Amplifier Timer

Turns-off your amplifier when idle for 15 minutes

Fed by amplifier tape-output

Circuit diagram:

Amplifier Timer


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 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.