21. Hot Water Tank Indicator

21. Hot Water Tank Indicator
Hot Water Tank Indicator
I used masking tape 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
2 - 3 years
depending on how often you use the push switch, SW1.
Save fuel bills and the economy of the planet with this electronic circuit. SW1 is a press
button switch which
allows you to view the level of hot water in a hot water tank. Thermistors NTC1-4 should
be spread evenly
over the height of the tank. I placed NTC1 roughly 4 inches from the top of my tank and
the others spaced
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. "Hot" can be translated as 50C to 80C
the presets P1-4
allow adjustment of this range.
I have used a quad version of the LM324 but any quad opamp can be used or even four
single LM741's.
R2-R5 I used 330ohm ,but not critical.
NTC1-4 The thermistor resistance governs the value of R1 R6 and the presets. I used a
thermistor from
the Maplin catalogue. Cold resistance was around 300K, hot around 15k
R7-10 series resistance, to prevent a short circuit only needed if your thermistors
resistance is several ohms
P1 - P4 I used 100k as this range corresponds to the resistance of the thermistor at the
required water temp
R1 & R6 are equal and bias the supply voltage to half supply. I used R1=R6 = 100k
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