Frost Detector Temperature Sensor Circuit

To know whether it is freezingyou only need tomeasure the temperature. This has to be done accurately, of course, and therefore we need to choose atemperature sensorthat we have some confidence in. The choice has again been made for a type that we have already used in many previous Elektor circuits, theLM35CZ(-40 to 110 °C). Thissensor is not exp

ensiveand generates an output voltage that is proportional to thetemperature in degrees Celsius(10 mV/ °C). AnLM35is normally powered from a single-ended power supply and 0 °C corresponds to an output voltage of 0 V. It is therefore not possible to measurenegative temperatureswith an LM35 in the standard application circuit. It is however possible to measure negative temperatures if its output is connected to a negative supply voltage via a resistor. There needs to be a current of 50 A through this resistor (R2 in the schematic). We only need todetect the freezing pointwith this circuit. That is why there is acomparatorafter thetemperature sensor, which turns an LED on if the temperature has dropped below 0 °C during the course of the night. To ensure that the comparator operates properly it is necessary that the measurement value can become slightly more negative with respect to the input. To solve this problem, a diode (D1) has been connected in series with the ground connection of the LM35. The voltage drop across D1 (because of the small current through the LM35 this is only 0. 47 V) acts as `negative` power supply. Since the non-inverting input of comparatorIC2 is connected via R3 to the anode of D1 it functions as the 0 °C-reference level for the comparator. connecting the bias-select input (pin 8) to the power supply voltage. There is no need for the detector to be fast and it will therefore work well with the opamp...

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