The scenario
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.
What is in this drawing
Read the decisions behind it.
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.
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