IC555 can also be used in monostable configuration for this purpose.
its very sensitive.
just tie IC555 trigger input to 5 V supply through 1K resistor.
then make close gap between trigger input pin and grond connection. you can make cross hatch pattern (like found in calculator keypads)
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| ______
|_____ |
| _____|
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to gnd to trig input
when rain drop falls on such pattern IC555 gets trigger and its output becomes high