AC-power monitor uses remote sensing

Sanjay R Chendvankar, Tata Institute of Fundamental Research, Colaba, Mumbai, India -- EDN, 4/26/2001

The detection circuit in the Design Idea "Circuit monitors ac-power loss" (EDN, Nov 24, 1999, pg 172) requires a physical connection with the mains to sense the power loss. The circuit in Figure 1 senses the power loss through the radiated power-line signal. The battery-operated circuit has a quiescent-current drain of approximately 2 µA. The antenna, which is either a telescopic antenna or simply an approximately 2-ft-long wire, intercepts the radiated power-line signal. The CMOS inverters, IC_1A and IC_1B, amplify this weak signal and convert it into a digital signal. D₁ and C₁ generate a steady dc voltage at the input of IC_1C. D₁ prevents discharge of C₁ through the output of IC_1B when the square wave at this output periodically goes to a low level. Inverters IC_1D, IC_1E, and IC_1F connected in parallel enhance the current-sink capacity for sinking the piezo-buzzer current. When the ac mains is present, the output of IC_1C is low; hence, the levels of IC_1D, IC_1E, and IC_1F are high, and the buzzer is off. When the ac power fails, the output of IC_1B goes low; C₁ discharges through R₁; and IC_1D, IC_1E, and IC_1F go low. This level activates the piezo-buzzer and warns of ac-line failure. Switching off the battery power deactivates the buzzer. You can turn S₁ on after ac power resumes.