A low power circuit which detects low level RF signals and RF signals of varying strength and, upon detection, activates or "wakes up" RF equipment, such as an automatic vehicle identification tag, that is normally kept in a dormant state to conserve operating power between active operations. The circuit converts a detected RF signal from a dc voltage signal to an ac voltage signal by "chopping" the dc signal to a "chopped dc" square wave, the amplitude of which is proportional to that of the original dc signal voltage, which in turn is proportional to the level of the received RF signal. The chopped dc square wave is then linearly combined with a threshold reference level signal of opposite polarity and the combined signals form the input to an ac-coupled limiting amplifier. The polarity of the amplified signal is identical to that of the larger of its two input signals. A phase detector determines the polarity of the amplified signal. When the phase detector discerns the detected RF activation signal, it activates the RF equipment.