I added a swarf-catcher tray to my lathe's carriage. It can be seen here...
Although the tray clears the bottom of the chucks, when working near to the chuck face the tray can hit the face of the headstock behind the chuck. If I'm threading this means that the leadscrew can try to drive the tray through the headstock if I'm not watching closely. (Don't ask me how I discovered this.)
This and other situations convinced me that the lathe needed an alarm to warn me of impending disasters. The device I made...
is fairly simple. A nine volt buzzer alarm and its battery are housed in a plastic container. If the two copper wires emerging from the top of the container are shorted the buzzer sounds loudly enough to get my attention. A microswitch with a long actuator is wired via bell wire across these contacts; pressing the actuator sounds the alarm.
In front is a piece of brass with a neodymium magnet cemented to it. The microswitch will be attached to the brass flag. Then it's just a matter of sticking the switch under the swarf tray so the actuator contacts the headstock early enough to trigger the buzzer hung somewhere near my ear via one of the hooked copper wires.
The switch can also be stuck to the lathe ways to announce other potentially dangerous positionings of the carriage.