voltage at 0 V. No input reaches voltage regulator IC₁, and, thus, the system's microprocessor, IC₂, also remains off. When the operator presses the normally closed momentary-contact pushbutton switch, S₁, current flows through R₁ and R₂ to ground, developing sufficient gate-to-source voltage to turn on Q₁ and apply power to voltage regulator IC₁ and the microprocessor. Capacitor C₁ debounces the switch contact and ensures that Q₁ remains on long enough to start the microprocessor, regardless of how quickly the user presses and releases the switch. In addition, as its final task, the start-up firmware initializes the system's LCD, thus reinforcing the operator's tendency to hold the power switch in its on position long enough to ensure full start-up.

Figure 1.	A single pushbutton switch can control power and select among operating modes in a simple microprocessor-based system.

Immediately after the microprocessor powers up, it begins executing its firmware and turns on Q₂, an N-channel MOSFET, by delivering a logic one of more than 3 V to Q₂'s gate. In turn, Q₂ keeps Q₁ switched on, and the system runs under software control. If the operator again presses the on/off button, Q₁ remains on, and the microprocessor continues to run but pulls its mode line high. The mode line drives an interrupt input pin, and the software can use the interrupt as a toggling function or to access a wraparound, multiple-choice menu. After a suitable preprogrammed time interval, the microprocessor system turns itself off by placing a logic zero on Q₂'s gate. In turn, Q₂ switches off Q₁ to remove power from the system.

Reference

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