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Okay, when Hurz is right he's right. Basically the previous version was prone of amplifying noise and everything that got through to the input, and could cause a false triggering of the standby system even without an input signal. A simple bypass to this problem is a Schmitt Trigger based system. Basically no signal below 22mV will trigger the output, but all signals equal and above 22mV will sharply trigger the output to change. This is due to the hysteresis of the Schmitt Trigger. Let me elaborate. In the previous version all amplification was done by a comparator, which was actually an open loop gain op amp. So, miniscule signals still got amplified and the end game was not ensured rail to rail switching, and noise induced instability issues. In this circuit, as the input signal touches one of the thresholds of the Schmitt Trigger the positive feedback (the cause for the hysteresis) further pulls the threshold level into the driving signal, thus reinforcing the amplification of the op amp to always be full, and the output to always stike from rail to rail (if the opamp permits it). This solves some stability problems that had been eating at me for a few days, and I can finally say this is almost certainly the final version (except if I think of some other feature to add, which I always do). By using a Schmitt Trigger, one must be aware that the maximum switching frequency is somewhat limited (mostly from the slew rate of the op amp chosen), so good op amps should be used. A dual op amp TL072 should be good enough, as it has a slew rate of 13V/us.
The 470k resistor also changes the input sensitivity. At 1Mohm the input sensitivity is 20mV, at 110k it's around 35mV. Further decreasing the resistor value will cause the Schmitt Trigger to enable tripped and will cause the system to become unstable. Recommended operation is as far as possible for this point as feasible. A 470k resistor is an optimal value.
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