An LM35 simulator: a temperature sensor on an Arduino
Mokxi is a free online LM35 temperature sensor simulator that runs in your browser, with nothing to install and no account needed. An LM35 on A0 of an Arduino Uno, read with analogRead five times a second. Drag the temperature and the serial monitor prints the raw count and the millivolts, so you can check the math in your sketch before the sensor is on a breadboard.
- 269parts on the bench
- 32boards running now
- 1.00xreal time, on every board
What you can do here
In the circuit above, or in the editor it opens.
- Set the temperature anywhere from -55 to 150 °C
- Read the raw analogRead count and the millivolts on the serial monitor
- Turn millivolts into degrees: divide by ten
- See why one count is about half a degree on a 5 V Uno
- See what a single supply does below 0 °C
- Add an LCD or a fan and build a thermostat
What the running circuit shows
10 mV a degree, with no offset.
An LM35’s middle leg puts out 10 mV for every degree Celsius, starting from 0 V at 0 °C. At the 22.5 °C it starts on, that is 225 mV, and the Uno reads a count of 46, which the sketch prints as 224 mV. Drag it to 25 °C and the line becomes a count of 51 and 249 mV; at 37 °C, 76 and 371 mV; at 100 °C, 205 and 1001 mV. Divide the millivolts by ten and you have the temperature.
Those readings sit a little under the truth, and the reason is the useful part. analogRead splits 0 to 5 V into 1024 steps, so one count is about 4.9 mV, nearly half a degree. 225 mV falls between counts 46 and 47 and reads as 46, and no reading can be finer than that half degree. A sketch that prints 22.46 °C is printing digits the hardware never measured.
Drag the slider below zero and the reading stops at a count of 0. With its ground leg on ground and one supply, an LM35 cannot go below 0 V, so it reads 0 °C for anything colder. The datasheet’s way to read below zero needs a resistor to a negative supply.
How the sensor is modeled
The datasheet’s straight line, and what it leaves out.
The model is the datasheet’s transfer function: 10 mV a degree from 0 V, across the rated -55 to 150 °C, on a supply of 4 V or more; below 4 V it does nothing at all. That is the LM35’s main difference from the TMP36 beside it in the parts bin, which reads 500 mV at 0 °C so that it can go below zero on one supply. Use one part’s formula on the other and the reading is 50 degrees out.
What it leaves out: the sensor’s own error, about half a degree at room temperature; self-heating; the time it takes to follow a change; and noise. The model has none, so the count holds still. On a real bench a reading that jumps by a count or two usually comes from a long wire to the sensor or a motor on the same supply, and averaging a few readings settles it.
Questions
Is it free?
Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.
How do I convert LM35 output to Celsius on an Arduino?
On a 5 V Uno, multiply the count by 500 and divide by 1023: a count of 51 is about 24.9 °C. That is the count turned into millivolts, then divided by the LM35’s 10 mV a degree.
What is the difference between an LM35 and a TMP36?
Both give 10 mV a degree. The LM35 reads 0 V at 0 °C and needs 4 V or more; the TMP36 reads 500 mV at 0 °C, so it can measure below zero on one supply, and it runs from 2.7 V. Use the formula for the part you have.
Can an LM35 read negative temperatures?
Not wired the usual way, with its ground leg on ground: it stops at 0 V, and here the count stops at 0. The datasheet’s full-range circuit adds a resistor from the output to a negative supply.
More simulators
Each one opens on a circuit that is already running.
Take a temperature
Open the circuit, drag the temperature and read the count. Nothing to install, and no account needed.