Simple and easy to make zero degree detector

To know if the temperature is below zero, the only thing to do is to measure the temperature. This needs to be done very accurately, of course, you have to choose a reliable temperature sensor. The sensor we choose here is LM35C2 (-40CC~1100C), which has been used in many previous Elektor circuits. This sensor is not expensive, and its output voltage is proportional to the temperature (10mV/oC).

sensor

LM35 is usually powered by a single power supply, and the output voltage corresponding to 0°C is 0V. Therefore, it is impossible to use LM35 to measure temperatures below 0°C in typical application circuits. However, its output can be connected to the negative pole of the power supply through a resistor, so that temperatures below zero can be measured. This requires a resistor that passes 50μA current (R2 in the figure below).

We only need this circuit to detect freezing point. That is why there is a comparator after the temperature sensor, which lights up an LED if the temperature drops below 0℃ at night. In order to ensure the normal operation of the comparator, it is necessary that the measured value of the input can be slightly negative. To solve this problem, a diode (Dl) is added between LM35 and ground. The voltage through Dl (because the current voltage through LM35 is only 0.47V) acts as a negative voltage. The positive input of comparator IC2 is connected to the positive pole of Dl through R3, which acts as the reference point of the comparator 0℃.

Comparator

The comparator is a standard operational amplifier TLC271, whose minimum current consumption can be set by connecting its input bias select (pin 8) to the power supply voltage. The detector does not have to work at a very high frequency, because it can be stably operated in the most "economical" mode while still amplifying.

The LED D3 is used to indicate whether the temperature is below zero. It will light up continuously when the temperature in the room begins or has dropped below 0°C. To achieve this, an asymmetric delay circuit is formed by R3, R4 and D2. When the output becomes high, the positive input is positively fed back through D2 and R4, and the output remains high. When the LED automatically turns off, it means that the temperature is above 30°C, which in practice may mean that it is summer and it will not freeze anyway. If necessary, the delay can be increased by increasing the resistance value of R3.

The function of capacitor C2 is to ensure that the LED remains off after the system is powered up. The positive input of the op amp is directly connected to ground so the output is low. Rl and Sl are only used when the circuit needs to be restarted after installing the battery. A power switch can be used instead, or the battery can be removed for a while to turn Sl on and off.

Low power supply

Since the circuit is battery powered, we have tried to minimize the power consumption as much as possible. The prototype circuit consumes less than 120μA when powered by a 6~9V power supply. Because a low-power LED is used, the current consumption when the LED is on is ImA (6V) and 1.8mA (9V). In our prototype circuit, a green low-power LED is used. If 4 AA batteries (capacity of about 2Ah) are used, the circuit can operate normally for about 2 years. However, if the LED is always on, the operating time will be shortened. (About two months is enough to survive a severe winter.) A standard 9V battery can also last a winter, provided that the LED light is checked frequently.

Finally, let me explain why the TLC271CP is used here. The suffix C specifies an operating range of 0°C to 70°C, but it will continue to work at lower temperatures, especially considering that the IC has no linear applications. If you question its performance, you can use a device with the suffix I (this is the TLC2711P: -40°C—125°C), provided that you are sure that the room being monitored is indeed cold.

Quick assembly

The circuit contains very few components, so they can be easily mounted on a small board and no measurements are required. Once completed, it is ready for use.