I'm trying to build a simple water level detector using BC557 PNP transistor (there's already a similar question here but doesn't solve my problem...)

Circuit 1 below is what I'm thinking to build based on the standard biasing of PNP circuits (Circuit 2) as there's only 1 power source available. However, without a power source to reverse bias Emitter, I'm not able to have a Collector current. I think I'm doing something wrong here as it should work based on this for example. I can simulate an emitter current in Circuit 1 but no matter what resistors I use on the Emitter end (using 20k and 200k here) I cannot get a Collector current. Any idea what I'm doing wrong? This is my simulation.



1 Answer 1


Problem: Learning curve error with Falstad drawing.

You had Red Dot below LED indicating it was not a junction (disconnected) So shink line and add another wire as in my comment link fixed.

But you had 12V with a pretty big LED current which only uses a couple volts which is very inefficient. So I assumed 20mA here.

Also remember to reduce hFE to 10% of maximum when using a switch.


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