Detect if the load is present on this circuit
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Current indicator.

Something like this may work depending on your load current. The diode bridge will drop about 1.4 V at a reasonable current and this should be enough to light an infrared LED as found in an opto-coupler. I've shown two arranged back to back so that a pulse could be detected on each half-cycle.

I have not tried it.



Figure 2. A current transformer and indicator lamp.

Devices such as that in Figure 2 are a safer means of accomplishing the task. In this case the visible LED can be replaced by an opto-isolator LED but get a back-to-back type or put a reverse diode in parallel so that there is a current path on both half-cycles.

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