По принципу действия датчик LM представляет собой стабилитрон, у которого напряжение стабилизации зависит от температуры. При повышении температуры на один градус Кельвина напряжение стабилизации увеличивается на 10 милливольт. Для измерения температуры ис- пользуются 2 вывода, третий нужен для калибровки датчика. В качестве примера использования датчика LM создадим индикатор температуры окружающей среды на RGB-светодиоде. Схема подключения показана на рис.
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This article includes additional downloadable resources. Please log in to access. Log in Measuring temperature in your project can be made easy using the tiny LM sensor.
In this issue of The Classroom, we take a look at the geometry of the LM, what makes it work, and how you can use it. On the subject of datasheets, they are generally produced by the original design team, and then provided to other manufacturers as part of production licensing. If you look closely at the first page of the LM datasheet from Texas Instruments , you may notice that it is word-for-word the same as the datasheet from National Semiconductor.
The LM is set apart from most other temperature sensors available to the maker. Most of these devices have some sort of response curve or slope. The combination of this means that a user has to figure out what voltage or current corresponds with a certain temperature, or what part of the response line is relevant to their set of circumstances. There is often little certainty of this for the maker. The LM, on the other hand, is truly linear within its operating range, but more importantly, its output is defined.
The Kelvin and Celsius scales share the same sized unit of measure unlike Fahrenheit , but the Kelvin scale starts at Absolute Zero, the coldest temperature currently known to science, where all movement of matter stops. But the size of the unit, the degree, is the same for both scales. Stay with us, this does have a point. The voltage output is x 10mV, giving us 2. This means that the voltage at the output can be read to give a meaningful, calibrated number that can be used by a block of code, or by eye.
Even a simple voltmeter would work as a thermometer display, as long as you think in Kelvin or remember to subtract from the number, and ignore the decimal point in both cases. You could also replace the dial card if you are using an analogue meter. This is what sets the LM apart from most of the usual slew of temperature sensors available to the maker.
Other packages are available but are unlikely to be encountered by makers. The schematic symbol of the LM shows an adjustable zener diode, but the functional block diagram which you can find in the datasheet reveals that in fact there is no zener diode. There is, instead, a network of transistors, resistors, and capacitors which make the IC function as though it were a specialised zener diode.
A zener diode normally has a fixed breakdown voltage. When reverse-biased connected with the cathode more positive than the anode it conducts but has a fixed voltage drop according to its specification. The temperature defines the voltage drop, rather than a relationship between components such as in the case of a resistor voltage divider.
Depicting the IC with a zener diode makes sense. This also reveals why the LM is used with a resistor and connected to ground, like a voltage divider would be, as above in the simplified sensor. Because the voltage drop changes, and the resistor is able to limit the current through the device to safe levels, the junction can be read like a voltage divider.
It is just that this one is temperature-dependent and highly accurate. This is achieved using the calibrated circuit here. In all cases, temperatures are in Kelvin. The neat thing with the LM is that its error is a slope error, which means the position of its linear response is shifted by the adjust pin. Getting it right for a known temperature means that the output is correct at all other temperatures within the range.
So the easiest way to calibrate the device is to use a known temperature and match the output. However, getting an accurate enough reference temperature will be the hard bit. Most thermometers have less accuracy as is, unless you spend lots on a professional grade model. Getting a temperature from, say, boiling water or ice, depends on other factors such as atmospheric pressure or purity of the water, respectively.
We are fortunate enough to have an accurate thermocouple-based thermometer here for testing, and used that to match ambient air temperature. For those without such a test instrument, the best way to calibrate is probably going to be with a medical thermometer. These are available from chemist shops, and are quite accurate within the temperature range they are intended to be used, which is within a few degrees of regular body temperature.
They are affordable because they are made to be accurate only within this range. We suggest using warm water with the tip of the LM dipped in it beside the thermometer see the next section about waterproofing sensors. This will be more consistent than trying to use your finger tips or any other bodily source of warmth. Remember, the reading on the LM is in the Kelvin scale, so add to whatever your Celsius thermometer displays.
This is where the resistor often depicted with the LM comes into play. It provides the limited current, as anything over 10mA will destroy the device, and even currents over 5mA for sustained periods will cause harm. The datasheets recommend an operating current of 1mA for highest accuracy.
Traditionally, an op-amp has been used as a comparator to make the LM into a temperature switch. Diagrams for that are to be found in the datasheets and can be used in conjunction with the classroom article on comparators in DIYODE issue However, the real value for the maker in the LM is the numerical output.
An accurate sensor with a comparator switch is great, and you can use trimpots to calibrate such a circuit against a known source, but using the LM as a sensor will feed a usable number directly into code for Arduino , Raspberry Pi , or other programmables, in a way few other sensors can.
Reading an analogue pin will yield a usable result straight away, and the voltage is in the usable range with no further need for scaling or buffering. The LM itself can be attached to a cable and mounted away from the circuit that uses it. Even for 24AWG wire, which is rather fine, this figure is over one hundred metres.
If you use reasonable gauge wire and keep lengths no longer than needed, less than a couple of metres, you can ignore this factor. Take care that your wire order remains consistent, as mixing it up could easily destroy the LM Make sure you carefully insulate connections from each other on the three pins of the device.
On that note, the sensor will need to be waterproofed for the sake of its metal legs and the connections to them. Choose a material that does not insulate thermally. The datasheets recommend glue-lined heatshrink for this task. Some of these products are thinner than others, and this would affect how thermally insulative the heatshrink becomes when the glue is set. You may wish to physically mount your sensor instead of leaving it dangling.
A short length of copper bar is ideal. The sensor needs to be in firm contact with the surface, so using a pair of bolts with another small length of metal will clamp it securely. Hands On:.
The LM335 Temperature Sensor
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