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THE CIRCUIT'S FUNCTIONING This circuit is a rain water sensor circuit that buzzes when there's water on a probe.I found it on net.1


  • 3 Volts Battery
  • BC 558 Transistor(PNP)
  • BC 548 Transistor
  • 330K Resistor
  • A Water Prob
  • 10K Resistor
  • .01 Microfarads Capacitance
  • Any Speaker


If NPN transistor(BC 548) in this circuit is working properly,emitter should have less current than collector,which is enough for speaker.Collector-side gets a high voltage which gets grounded by PNP transistor's collector.(BC558)

If there's a problem with Bc548 then emitter will have lots of voltage,thus collector will have less.Which means PNP transistor on there is used to shut down the circuit if there's a problem.(If a PNP transistor gets low voltage from base,the current from emitter starts to flow to collector which means shutting down the circuit,causing all coming current to go to grounding for avoiding burning the speaker.)

Is my understanding of transistors roles correct?

Thanks for the help.


  • $\begingroup$ I'm not an EE, but it may be easier if you logically break it down into two circuits. One detects the water (the 330K and the BC548) and the other creates an oscillating amplified signal for the speaker (capacitor creates the oscillation feedback). Here is simpler circuit that could be used with an off the shelf buzzer: 42bots.com/tutorials/simple-water-detection-circuit. If you don't get any direct answers here, try the EE stack exchange (thats where I post all my EE questions) electronics.stackexchange.com $\endgroup$ – ericnutsch Sep 25 '20 at 2:06

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