Please read the description/textbox first. With this 1 transistor (or 2 transistor Darlington) circuit you can make a lie indicator (the lie level can be attenuated via a potentiometer), a moisture indicator or a water-overflow indicator/beeper.
Perhaps even a circuit that switches a pump “on” when the water level gets too high.
Please note: the fluid between the 2 electrodes must be conducting electricity and be NOT flammable. Water is in general a fluid that conducts electricity.
Warning/disclaimer: this a hobby circuit, NOT (!) made for professional applications and (thus) NEVER use this circuit to watch over fluids (via the 2 showed electrodes) that are flammable (!!).
Corrections to the video: on 10.21 I say “not a protective resistor” but I mean (of course) “not a protective diode”. Usable Si diode is, in this case, the 1N4002 or any Si diode that can handle minimum 300 Volt.
On 12.47 I say “you can hear the transistor hum”, but that is of course not the case. You hear the relay hum, here on 50 Hertz. Somewhere else on 60 Hertz, where the AC mains differs.
My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in.
My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.
https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=Ko+Tilman
I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc.
Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 12 May 2022.