Question about simple transitor circuit

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Cassius

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I recently found a circuit on Electronicshub.org like this

I really wonder why they need to use the transitor here, because the idea of the circuit is just:

  • The circuit is designed to indicate three levels of water stored in the tank: low but not empty, half and full but not overflowing.
  • When there is no water in the tank, all the LEDs are off as an indication that the tank is completely empty.
  • When water level increases and reaches M2, the contacts M1 and M2 get shorted as water acts as a conducting medium between M1 and M2.
  • This will turn on the transistor Q1 and the Green LED starts to glow. As the water level continues to rise and reaches half the tank, M3 will come into contact with water and receives a small voltage from M1.
  • As a result, Q2 is turned on and Yellow LED will glow. When the water in the tank rises to full tank, M4 is also shorted with M1 and both Q3 and Q4 will turn on.
  • The Red LED glows and also an alarm is made by the buzzer as an indication that the tank is full and the water pump or motor can be turned off.
In my opinion, the circuit can be basically like this:

I think that is enough for the LED to light up
Can someone explain for me if i am understanding it wrong or something?? Thank you so much
 
The resistance of the water is too light to light the LED's very much, if at all - the transistors are used as current amplifiers.
 
Cassius said:
I think the transistors here are only used as switches, sir.
Click to expand...

Only if there's enough base current to saturate them, which depends entirely on what contaminates are in the water - pure water is a very poor conductor - salt water is pretty good.
 
Cassius said:
In my opinion, the circuit can be basically like this:

I think that is enough for the LED to light up
Click to expand...

That "circuit" will never work.
Why?
Because it is not a circuit, it is just a bunch of stuff connected to the Vcc supply, there is no current path to 0v.

Also note, that this common circuit;
Cassius said:
I recently found a circuit on Electronicshub.org like this
Click to expand...
has problems, often explained here on ETO.
Because the electrodes in the water are connected to a DC supply, there will be electrolytic corrosion which well eat away the electrodes.

JimB
 

As I've already said, the liquid probably isn't conductive enough - although feel free to try it, and see if it works or not, if it's got a high salt content it may do?. That's why the transistors are there - it's already a very simple and crude circuit.

To prevent corrosion you need to use AC rather than DC, which is much more complicated than the simple transistor circuit you originally posted.
 
Cassius said:
I think the transistors here are only used as switches, sir.
Click to expand...
Yes, it's a switch, but the transistor amplify the low level signal to switch a higher current load, and that's an amplifier.
An amplifier can amplify more than just AC.
 
I agree an AC signal will reduce or eliminate corrosion.
You did not say what contaminant is in the water.
The project is shown in Electronics Hub which is from a country where students (?) spell "CIRCUITS" wrong on the home page. They might also post circuits with similar errors.
 

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JimB said:
That "circuit" will never work.
Why?
Because it is not a circuit, it is just a bunch of stuff connected to the Vcc supply, there is no current path to 0v.
Click to expand...
Not needed. The current path is from the 9 V through M1, through the fluid, to the three transistor bases. Granted there is no path from the bases to GND to assure a crisp turn-off, but that bad design practice does not affect the turn-on mechanism.

And though it is not 100% reliable, a BC548 with a floating base almost certainly will not leak enough to light the LED.

ak
 
AnalogKid said:
Not needed. The current path is from the 9 V through M1, through the fluid, to the three transistor bases.
Click to expand...
What Nigel said.

Please look at the picture immediately before the words "That "circuit" will never work".

JimB
 
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