Electronic projects

Turns-off your amplifier when idle for 15 minutes
Fed by amplifier tape-output

Circuit diagram

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
SK1 Female Mains socket

Circuit operation:
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 IC2 A & B 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 at 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.

Notes:
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 with the device's supply.

author: RED Free Circuit Designs
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web site: http://www.redcircuits.com/