A good way to spend a relaxing afternoon is to be in a quiet place with just the right amount of sun or shade, drinks within reach and listening to your favourite songs on MP3 or CD. You doze off and by the time you wake up again the audio equipment has dropped silent due to flat batteries. What a pity! The simple circuit shown can prevent this embarrassing situation by de-actuating a relay when no audio signal is detected for about two seconds. The circuit consists of a sensitive LM358 based comparator, IC1A, which keeps monostable IC2A (a 4538) triggered as long as an audio signal is detected at the input.
Via coupling capacitor C1 the circuit takes its input signal from the ‘hot’ side of the loudspeaker or headphones in your audio gear. The monostable will time out 2 s after being triggered, the delay being determined by R6 and C3.