Digital Thermometer

Another simple project, using the National Semiconductor LM35 pre-calibrated Digital Thermometer that had been sitting in my junk box for some time. The LM35 sits on the end of the probe, which is made out of an old ballpoint pen. The LM35 delivers a pre-calibrated output with accuracy within a degree over the full range of -50 to 150 degrees celcius (though my plastic probe may not withstand this range of temperature!). It delivers 10 mV per degree Celcius, with zero degrees being zero Volts. The output is divided by 10 to drive a standard 200 mV digital panel meter. I obtained a meter for $14.50 from SurplusTronics (search for item LA0310) which works fine.

The LM35 seems to be hard to get. There are other cheap temperature measuring devices such as the DS18B20 which is popular with PicAxe educational projects - however, the DS18B20 requires a little micro to drive it, and a digital meter for readout (not a 200 mV analog panel meter).

Digital Thermometer

Referring to the circuit below, the panel meter must be driven from a separate supply from that which it is measuring (not all panel meters have this limitation, but many do), so rather than having two batteries I used a small inverter to provide an isolated power source for the LM35. The little inverter runs at about 100 kHz and draws about 7 mA. The panel meter draws about 1 mA, and the LM35 less than 1 mA. I don't have any means for verifying the accuracy of the thermometer but it is adequate for my purposes.

Circuit diagram

Inside of case showing probe, inverter and panel meter

Reverse side of panel meter

Close-up of inverter and toroid inductor