Amplifier Timer Schematic Diagram


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.


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.