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I have built an automatic water tank using the circuit given below:

enter image description here

But this circuit was working well for 2 days. But after that, it triggers falsely. When a fan in another room is switched on, this circuit trigger the relay and motor will turn on.

I think this is due to AC noise. I was using 12v AC to DC converter supply, i.e, 12DC adapter.

Can anyone help me to avoid this situation?

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  • \$\begingroup\$ What’s your power supply situation? Show how Vcc is generated and connected. Show layout. Why are you missing decoupling? Why the hassle in the first place instead of a electromechanical float switch? \$\endgroup\$ – winny Mar 30 '19 at 8:08
  • \$\begingroup\$ Its an DC adapter supply bought from store. \$\endgroup\$ – vishnu m c Mar 30 '19 at 8:23
  • \$\begingroup\$ Neat. Which model? Show layout. Why no decoupling? \$\endgroup\$ – winny Mar 30 '19 at 9:07
  • \$\begingroup\$ I am not an expert. I do not no the circuit. where to give decoupling? \$\endgroup\$ – vishnu m c Mar 30 '19 at 9:11
  • \$\begingroup\$ Across your IC but depending on your layout which you haven’t shown yet, more places too. \$\endgroup\$ – winny Mar 30 '19 at 9:12
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Decouple the IC as said in the remarks above: 100 nF close to the IC across Vcc and GND. A 10uF or 100uF cap next to it will help too if the 12V power supply lines are long.

Moreover make sure the 100 ohm resistor (connected to up) is close to the transistor and both are close to the 555. And most important, add a 100 ohm between Low and pin 2 of the 555. The resistor should be close to the 555 again.

It is most likely the wire to pin 2 of the 555 picks up noise and makes the 555 trigger incorrectly. Adding a resistor close to the 555 willl attenuate that noise. If that resistor is far away, noise can still couple into the line and triggger the 555 incorrectly because it won't 'see' the 100 ohm resistor.

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Your water tank has lots of capacitance to ground .Your high and low sense wires are antennas .Deal with these wires with 100n bypass caps to shunt any AC noise .Your SMPS makes lots of common mode noise .Try something thats properly grounded or a linear supply .

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Pin 2 is very sensitive, as stated add a 100R is series, I would also try a large pull up to Vcc, 220K or higher. this means gnd (0v) to you tank not Vcc and a pnp for the reset transistor.

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