Keywords: LM35, TLC271, Temperature Detection Circuit
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Author: circle flying
To know if the temperature is below zero, the only thing to do is to measure the temperature. This requires a very accurate, of course, you have to choose a trusted temperature sensor. Here we choose the sensor is LM35C2 (-40 ° C ~ 100 ° C), which is used in many ELKTOR circuits. This sensor is not expensive, and its output voltage is proportional to the temperature (10 mV / ° C). The sensor LM35 is typically powered by a single source, and the output voltage corresponding to 0 ° C is 0V. Therefore, it is impossible to use LM35 in a typical application circuit to measure the temperature of 0 ° C or lower. However, its output can be connected to the power supply negative through a resistor so that the temperature below zero is measured. This is desirable - a resistance through 50 μA (R2 in the figure below). We only need to use this circuit to detect freezing points. That's why there will be a comparator after the temperature sensor, if the temperature is low at night below 0 ° C, it will light a LED. In order to ensure that the comparator can function properly, the input measurement value can be negative, in order to solve this problem, a diode (DL) is added between the LM35 and the ground. The voltage passing through D1 (because only 0.47V of the voltage currently passed through LM35) is used as a negative voltage. The forward input of the comparator IC2 is connected to the positive electrode of D1 via R3, and its effect is the reference point of 0 ° C as a comparator.
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The comparator comparator is a standard op amp TLC271, we can set its minimum power consumption by connecting its input bias selection (pin 8) to the power supply voltage. The detector does not have to work at a very high frequency because it can operate stably in the most economical mode in an enlarged state. The light emitting diode D3 is used to indicate whether the temperature is below zero, and when the temperature in the room begins or has been reduced below 0 ° C, it will continue to light. To achieve this, an asymmetric delay circuit is constituted by R3, R4, and D2. When the output becomes high, the forward input is fed back by D2 and R4, and the output remains high. When the LED is automatically extinguished, it means that the temperature at this time has exceeded 30C, which may mean that it is summer and no matter how it is. If necessary, the delay can be increased by increasing the R3. The function of capacitor C2 is to ensure that the LED is still extinguishing after the system is added. The forward input of the op amp is directly connected to the ground therefore the output is also low. RL and SL are used only when the circuit needs to be restarted after the battery is installed. You can use a power switch, or remove the battery for a while to replace SL open, off. Low power consumption is powered by the battery, so we should reduce power consumption. The power consumption of the prototype circuit is not 120μA when power is powered by a power supply of 6 to 9V. Because use low-power LEDs, it is 1mA (6V) and 1.8mA (9V), respectively, when LED is lit. In our prototype circuit, a green low-power LED is used. If you use a 4-section AA battery (about 2ah capacity), the circuit can operate normally about 2 years. But if the LED is often bright, the runtime will be shortened. (For about two months, it is enough to have a strict winter) One standard 9V battery can also stick to a winter, and the premise is to check if the LED light is on. Finally, explain why TLC271CP is used here. The complex c specified by the suffix C is 0 ° C to 70 ° C, but it can continue to work at lower temperatures, especially in view of the IC without linear applications. If you question its performance, you can use the suffix i (which is TLC2711P: -40C-125C), provided that it is confident that the room monitored is really cold. Quickly assemble this circuit contains few components, so they can be easily mounted to a small plate, nor does it need to measure any devices. Once completed, you can use it.
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