Wiring Diagram

This is a circuit that serves to indicate various levels of hot water in a tank. SW1 is a normally open press button switch which allows you to view the level of hot water in a hot water tank. When pressed the voltage difference at the junction of the thermistor and preset is compared to the fixed voltage on the op-amps non-inverting input. Depending on the heat of the water in the tank, the thermistors resistance will toggle the op-amp output to swing to almost full voltage supply and light the appropriate LED.

Note:

    * Op-Amp: LM324 or any quad opamp can be used or even four single op-amps.
    * R2-R5: 330ohm resistors, but Lower values give brighter LED output.
    * NTC1-4: Cold resistance was around 300K, hot resistance 15k. Alternative thermistors may be used with different resistance ranges, but the presets P1 to P4 must also be changed as well.
    * R7-10: only required if your thermistors resistance is several ohms at the hottest temperature.
    * P1 - P4: Chosen to match the resistance of the thermistor when cold.
    * R1 & R6: 100k Resistor


Masking tape was used to stick the bead thermistors to the tank. Wires were soldered and insulated at the thermistors ends. A plastic box was used to house the circuit. Battery life will probably be 4 to 5 years depending on how often you use the push switch, SW1.