Calibration:
With a full tank of hot water adjust P1-4 so that all LED's are lit.
As hot water rises, the sensor at the bottom of the tank will be the maximum level of hot water. "
Parts:
- I have used a quad version of the LM324 but any quad opamp can be used or even four single op-amps.
- R2-R5 I used 330ohm resistors, but value is not critical. Lower values give brighter LED output.
- NTC1-4 The thermistors maximum resistance must roughly equal the resistance of the fixed resistor and preset. As negative temparature coefficient (NTC) thermistors are used, then their resistance decreases for increases in temperature. I used a thermistor from the Maplin Catalogue. 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 series resistance, 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. These resistors are equal and bias the op-amp inverting input to half the supply voltage. I used 100k.
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