AC-power monitor uses remote sensing
Sanjay R Chendvankar, Tata Institute of Fundamental Research, Colaba, Mumbai, India -- EDN, April 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, IC1A and IC1B, amplify this weak signal and convert it into a digital signal. D1 and C1 generate a steady dc voltage at the input of IC1C. D1 prevents discharge of C1 through the output of IC1B when the square wave at this output periodically goes to a low level. Inverters IC1D, IC1E, and IC1F connected in parallel enhance the current-sink capacity for sinking the piezo-buzzer current. When the ac mains is present, the output of IC1C is low; hence, the levels of IC1D, IC1E, and IC1F are high, and the buzzer is off. When the ac power fails, the output of IC1B goes low; C1 discharges through R1; and IC1D, IC1E, and IC1F 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.
