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enter image description here

I have built a water level indicator with a 555 timer. The supply is a 12 V SMPS adapter, the output of 555 timer is a square wave with a 21 kHz frequency. I pass this signal into AC coupling to convert it into an alternating square wave between +/- 6 V through a 10 μF cap and 1 kΩ series resistance.

This +/- 6 V signal is then sent to my overhead tank at the very bottom; I have also installed other probes at 25, 50, 75, and 100 percent level.

My question is why all level LEDs lights up even though the water level in the tank is only 25 percent?

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    \$\begingroup\$ Can you please provide a schematic of your circuit and the wiring of the probes? I did not understand where the LEDs are connected at all. Are these probes just direct contacts to the liquid or capacitive couplers? \$\endgroup\$
    – Jens
    Jul 10, 2022 at 14:09
  • \$\begingroup\$ Sorry i forgot to attach schematic , i have attached now \$\endgroup\$
    – Dhaval
    Jul 10, 2022 at 14:11
  • \$\begingroup\$ Are you trying to pass enough current through the water to light up a LED? What won’t work. You need to detect the uA current flowing and trigger the LED with a separate drive. Try to split detecting the level and lighting the LED into two separate circuits. \$\endgroup\$
    – winny
    Jul 10, 2022 at 14:44
  • \$\begingroup\$ Led is glowing with enough brightness probably around 5 - 7 mA of current is flowing through LED . Is there any way that i can directly turn the led on (with this setup) without sensing any current and using different drive to turn on the LED \$\endgroup\$
    – Dhaval
    Jul 10, 2022 at 14:56
  • \$\begingroup\$ With high enough drive voltage you can do anything you want, but why would you want to? Please show the physical setup. \$\endgroup\$
    – winny
    Jul 10, 2022 at 15:09

1 Answer 1

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The root cause of your problem is cable capacitance.

enter image description here

Introducing the 0.1 μF capacitors will help. The values of the current limiting resistors, for the LEDs, may also be lowered.

The schematic of the 555 IC - based 20kHz generator has been incorporated.

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  • \$\begingroup\$ I also think so but i can't eliminate it , what can i do so that this setup works ? \$\endgroup\$
    – Dhaval
    Jul 10, 2022 at 15:46
  • \$\begingroup\$ My answer has been edited. \$\endgroup\$
    – vu2nan
    Jul 10, 2022 at 18:52
  • \$\begingroup\$ I added capacitor but other LEDs are still glowing . I also tried reducing the frequency of 555 timer to 42 Hz but still LEDs are glowing with low brightness \$\endgroup\$
    – Dhaval
    Jul 11, 2022 at 4:42
  • \$\begingroup\$ My answer has been further edited. \$\endgroup\$
    – vu2nan
    Jul 11, 2022 at 6:49
  • \$\begingroup\$ You're putting capacitors in series with a diode. In other words, they can charge but not discharge. Now, it is true that the LEDs do have a reverse-breakdown voltage and thus it possible for discharging via breaking down the LEDs, but I would avoid that. \$\endgroup\$
    – Atomique
    Jul 12, 2022 at 19:25

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