A Super Easy-to-Build Electronic Thermometer
| Polish version is here |
It Couldn’t Be Simpler
In a home laboratory, the need for precise temperature measurement often arises. A mercury thermometer (not a medical one) is excellent for this task. But what should an experimenter do if they need to measure the temperature of an experimental setup and don’t have access to a mercury thermometer? I suggest building what is probably the simplest electronic thermometer.
Construction
The core component of this thermometer is the LM35 integrated circuit. According to its datasheet (available here), it is a complete electronic thermometer.
The LM35 is housed in a standard three-pin plastic package similar to that of a transistor. In its simplest configuration, the circuit requires only a power supply and a measuring device. Below is the simplest application schematic, taken from the datasheet:
Taking measurements is straightforward and simple: the voltage at the middle pin of the LM35 is directly proportional to the surrounding temperature at a ratio of 10mV/°C. For example, if the voltmeter reads 245mV, dividing by 10mV gives a temperature of 24.5°C (76.1°F). Simple, right?
The circuit above can be assembled without a printed circuit board by connecting the LM35 to a three-wire cable:
Measurements can be taken using either a digital or analog voltmeter (multimeter).
In both cases, the reading is approximately 24°C (75.2°F). Both multimeters are set to measure millivolts—the analog one on the 1V range and the digital one on the 2V range. Because of the analog meter's scale construction, the reading (about 2.4) should be multiplied by 100 (for the 1V range) and then divided by 10 (due to the voltage-to-temperature ratio), resulting in a simple multiplication by 10 (2.4 × 10 = 24°C or 75.2°F). In the case of the digital multimeter, the displayed value is already in millivolts, so you only need to divide it by 10 (240/10 = 24°C or 75.2°F).
This simple temperature measurement setup is sufficiently accurate for most applications.
Enjoy experimenting with this educational project! :)
Further readings:
- Praca zbiorowa, Słownik fizyczny, Państwowe Wydawnictwo „Wiedza Powszechna”, Warszawa, 1984, pp. 403-404
- Middleton W.E.K., A history of the thermometer and its use in meteorology, Johns Hopkins Press, 1966
Marek Ples