Amplifier Timer Circuit Schematic


Description

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.

Circuit diagram:

Circuit

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

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 to the device supply.

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