Standby level sensors are a most tricky affair, it is very hard to find a sweet spot that works for most scenarios.
IME, one needs several things:
- restricted bandwidth, notably down low, to avoid nuisance triggering from mains hum (when upstream gear goes into standby, signal conductors often go open circuit and thus become prone to pick up hum)
- integrating behavior, a single event, even when large (like a turn-on thump from upstream gear), shall not trigger
- hysteresis, so that turn-off threshold is quite a bit lower than turn-own threshold
- hold time that keeps the unit active for some time even after turn-off threshold has been reached
- and preferably, adjustable overall threshold, like in three steps 10dB each, as mentioned by
@mcdn .
- finally, the sensing circuit must not affect the main signal (learned that the hard way ;-)
Doing all that with an analog circuit is quite a challenge. Low power uC with some proper analog front end is much more convenient and may even result in lower BOM cost.
There is a reason why many gear has moved to 12 triggers, then only one device, the source, usually digital, can implement the control in the best, simplest and most reliable way.