A couple of days ago the switches I plan to use to activate my on-off switch arrived. I had expected them to be DPST switches, but they turned out to be SPST switches. This required some redesign of my on-off switch to accommodate the fact that I only have one switch. To do this, I am feeding the output of the switch into a pair of Schmitt trigger inverters (in series to act as a buffer) as well as into a NPN transistor. The output of the buffer is then fed to my GPIO input, after first being adjusted to a 3.3V level using a zener diode. The reason for using two Schmitt trigger inverters is so that my output is active-high, meaning that the output only exceeds the zener voltage of the diode for a short period – meaning that my power loss on the voltage adjustment is realistically negligible because the output is so seldom high.