Here's a method to create a permanent "latching" action from a switch and a relay:
simulate this circuit – Schematic created using CircuitLab
Once the button (SW1) is pressed, it charges the relay coil and closes the relay contacts. As usual, this bring power to the load. But, in this case, it also brings power back to the relay coil. So, even when the button is released, the coil is still energized and the load stays powered.
The load will stay powered until you remove the voltage source. If you need a method to turn off the circuit, you could add a normally-closed pushbutton in the path from the load back to the relay coil.
This could easily be modified to use BJTs or MOSFETS, if you would like a solid-state solution.
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