Trigger for a water sensor

Question

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 :))

^{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?

Answer 1

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.

Answer 2

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.

^{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.