Thursday, May 26, 2011
Hot Water Indicator Using NTC Thermistors
A simple circuit 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.
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.
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.
This hot water indicator circuit from : zen22142.zen.co.uk
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