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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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.

schematic

simulate this circuit

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

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  • \$\begingroup\$ ARe you sure you have it wired right. BC transistors have a different pin-out \$\endgroup\$ – Trevor_G Dec 21 '17 at 16:18
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    \$\begingroup\$ Y i followed the datasheed if the flat side is facing me starting from left to right Collector, base, emiter \$\endgroup\$ – daniels_pa Dec 21 '17 at 16:19
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If it still turns on with R1 removed I suspect a wiring issues.

However, you need to move R1 left or R2, as shown below, so it does not form a voltage divider which will allow the base to still be biased on a little.

If it still turns on with a hard short across the probe something else is up.

schematic

simulate this circuit – Schematic created using CircuitLab

These circuits are VERY dependent on the resistance of the probe though, especially top side like that. You might be better off doing it this way.

schematic

simulate this circuit

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    \$\begingroup\$ Awesome that fixes it i will try the second circuit also! Thanks, i also had resistors R1 and R2 flipped as well as R1 was of value 68k instead of 680k your first circuit with the right resistors fixed it. Thanks \$\endgroup\$ – daniels_pa Dec 21 '17 at 16:35
  • \$\begingroup\$ Actually the first circuit behaves oddly. It glows then stops when the probe connects with water then all of the sudden starts glowing more. However hard shorting them works and then the LED doesnt glow. 2nd circuit works but only issue i have is it draws 1mA when the LED is not glowing which is a lot imo. \$\endgroup\$ – daniels_pa Dec 21 '17 at 17:21
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    \$\begingroup\$ @daniels_pa yes that is not uncommon with this type of thing. The resistance of the fluid can and will change with the current through the probe hence the 2nd circuit is better. The later should draw 0.5mA when the LED is off. If you really want low currents you might want to consider using MOSFETs instead, then you can make R3 MUCH larger. \$\endgroup\$ – Trevor_G Dec 21 '17 at 17:25

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