상황
A blind person fills a cup and needs an audible signal when liquid reaches the desired level, using two probe contacts that close through the water.
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01
Used an NPN transistor as a low-side switch triggered by water contact across probe gaps
02
Added a base pulldown resistor to keep the buzzer off when probes are dry
03
Included a flyback diode across the piezo buzzer to protect the transistor
Reuse for any conductive liquid level detection where a battery-powered audible or vibration alert is needed.