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I want to build a water sensor that disconnects the motor when:
1. tank is full
2. water in input pipe is missing.

I came up with the following schematics (haven't practiced much electronics in last 10 years :))

schematic

simulate this circuit – Schematic created using CircuitLab

Sensors
The sensors are just 2 wires (R1, R2).

  • R1 (priming water sensor) is the resistance of input water (let's say 2K when water present and 1M when water missing). This makes sure the motor is not running without water.
  • R2 (tank full sensor) is the resistance of second water sensor.

Seems to work but I see a residual current (few mA) in the relay. The relay makes contact at 15mA, but I think firm contact is at 25mA.

I need to know if the relay might oscillate. Do I need some kind of digital trigger to make sure the relay is either on or off, but not in the middle?

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  • \$\begingroup\$ You need to fix the link to your schematic. \$\endgroup\$ – Jack Creasey Apr 28 '17 at 18:46
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Simple and straightforward would be using float switches. A float switch has a metal ball inside that pushes a micro switch. With regard to the inclination of float switch the contact changes to on/off. Almost every pump is equipped with such float switch for preventing dry running.

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  • \$\begingroup\$ Thanks. I could install a float switch in the water tank but not on the input pipe. Or? \$\endgroup\$ – WeGoToMars Apr 28 '17 at 19:00
  • \$\begingroup\$ wow - those switches are around 10 euros on Amazon.de! \$\endgroup\$ – WeGoToMars Apr 28 '17 at 20:04
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You may be better to use a cheap comparator like an LM339 with some hysteresis on it. Otherwise this is going to turn on and off rather a lot. You also don't really need a relay, a MOSFET driver would be better.

Perhaps something like this.

schematic

simulate this circuit – Schematic created using CircuitLab

Note I added a small delay filter R19 and C1 which you may need if the signal from the tank-full sensor is erratic while the pump runs.

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  • \$\begingroup\$ Thanks, but that is too complex for me to design :). But the FET idea is great. I want to build the circuit with whatever parts I have home and I disabled some TVs and a computer. There must be some FETs there :) \$\endgroup\$ – WeGoToMars Apr 28 '17 at 18:55
  • \$\begingroup\$ Why was this answer downvoted? It is a very good answer. \$\endgroup\$ – WeGoToMars Apr 28 '17 at 19:59
  • \$\begingroup\$ @BorlImpriceGoGearEmbaIdera who knows.... \$\endgroup\$ – Trevor_G Apr 28 '17 at 20:10

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