How do these modern power buttons on devices work

Question

In the older days and still now we have buttons where one way is off and the other way is on, its mechanically built to connect or disconnect an electric path physically.

On modern devices like laptops the power buttons work differently. Push once, it's on, Push again, it won't turn off and will go to sleep mode instead. Keep the same button pressed for around 3 seconds and the laptop shuts down as if the power was literally cut off.

These buttons surely have a more complex mechanism than just connecting or disconnecting a wire.

Does anyone know how those circuits have been designed? I am quite curious as to how these power switches work.

Answer 1

The modern switches don't contain any magic. In fact, they are less complicated and expensive than real physical on/off switches.

These switches are just inputs to a microcontroller. The microcontroller can tell when you push the button, and the rest is policy encoded in the firmware to decide what to do about it. The power is usually switched with transistors. This means the button itself doesn't have to handle high voltage or high current, so there are a lot more options to make it and for it to be small. It could be a membrane switch, for example, which you'd never use to switch wall power.

This does mean that a little bit of the device is usually on, at least enough to power the microcontroller. However, modern microcontrollers can take such tiny amounts of power when doing nothing but waiting for a switch signal that this power is irrelevant in most cases.

In some cases, the button actually causes the micro to get powered up when pressed, which then turns on some transistors or a relay or something to keep the power on. When you press the button to turn the device off, the micro shuts down everything, including itself.

Answer 2

In simple devices you might have a microcontroller in low-power mode which wakes up (generating the enable signals for other devices) when the button is pressed, then goes back to sleep when the button is pressed again. This after considering debouncing, the fact that the user might press the button many times in a row etc...

When you already have a power management framework (as in laptops) it's easy to integrate those features. Besides, there are integrated circuits (like the LTC2950) that can handle power buttons (e.g. debouncing, handling quick successive pushes etc...).