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Figure 14-AC Detect snippet
Q3 turns on whenever the voltage Across C2 is greater than 1.2 volts. So we see there is
an element of timing involved here. Every time a pulse at X1-X2 exceeds 40 volts, C2
discharges. If the power switch is turned off (or the plug is pulled), then there will be no
input pulses, C2 charges, and C3 turns on, resetting C1.
All Together Now
So we see that to allow the time delay that leads to relay turn-on to begin:
1.
The DC voltage must be high enough
2.
Pulses must be present on the AC input voltage
When either condition is false, the time delay capacitor is reset, and the relay turned off,
preventing the speakers from being connected to the load. This keeps turn-on/turn-off
pops and clicks out of your speakers.
