Load Sensing Automatic Switch

Posted on Dec 9, 2012

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.

Load Sensing Automatic Switch
Click here to download the full size of the above Circuit.

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 t

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