3) Power Switch Circuit: We also use a power switch circuit
to turn on/off the power supply, which is a simple p-MOS
FET and n-MOS FET circuit. The switch is controlled by the
transmitter tag, which uses a pin to pull down to the ground
to turn off the circuit or pull up to 3V to turn on the circuit.
In this way, we can conserve a significant amount of energy
when the measurement is not needed.
The whole circuit is shown in Figure 4.