The circuit is able to sense a mains load as low as 50mA, so even the smallest preamp will trigger it reliably. Using a light dependent resistor (LDR) as the sensor.
The circuit of the mains current detector is shown in Figure 1. By using a few cheap diodes, a resistor, LED and LDR, a simple opto-isolated detector can be created. This entire circuit dissipates very low power, and can safely be housed in a heatshrink wrapper to ensure that contact with live wiring is not possible. This will also keep light away from the LDR. These are cheap and easy to use. I found that I could detect as little as 10mA of mains current with this circuit, and no distress was created at 0.5A. The diodes will get very warm at higher currents.
Note that because of the 1A diodes used, this is the absolute maximum current of the switched load. If a higher current is expected, you must use high current diodes to prevent failure. I shall leave it to the reader and the local electronics supplier to select suitable devices. Voltage rating is not important, as they are in series with the load. Don’t be tempted to use shottky diodes to reduce the loss – you need the loss to turn on the LED.
This circuit introduces a small amount of distortion into the mains waveform, but the added distortion is quite low, and the difference is completely inaudible in a preamp or other audio equipment. I measured the AC mains distortion with and without the diode string in circuit, and could not detect any difference – mind you, the distortion was 5.6% at the time – somewhat higher than I would have expected, but it remained unchanged with this circuit in or out. Since all of this occurs on the household AC side of the equipment, it is of absolutely no consequence to the final sound quality (despite what you might have heard or read).
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