This circuit detects the 1050-Hz tone sent by the NOAA (National Oceanic and Atmospheric Administration) Weather
radio stations that operate from 162.40 to 162.55 MHz. This tone lasts for several seconds. Ql is an amplifier that feeds tone detector Ul, an NE567 detects this tone and produces a low on pin 8. This is coupled to a 555 timer (U3), which produces a high on its pin 3, sounds BZ1, triggers SCR1, and lights the LED. S2 is used to rest the circuit. Audio is taken from the receiver that is used with the device. SI is used to test the device and it will sound the alarm in two seconds if all is OK.