NTC Thermistor Water Level Detector.

Many of you want a reliable and simple water level sensor and, as many of you have looked at the worst Instructable I have made, the "Water Tank Float Switch", I offer the following to make amends.

2 x Small signal Transistor; any NPN with a gain of 100 or more.

1 x 10K ohm Resistor

1 x 220 ohm Resistor

1 x 500 ohm variable Resistor (TrimPot) - Optional

1 x NTC Thermistor

1 x circuit mounting board

Thermistors have the benefit of being simple and cheap devices but the non-linear response can make design choices difficult.

This idea uses two characteristics that are normally considered problems; the non-linearity and self heating.

If a Thermistor is powered to raise it's internal temperature in free air then immersing it in water will cool it giving a significant change of resistance due to the non-linear response.

Using a NTC (Negative temperature coefficient) Thermistor with self heating could cause it to over-heat if not controlled i.e it gets a little hotter; the resistance goes down and current increases making it hotter and hotter until it fails.

I suggest using a constant current source due to W=I^2 x R and that will prevent thermal runaway: If the Thermistor gets a little hotter then because W=V^2/R as the resistance decreases it becomes self-regulating.

The constant current source diagram shown is set at 4mA by the 220 ohm resistor and gave me a voltage change of 11.6 volts from a 22v supply voltage (i.e about half the supply voltage). A variable resistor could be used to find a suitable current for any particular thermistor but I recommend a resistor "chain" that can have each element short-circuited to find the best value. A "chain" could be a combination of 56 and 100 ohm etc. but be sure to start at the highest total resistance.

The difference change in voltage across the device is relatively large so a simple transistor switch or comparator circuit could drive a relay to control water level.

A simple way to check if the Thermistor is self-heating in free air is to pinch it with your fingers then, the resistance will go down if your fingers warm it or will go up if they are taking heat away.

Be careful not to burn yourself! The resistance values I used suggested an internal temperature upto 70°C = 158°F

Be sure to seal the Thermistor connecting wires from water as the voltage would cause corrosion.