It may seem a little excessive, but since I had the programmer, the C compiler and some spare PIC16F627 chips, this solution to handling power on mute, 12V trigger powering and LED for status would be small, reliable and cheap.
Three relays would be involved.
Mains switching relay, to switch in the 225VA toriodal transformer
Output switching relay, to disconnect the speaker from the amp
Input switching relay, to remove any input so the output switching relay switches a very small current, prolonging its life (thanks to Mark Hennessy for this idea).
Two LEDs would be on the front of the box. One LED for standby, one (blue) LED for power on. The would both flash in turn when the power on sequence runs.
Finally, there would be two inputs. One small push button for mounting on the front of the case for manual power on/off, and one 12V trigger input, isolated using a standard opto-isolator in order to prevent earth loops.
Thats all there is to it. Without the PCB mount transformer, the PCB turns out pretty small. It has been desgined to fit my relays (and other components) I had available to me, but is provided as a scan below if you want to use (and modify it in your subwoofer/power amplifier project. The C file, and HEX file for the PIC micro is also provided.
Origial Article sourse e-dan.co.uk
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