I wanted to create a circuit that Lit's up an LED when water level rises to a certain level.
I have created the following schematics following the https://www.electronicshub.org/water-level-alarm-using-555-timer/ However since the LED lit up even though there was no water i needed to add the R1 resistor.
Now i want to do the opposite. I want the LED to not light up when both probes are connected with water and lit up only when water level drops.
For that i have created a following circuit swapping the BC546B (NPN) with BC556B (PNP) transistor.
However this lights the LED when the probes are not connected with water but even shorting them doesn't turn off the PNP transistor (that is LED is still glowing). Removing the R1 and path to ground from base of the PNP makes it glow dimly (it glows more when i hold the water probe) and connecting them then does turn off the PNP.
Why is that ? I thought i can turn off the PNP by applying a small amount of voltage to its base?
How can i fix my circuit so that it works ?
Ps. i have also tried with a darlington PNP transistor TIP 125 and it doesn't work