This circuit turns-off an amplifier or any other device when a low level audio signal fed to its input is absent for 15 minutes at least.
Pushing P1 the device is switched-on feeding any appliance connected to SK1. Input audio signal is boosted and squared by IC2A & IC2B and monitored by LED D4.
When D4 illuminates, albeit for a very short peak, IC3 is reset and restarts its counting. Pin 2 of IC3 remains in the low state, the two transistors are on and the relay operates.
When, after a 15 minutes delay, no signal appeared at the input, IC3 ends its counting and pin 2 goes high. Q1 & Q2 stop conducting and the relay switches-off. The device is thus completely off as also are the appliances connected to SK1. C5 & R9 reset IC3 at power-on. P2 allows switch-off at any moment.
Parts:R1,R8 - 1K 1/4W Resistors
R2,R3 - 4K7 1/4W Resistors
R4 - 22K 1/4W Resistor
R5 - 4M7 1/4W Resistor
R6,R9 - 10K 1/4W Resistors
R7 - 1M5 1/4W Resistor
R10 - 100K 1/4W Resistor
R11 - 15K 1/4W Resistor
R12 - 10M 1/4W Resistor
R13 - 1M 1/4W Resistor
R14 - 8K2 1/4W Resistor
R15 - 1K8 1/4W Resistor
C1 - 470µF 25V Electrolytic Capacitor
C2,C3,C6 - 100nF 63V Polyester Capacitors
C4,C5 - 10µF 25V Electrolytic Capacitors
D1 - Diode bridge 100V 1A
D2,D7 - 1N4002 100V 1A Diodes
D3 - Red LED 5mm.
D4 - Yellow LED 5mm.
D5,D6 - 1N4148 75V 150mA Diodes
IC1 - 78L12 12V 100mA Voltage regulator IC
IC2 - LM358 Low Power Dual Op-amp
IC3 - 4060 14 stage ripple counter and oscillator IC
Q1 - BC557 45V 100mA PNP Transistor
Q2 - BC337 45V 800mA NPN Transistor
J1 - RCA audio input socket
P1 - SPST Mains suited Pushbutton
P2 - SPST Pushbutton
T1 - 220V Primary, 12V Secondary 3VA Mains transformer
RL1 - 10.5V 270 Ohm Relay with SPST 5A 220V switch
PL1 - Male Mains plug
Simply connect left or right channel tape output of your amplifier to J1.
You can employ two RCA input sockets wired in parallel to allow pick-up audio signals from both stereo channels.
The delay time can be varied changing R13 and/or C6 values.
Needing to operate a device not supplied by power mains, use a double pole relay switch, connecting the second pole switch in series to the device supply.