ZeroPower IoT devices are a class of wireless sensors that are extremely low power and self-sustained in energy. Completely passive sensors, such as the reed switch sensor, are ideal candidates for such applications.However, in a back-scattering communication scheme, the switching should be sustained (dynamic, continuous switching) to yield any valuable modulation, which excludes many sensor switch scenarios.This paper proposes a rectifier-powered sub-carrier modulation scheme capable of differentiating the static switch positions and operating below 200mV and 0.26mW of harvested power. The designed system comprises an antenna, an oscillator, and an RF-DC converter. The characteristics of the tunnel diode are harnessed to design an extra low-power oscillator, while the non-linearities of the Schottky diode are utilized to create both rectification (RF-to-DC) and modulation functions. In this scheme, the sub-carrier LO frequency can be hardware-tuned for tag identification.