Summary of Simple LM335 Thermometer using microcontroller
### Summary This article details a simple LM335 thermometer circuit based on a National Semiconductor application note. The project uses the LM335 sensor, which outputs 10 millivolts per degree Celsius. By establishing a zero reference voltage of 2.832 volts and measuring the difference against a high-impedance digital voltmeter connected to specific pins, the device displays temperature directly in degrees Celsius. The text also explains the relationship between Celsius and Kelvin scales regarding the sensor's output factor.
Parts used in the LM335 Thermometer:
- LM335 Temperature Sensor
- Reference Circuit for Zero Voltage
- High Impedance Digital Voltmeter (DVM)
- Output Connector with +5V Input
Not too many parts. When a voltmeter is connected across the outside terminals of the output connector, its display reads out in degrees C.
I’ve been fascinated by the LM335 for some time -maybe my obsession with stability finally gave way to my fascination with a circuit with intentional thermal drift. I finally bought some the other day, and to check one out, I hooked it up as a thermometer, based the National Semiconductor application note.
The +5V on the “input” connector is as a convenience in case I want to power the circuit from +5V someday, or use the +5V as an output to drive a meter or microcontroller.
The circuit consists of two parts: The LM335 and its adjustment. The output of the LM335 is 10 millivolts per degree C, with 25 degrees C corresponding to 2.982 VDC. A reference circuit provides a zero reference voltage. It is adjusted to (2.982 volts – (25 degrees x 10 millivolts/degree) = 2.832 volts. To read the temperature of the LM335 directly in degrees C, connect the + lead from a high impedance DVM to the output pin and the – end of the DVM to the 2.732 volt pin.
The factor of 10 millivolts per degree C is equivalent to 10 millivolts per degree K, since a change of one degree C is equal to a change of one degree K. The difference in the two scales is only their offsets. The melting point of water ice is 0 degrees C and 273.15 degrees K. The boiling point of water is 100 degress C and 373.15 degrees K.
For more detail: Simple LM335 Thermometer using microcontroller
- How does the LM335 sensor output relate to temperature?
The output is 10 millivolts per degree C, which is equivalent to 10 millivolts per degree K. - What voltage corresponds to 25 degrees C?
25 degrees C corresponds to 2.982 VDC. - How is the zero reference voltage calculated?
It is adjusted to 2.832 volts by subtracting 25 degrees times 10 millivolts from 2.982 volts. - Where should the positive lead of the DVM be connected?
The positive lead connects to the output pin. - Where should the negative lead of the DVM be connected?
The negative end connects to the 2.732 volt pin. - What is the purpose of the +5V input connector?
It serves as a convenience to power the circuit or drive a meter or microcontroller. - What is the melting point of water ice in this context?
The melting point is 0 degrees C and 273.15 degrees K. - Does a change of one degree C equal a change in another scale?
Yes, a change of one degree C is equal to a change of one degree K.
