Use a capacitor with a large bleeder resistorby Clippy (Login burger2227)
You will get a pulse only when the switch closes. Once the voltage evens out across the capacitor, there is no change. When the switch is open, the resistor will pull the capacitor voltage to ground and it is ready for the next pulse.
I hope the voltage is pretty low. You could try using an opto-coupler if it is low enough to light an led.
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