An “Autopower” Circuit…

This is another Rod Elliot circuit (from here) but as usual I have done my best to add my own personal touches to it.

Basically this is a signal-activated trigger circuit (such as you would expect to find on just about any active subwoofer). The board turns on the power when a signal is detected and switches it off again when no signal has been detected for a while (usually 10-30 mins). For schematic and details on how the circuit works I’d refer you to Rod’s original page.

What I have done to make my mark is partly to keep the relay offboard and make a matching relay board instead, and then of course to actually lay out a PCB for the design. Both the relay board and the main PCB are in my own industry-standard of 2” x 2” 😀

The main connectors on the control board and the relay board line up so the boards stack perfectly on top of each other. The relay board also includes a separate regulator (mostly because there was a bit of space left :D), so the board can be powered from any DC voltage above app. 15V. If you have no suitable standby voltage available in the device, I have also made a small unregulated PSU that can provide power to the circuit using a small (app. 1.5-2.5VA) mains transformer.

I’ve done some basic testing on the prototypes and it seems to work fine as such. However, one potential issue needs further exploration: The time is set by an electrolytic discharging through a very large resistor (10M). This makes the delay time a bit unpredictable. Also because the “must-release” voltage of most relays is around 10% of the nominal coil voltage the relay doesn’t actually release until very late and the indicator LED is therefore slightly pointless.


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