A digital multimeter is a very useful instrument that combines several measurement functions in one unit. A typical multimeter includes features of a variable-range ohmmeter, voltmeter, and ammeter. Some of them also include capabilities of testing diodes and transistors. In this article, I am going to talk about a technique of adding thermometer feature to a regular digital multimeter. The technique is very simple and uses one temperature sensor along with two resistors and a DPDT slide switch.
Tag Archives: LM35
There are quite a variety of active analog temperature sensor ICs that provide an output voltage proportional to the temperature. They usually don’t require any external calibration and signal conditioning, and as such their output can be directly fed to the input of an ADC for digital processing. A few examples of such sensors are LM34, LM35, TMP35/36/37, and MCP9701. If you are having any trouble using any of these sensors in your project, here is a quick way to test if your sensor is working or not.
This is a revised version of my LM35 based digital thermometer project that I posted last year. Although it is one of the simplest projects, it is very popular among newbies who are just starting to learn microcontrollers. There was a little flaw in the original project as pointed by some readers. I was using a 1.2 V reference for A/D conversion with PIC16F688 microcontroller. However, the PIC16F688 datasheet says Vref should be equal to or higher than 2.2 V to ensure 1 LSB accuracy of A/D conversion. Here, I am rewriting the same project but this time I am using a MCP1525 IC to generate a precise 2.5 V reference for A/D conversion. This will improve the accuracy of temperature measurements.
A digital thermometer is a good choice of project for beginners who just stepped in to the world of microcontrollers because it provides an opportunity to learn using sensors to measure the real world signals that are analog in nature. This article describes a similar project based on a PIC16F688 microcontroller and an LM35 temperature sensor. LM35 is an analog sensor that converts the surrounding temperature to a proportional analog voltage. The output from the sensor is connected to one of the ADC channel inputs of the PIC16F688 microcontroller to derive the equivalent temperature value in digital format. The computed temperature is displayed in a 16×2 character LCD, in both °C and °F scales.
The LM35 series of temperature sensors are produced by National Semiconductor Corporation and are rated to operate over a -55 °C to 150°C temperature range. These sensors do not require any external calibration and the output voltage is proportional to the temperature. The scale factor for temperature to voltage conversion is 10 mV per °C. The LM35 series sensors come in different packages. The one I used is in a hermatic TO-46 transistor package where the metal case is connected to the negative pin (Gnd).