Soft switching allows a single button to be used as both an input and a power button. An example of this is the LCR45 LCR meter from Peak Electonic Design which has two buttons – on/menu and enter/off. As you can tell from the names, both buttons operate to turn the device on/off and also trigger the menu and enter functions in the interface.
This could be achieved by using both buttons only as inputs to the processor (a PIC in this case) and never turning the chip off, simply putting it into low power mode and using the ‘on’ button to wake it from this sleep. But even the best low power mode uses more power than actually turning the device off and the battery would eventually drain. So how can we achieve a multipurpose button and still use no power when turned off? To answer this I reverse engineered the LCR45 and drew out the circuit which I will now explain.