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Water Level Controller BTC L500 Repaired/

By on April 28, 2016
Water Level Controller BTC L500 Repaired/











One of my friends that have a garden brought me this water level controller and asked me why this water controller was not functioning?


But what is water level controller?

In many homes and public places and gardens , ground water is used, which must be pumped up to overhead tanks to give enough pressure to water and by using water pumps which are controlled by electric motors the water goes to the tank. Controlling the pumps is a necessity to avoid wastage of water.

To control of the water level in the tank a water level controller is required.


 Aerial water tank

How this controller works?

This controller has three copper wires that act as sensor and installed inside the tank


With the help of a CMOS IC that is installed on the pcb ( IC CD4011 )-see next photo, low water sensor or a high water sensor both Via 4 resistors and caps are bringing data for IC CD4011 by processing the data and give order to relay (That installed also on pcb ) to run the pump or stop the pump 1- water level is low, the input pin 1 of the IC is at logic high and according to NOR gate truth table, the output at pin 3 will be at logic low.

Since pin 3 is shorted to pins 5 and 6, hence the input to other NOR gate will be logic low signals. This gives a logic high signal to the corresponding output pin 4. As current flows through the resistor to the base of transistor, it starts conducting and acts as a closed switch. The relay connected to the collector of the transistor gets energized and the NO contacts get connected to the common contact and the water pump gets power supply from the mains and starts working.

Now when water level rises in the tank rises such that probes low and high are connected through water, current flows through them (As water is a conductor) and the pins 1 and 2 are connected through the negative side of supply.

The output pin 3 is thus, at logic high level, causing the input pins of the other NOR gate to be at logic high level and thus the corresponding output pin 4 is at logic low level. The transistor gets cut off due to lack of bias current and the relay gets correspondingly de-energized and the power supply to the water tank gets cut off.

CMOS IC CD4011: It is a versatile 14 pin IC which contains 4 NOR gates. Each NOR gate has two inputs and one output. Thus the IC has 8 input pins and 4 output pins, one Vcc pin (connected to positive voltage supply) and one Vss (connected to negative supply). Its basic features include – Maximum supply voltage: 15 V, Minimum supply voltage: 3 V, Maximum speed of operation: 4 MHz. It can be used in tone generators, metal detectors etc.

CD4011 IC

Transistor BC945: It is a NPN bipolar junction transistor and it is used mainly for amplification and switching purpose. Its features include maximum current gain of 800. It is used in CE configuration when used as an amplifier.



After opening the case of water controller I found the following components on the pcb:

  • One transformer that act as step down transformer 220 v AC to 9 volt ac
  • 2 x 4007 diodes  to rectify the AC to DC
  • Electrolytic cap as filter of rippled DC product of power supply
  • A relay 220/24v dc as key for turning off and on the pump
  • Transistor NPN BC 945 that amplifies signals for relay
  • Cmos IC 4011 that translate the signals that coming from sensors and with cooperation of Transistor and relay give command to the motor  pump for on and off
  • Zener diode 8.5 volt as regulator supply voltage for IC pin 14
  • Green LED as indicator that the pump is working
  • Red LED indicator of 220 AC is present in circuit
  • 2 x 220 k ohm resistor and 2 x 480 k ohm resistors as impedance matching parts of sensors to digital IC

water level controller circuit board


The only part that I found bad on this machine was the linear transformer. The primary winding of the transformer was opened circuit and by replacing with a new transformer, the controller return to service.

bad linear transformer

Note: Since I could not find the exact size of the transformer in the market so I have used the bigger one but with the same specification. Since the new transformer was big, I installed it on top of the box instead on the PCB. Rewinding of the old transformer was costly so getting a new transformer fixed the problem.

replace linear transsformer

Final installation of transformer on top of the water controller

I suggest to my friend to use one AC voltage regulator on the supply voltages of water level controller to prevent transformer from unwanted spike of voltages so that this can prolong the life of the transformer.


This article was prepared for you by Mr Beh from Iran.

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Note: You may check out his previous repair articles in the below link:




  1. George Greenfield

    April 28, 2016 at 7:25 pm

    Mr Beh,
    Pretty work ! I've mounted components externally a few times myself. Whatever it takes to make it work.
    Great job 1


    • Beh

      April 28, 2016 at 10:15 pm

      Thanks for supports .

  2. Albert van Bemmelen

    April 28, 2016 at 7:27 pm

    Good Job Beh! Probably the new and bigger Transformer will work better
    than the old one. It likely can handle more heat because the core is
    The 4011 I once replaced in my brothers Car-Alarm because it was not
    working. After I replaced the defect 4011 CMOS the Ultrasonic Transducers (operating probably around 40KHz) worked so good that even a fly or a
    mosquito was enough inside the closed Car to trigger the Alarm.
    And it was adjusted so sensitive that even a thunderstorm drove him
    out Bed in the middle of the night because the Alarm wake everyone up.
    Because it worked too good he decided to stop using it.
    It was working so successful that it drove him mad. I must admit that I
    felt sorry for him that he was not able to adjust it well enough. And
    the Car Alarm ended at a scouting flea sale in our hometown.

    • Beh

      April 28, 2016 at 10:11 pm

      Hi Albert
      Thanks for support and happy the alarm of your brother caris repaired too...beh

      • beh

        April 29, 2016 at 11:38 am

        Albert: the black pipes above photo are sensors of the water level controller

        • Albert van Bemmelen

          April 30, 2016 at 3:33 pm

          I thought as much Beh (;-) If those sensors keep working it should be okay!

    • Humberto

      April 29, 2016 at 12:50 am

      Hi Albert, I have your own opinion: bigger Transformer will work better
      than the old one.

      • beh

        April 29, 2016 at 11:41 am

        Humberto: yes but with same specifications the cost of this repair was 1 USD and thanks for comment

    • Robert Calk

      April 29, 2016 at 11:28 pm

      You couldn't adjust the sensitivity down on the car alarm?

      • Albert van Bemmelen

        April 30, 2016 at 3:48 pm

        Yes we could Robert, but somehow we couldn't get it adjusted good enough working under all circumstances. It is like with Hysteresis in
        electronics and other physical influenced areas, it never works
        exactly afterwards as you thought you had it set. As is the case with setting the wanted temperature in your home. There is always a difference between the wanted and the momentary room temperature and the time it takes to adjust to the set temperature depends on how big the Hysteresis is (if I remember it right). Maybe we could have made some kind of test sound checker to see if it would behave as we wanted it to work. But testing in a old closed car is no easy job either.

  3. moshe

    April 28, 2016 at 8:07 pm

    Hi Mr Beh
    Can you give a link to the suppliers/manufacturers of that level controller?


    • Beh

      April 28, 2016 at 10:13 pm

      Hi Moshe
      At the moment no is not possible but I will ask from gardener give this info for you sure.


      • beh

        April 29, 2016 at 12:24 pm

        MOSHE : this company (above link) in PUNE INDIA is manufacture same model controller as i repaired .

        • moshe

          April 30, 2016 at 8:53 pm

          thanks Beh, much appreciated.

    • Paris Azis

      April 28, 2016 at 11:43 pm

      Good job, Beh. I also have repaired some devices like this one. Very useful and simple circuits indeed.


      • beh

        April 29, 2016 at 11:45 am

        Paris: the price of this controller in our market is 15-20 USD but repaired with 1 USD and thanks for your confirmation.

      • Paris Azis

        May 3, 2016 at 9:16 pm

        Hey Beh

        I know that these controllers are cheap to buy because they are simple circuits, but I also don’t see any reason to throw them away (as you do, as I see and agree with you) instead of repairing them, given that their repair is equally easy, not time consuming and low cost as well. I would certainly repair it, exactly as you did!
        For your market the repair is even more worthwhile as I see. So, why not? And good for you!


  4. Parasuraman S

    April 28, 2016 at 9:50 pm

    Hitherto uncovered area, I think. I remember to have made one water level controller many years back! Very nice decision to mount the transformer externally, by wiring it inside for safety! Very professional work! The water level controller can have problems when working in hard water!

    • beh

      April 29, 2016 at 11:50 am

      Parasuraman: yes this was first time that i opened this device in my life and as you said it has very simple cir cute in it just by my general knowledge in electronic i solved the problem and thank s for your kind words.

  5. Humberto

    April 29, 2016 at 12:53 am

    Hi beh, I confess I've really enjoyed reading your article, reading step by step your repair procedure. By the way, did you draw the complete schematic diagram of this Water Level Controller? if yes, please, send it to me by means of Jestine Yong.

  6. Alvaro Rivillas

    April 29, 2016 at 9:47 am

    Hi Mr Beh It is a good article and I would like to have the schematic of the water controller. It is very interesting Thank you .

    • beh

      May 1, 2016 at 12:21 pm

      Alvaro Rivillas : you are most welcome

  7. beh

    April 29, 2016 at 11:54 am

    Humberto : i am really happy you like my article i wished one day to come to Havana and meet you there . no i did not draw any diagram of this controller and i am sorry about that.
    thanks for your warm words.

    • beh

      April 29, 2016 at 12:07 pm
      Humberto: above links are useful how this machine works

  8. Tesfaye kebede

    April 29, 2016 at 2:57 pm

    A good work Mr Beh

    • beh

      April 30, 2016 at 11:27 am

      you are welcome Tesfaye

  9. suranga bandara, Suranga Electronics

    April 29, 2016 at 3:40 pm

    hi, Mr- Beh,

    you have done Excellent Repair Work !
    Thanks for This Article.

    • beh

      April 30, 2016 at 11:29 am

      you are welcome Suranga

  10. Robert Calk

    April 29, 2016 at 11:31 pm

    Good job, Beh. Thanks for sharing the repair with us.

    • beh

      April 30, 2016 at 11:29 am

      you are welcome Robert

  11. Yogesh Panchal

    May 12, 2016 at 10:36 pm

    well done! Beh

    It is really fun while expanding area of knowledge thanks for sharing.

  12. Edwi n Kelechi Anaeme

    June 7, 2016 at 8:57 pm

    Hi Beh
    Great job. Your idea of improvising was great also. Thanks for the article.


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