Temperature x voltage converter with Lm35. To be used with a digital multimeter with a range from -50 °C to 150 °C. With a 9V battery and a home-built probe. And all this without any adjustments, thanks to the LM35!
Curiosity about degrees centigrade and Celsius.
Ice melts at 0 °C and the boiling point of water at sea level is 100 °C. Using these two reference points, the temperature scale is quickly calibrated. When we speak °C it is degrees Celsius and not centigrade as we can see in some literature, centigrade is a scale of 100. While temperature goes beyond 100. So, the correct is to speak degrees Celsius! Well enough blah blah…
Today, thanks to a specialized integrated circuit from National Semiconductor, called LM35, we don’t need any calibration to get the correct temperature.
Lm35 temperature sensor module for multimeter
For the circuit we need a 9v battery and a negative supply voltage. This is obtained according to the LM35 datasheet by 1n4148 diodes D1 and D2. Resistor R1 decouples the probe output from the high impedance output of the multimeter.
Resistors R2,R3(90K)/R4(10K) form a voltage divider so that 1 °C is equivalent to 1mV. So the reading on the multimeter display will be direct. Since the output of the Lm35 ⇒ 1 º = 10mV
The LM35 should be mounted in a metal tube of about 5 to 10 CM, connected to the main circuit by a three-wire cable no more than 1 m long. The tube should be hermetically sealed on both sides to allow the measurement of liquids.
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The main board and battery should be placed in a small plastic box. The board should be isolated from the battery to avoid short circuits and damage caused by the corrosive content of the battery.
The LM35 should be mounted in a metal tube of about 5 to 10 CM, connected to the main circuit by a three-wire cable no more than 1 m long. The tube should be hermetically sealed on both sides to allow the measurement of liquids.
Operating mode of the circuit with lm35 sensor:
Before making a temperature measurement test the battery by pressing S1, everything ok just connect the unit to the multimeter to make the measurements. Depending on the temperature, the multimeter on the scale Volt DC 200mV will indicate a voltage of -55mV to 150mV or the scale to be used depends on the multimeter, this voltage corresponds to the temperature of -55 to 150 °C. Or depending on the range of the Lm35 used!
PCB for lm35 temperature sensor module for multimeter
lm35 thermometer circuit components
| Part | Component | Value | Description | Amount |
| Resistors 1/4W | ||||
| 1 | R1 | 15K | Resistor 5% | 1 |
| 2 | R2 | 68K | Resistor 1% | 1 |
| 3 | R3 | 22K | Resistor 1% | 1 |
| 4 | R4 | 10K | Resistor 1% | 1 |
| Capacitors | ||||
| 5 | C1, C2, C3 | 100n | Ceramic Capacitor | 3 |
| Semiconductors | ||||
| 6 | IC1 | LM35 or Lm35A | Precision Temperature Sensor | 1 |
| 7 | D1, D2 | 1N4148 | Diode | 2 |
| Miscellaneous | ||||
| 8 | JP1, JP2 | Pin Header | Connect 9 to 12V power supply | 1 |
| 9 | JP2 | Pin Header | Connecting the probe tip | 1 |
| Solder, Wires, Pci, Box, Source, Male banana jack | ||||
For more information about the lm35, download the PDF datasheet of the lm35 integrated
Download PDF, PNG and Gerber files for this project.
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