Remote water level detector
Here's a circuit similar to Rain Detector in oscillator section but with a twist: it includes a radio transmitter.
This means you can check for high water levels or rain even if you're not in a place where you could hear an alarm. Just take along an AM radio and listen!
As you can see from the schematic, there's no power connection to turn this Project ON or OFF. But whenever water (or some other conducting substance) allows electric current to flow from the emitter to the base of the 2SA Transistor, the circuit will operate. You'll hear a tone from the radio when this circuit operates.
You use this Project by finding a clear spot on the AM radio dial like we've done in previous Radio Projects. And you can place the two long wires side-by-side as described in Rain Detector.
You can also use this as a remote door alarm by following some of the steps. Experiment to see the maximum range you can receive signals from this Project—it will likely be a few feet.
Of course, you can come up with some interesting applications for this circuit on your own. You might try adding the CdS Cell between the 2SA Transistor's emitter and base. Or you could add the 50K variable resistor and Key in the emitter base circuit or ... well, you get the idea. Be sure to keep notes of what you manage to come up with.