Low water level indicator alarm circuit for Water tank

Most of the time we wish to have a water low level alarm (liquid level alarm) circuit to know if the amount of water inside the tank has decreased in order to switch ON the water pump set. Don’t worry about that, we can implement a simple water low level indicator sensor alarm circuit for your home or office with the help of liquid level probes. Our water level sensor alarm detects the fluid level using a transistor switching circuit. We have already discussed about different types of fluid level alarms and water tank level indicators, they are mainly to detect overflow from the water tank. But this one indicates that the tank is near to be emptied.(This circuit is posted as per the request from one of our reader MR. Ramz)

Circuit diagram of water tank low level alarm

Components required

  1. Power supply
  2. BC 547 transistor
  3. Resistors ¼ Watt (100Ω; 1kΩ)
  4. Buzzer 6V

Working of fluid level sensor

  • When the probe that is connected to the BC547 transistor via 100Ω resistor is covered by water, the potential at the point ‘A’ is 0V (ground potential).
  • Thus the transistor remains in OFF state. So buzzer won’t produce any sound.
  • If the water level goes below the ‘Low’ liquid level probe, the potential at the point ‘A’ becomes almost equal to Vcc.
  • This turns ON the BC547 transistor, then a current will start to flow from Vcc to ground through the transistor.
  • Thus the buzzer produces a beep sound, indicating that the water level inside the tank has reached to  bottom and you need to switch ON the water pump set.

Related water level sensor circuits

Pinout of BC547

18 thoughts on “Low water level indicator alarm circuit for Water tank

  1. I had a problem with this circuit regarding the kind of water will be used for shorting and opening the circuit to trigger the alarm.

  2. sir the circuit is working properly only outside the water while connecting the ground and point A manually.It is not working in the water the buzzer is beeping continuously please help me to overcome the provlem

    • As all the others have commented the circuit does not work properly!! ie when the probes are in water the buzzer is on continuously. When I short the emitter of BC547 to point A then the buzzer stops. I have checked all the connections/wiring/components.
      Please help. Urgent!.
      Thank you.

  3. Good day sir, I am having trouble when putting water on it. it doesn’t turn off when the water comes in, it seems the conductance of the wire to water doesn’t work, how could i amplify it so that it will work when the water is in the tank comes in and when it reach the two probe which of the said diagram it will turn off, i Follow exactly the diagram, but it doesn’t work on water, pls help me sir, thank you and good day again.

      • It is sad to note that the problem of this circuit has not been rectified up to now !!
        In my opinion the transformer used must be an “Isolating Transformer” taking safety in to consideration.

        D C Jayaratne

  4. I want to run the circuit of the automatic low level indicator circuit for water tank using electronic workbench. However, I can’t see a symbol of water or water in a can so as to complete the circuit and simulate it. Kindly advice.

  5. sir the circuit is working properly only outside the water while connecting the ground and point A manually.
    It is not working in the water the buzzer is beeping continuously. Please provide the solution to overcome this problem

    • Replace the transistor BC547(P type) with transistor BC 557(N type), and connect the +ve terminal of buzzer to supply and -ve terminal of buzzer to the Emitter of BC557. Connect the collector with ground. Take one wire from base and one wire from supply into the water tank. When both wires are dipped in water, transistor is OFF(base gets +ve voltage), no buzzer sound. As soon as wires are out of water I.e. low water level, transistor is ON(base gets 0 voltage), buzzer starts buzzing.

  6. sir i used this circuit and faced the same problem as that of the ppl cmmented above the circuit works only outside the water even i tried using cnducting bowls its really urgent if u can suggest any sensor in the liquid this circuil would work i guess so pls help me out.

  7. I think the 1k resistance needs to be replaced by a 10M ohm preset.(variable resistance). Then on the circuit with both the probes in water, and start adjusting the preset till the beep stops. Actually the ‘value’ of resistance at this place will vary, from water to water.

  8. I think…The low power transistor BC547 should be replaced by a high power transistor i.e. from the BL series….since BC547 is so sensitive that it turns on even by a ‘touch’ at its base, if its collector is powered by 5volt dc and emitter grounded. Hence it keeps buzzing due to the slighest presence of current…a few miliampere current that reaches its base via 100 ohm resistor, even when point ‘A’ is approximately grounded by water ( plain water also has good enough resistance…approx 20k ohms…, it is a bad conductor of electricity, hence it is not a good grounding). As a result some current still flows through 100ohm resistor to its base, hence this fault. Please correct me if I am wrong …I will appreciate your expert opinion.

  9. I think we should use 557 pnp transistor instead of 547npn… and the sensor probes will be the positive end and the bade of the transistor.. i hv not tried but i think it will work.

  10. I think replacing 547 tr with 557 pnp tr will work…. the sensor probes will be the positive end and the base of 557…

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