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Battery changeover circuit required

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Hello, I am new here. I have a small query.
I have two different DC power supplies (3V & 24 V) coming from two different sources. An LED is normally connected to the 3V supply. I want a of circuit that when I press a push button the supply changes over to the 24V supply to run the LED and isolate the 3V supply. Can we make circuit without using relay?
Thanks in advance
 

Instead of an electromagnetic relay you can use a MOSFET switch, also optocouplers can be used for powering a LED from two separate sources.,

As a LED requires a series resistor to limit a specified LED current , then you need two such resistors, one for 3V, another for 24 V.

You did not indicate what other load is to be connected to you sources in addition to the LED. LOad current will be important to choose the switching element.
 
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Thanks for reply. Actually its just an LED at the output. The problem is that the ground is not common for both supplies (may need two BJT's??). Actually I wanna make a lamp testing circuit. In the field when an alarm comes, it sends a 3V signal to the LED(far end). I want the circuit so when I'll push the switch all the LED's on the panel (local) should ON just to show that the LED's are working fine. Locally I have only 24V supply available. Transistors would be great to use, can you make the circuit diagram for that?
 

This switch is called a "lamp test", it was commonly used when filament bulbs were fitted as these were unreliable. It is not worth fitting them to LED lamps because they are so reliable. If you are so worried that a LED may burn out or suppose it may become disconnected, its best to feed a little current through it which will not light it up through a high value resistor and check the voltage across it.
Frank
 
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