Smart digital thermometer - from conception to design

Smart Digital Thermometer

Ok before we start let me say this - this is going to be a multiple blog entry and it will be an ongoing project as I develop and modify the design criteria of the Smart Digital Thermometer.

So with that let us begin. Let me give a brief introduction to the project. The idea is to create smart thermometer for logging temperature in digital format. Sounds simple enough but I am going to pack the device with tons of features like a clinical mode with high precision operating between 36 deg C and 45 deg C as well as a normal 0 to 100 deg C. Device will have an internal memory users can create patient profile where the temperature will be logged automatically w.r.t. time. Device will have a real time clock There are also other feature which I will discuss as we go along.

Now one may ask there are tons of digital thermometers like this in the market why is this project special. That is because my soul idea behind this project is to design a high precision sensor with as low cost as possible. I will rarely be using high priced all in one package IC. For all the controlling, detecting and logging stuff  I will be using  of the shelf "jelly bean" stuff but ensuring the high precision is always maintained no compromise there.

The basic temperature sensor is going to be a LM335 IC from Texas Instruments. Initially I will be using a single IC but later upto 4 for averaging the temperature for increased accuracy. For more info please check the data sheet.

Basically it is a Zener diode with breakdown voltage function of temperature as 10mV/deg K. However current flowing through this device should be sufficient enough to cause breakdown but not too much to cause self heating which would interfere with the actual temperature.

However we can see a problem here we do not work with Kelvin scale generally. So for 0 deg C we will get 2.73V as the output i.e. when we say we are getting 2.73V at the output of the sensor we know the temperature is 0 deg C. Similarly 3.73V at 100 deg C. But we want 0V at 0 deg C and 1V at 100 deg C.

Now we can easily offset this in our software but again although we will be using a 10bit ADC which is more than enough but still we will loose some precision in this conversion so we will offset this in the analog domain and then sense the offset value via the 10bit ADC. So our first problem lies in designing the offset analog circuit. Solution very simple we will be using an op amp based subtractor ckt.

In the next post we will design and simulate this ckt and then implement it on a strip board.

to be continued...