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Wondering if the following would be ok. I'm trying to build an indicator that will test if my house mains power switch is ON or OFF. Can I add a 5v DC source to one side of the AC switch and when the switch is closed it would complete the DC circuit as well as the AC circuit? I'm thinking they are 2 separate circuits. Or am I missing something here.

While a DEL is a diode, it cannot withstand a lot of voltage in reverse: When the switch (I doubt that it is a DPDT) is open as in the sketch (modified with an SPDT switch), there is still a closed path for the AC through the added DC source: V1-V2-R1-D1-load-ground-back to V1, but D1 in reverse will blow off (240 VAC, that is even more peak to peak).

Using an apparatus like a clamp multimeter, around the live wire, would be safer, imvho.

I can dream up several minor modifications to this scheme which would make it work but I doubt that any one of them would pass an inspection to any code. This would leave you in a very bad legal position in case of any serious accident. The idea of a clamp-on multimeter would tell you if current is flowing but it's always possible to have the main breaker closed and no load turned on so the breaker would appear to be open when it is actually closed. A brute force method which I expect would be legal would be to connect a 240v relay to a dedicated branch circuit and use a relay no/nc contact to feed two indicator lights (led or other) with a dedicated power supply.

Would we really get a significative reading of an AC current, with a clamp meter, at the right of the switch, after the switch but before the load, when the switch if off? I assume that the load is resistive. I don't say it is impossible, just that I fail to imagine a scenario where that will occur.

Of course there would be no current reading from an ammeter, clamp-on or otherwise with the switch turned off. The whole idea of a clamp-on meter is impractical as the object of the game is to detect whether or not there is voltage on the load side of the switch. This should not be hard. If this "main switch" is the 240v supply to a dwelling I would be very concerned with safety as I've seen a couple of very impressive blowups on domestic equipment and it is wise not to set the stage for a problem.

I wouldn't attach any components directly to the mains, other than a transformer or relay. For one thing, your country/city electrical code will forbid it; for another, should anything happen, whether related to your circuit or not, your insurance company will cut you off without a pfennig.

Here in North America, there are usually two, sometimes three, 110-volt phases; going hot-to-hot gives you 220/240 for stoves and similar devices. Do you have only a single phase of mains? or more than one?

Take an old "wall wart" power supply, or a USB charger, and get someone to create a circuit with a resistor and an LED; install it near the main breaker. If the LED is lit, the mains is definitely on. If it is not lit, your circuit might be not plugged in, or the LED or resistor blew, or else it is safe to stick your hands in the breaker box. If you don't know which is which, I advice against going into the breaker box :-)

Tom