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Topic: Arduino-LM35 issues (Read 94 times) previous topic - next topic


Hi, I am currently making an incubator, using an Orpat fan heater.
I have an Arduino Uno, connected to an LM35. the readings are displayed on a 16x2 LCD.
I have no problems upto now.
The problem starts when i connect a 5v 20mA relay to the circuit.

I have connected a transistor SL100 through a 1k resistor to the arduino. The emmiter is grounded, while the collecter is connected to the 5V supply through a diode.

Across the diode is connected the relay.

I have put the relay's Normally closed and common across the live wire of the power lines of the heater.

Everytime i switch on the power to the heater, the LCD begins showing garbled digits....while my LM35 fluctuates within a 10-15 degree range.

Is this a programming problem or a physical one ?.
If its a physical problem, what changes should i make to handle this ?


Do you have capacitors across the power supply?
I use UnoJoy and Hoodloader for HID. No Leonardo!
Do not PM me for help.Only links to topics.



Is this a programming problem or a physical one ?.
Yes : physical one.
Welcome to the real world. :smiley-mr-green:
Are you perfect? Like all humans, including me, I think not.
For components it is the same.
Copper cables do not have zero resistance and the Ohms law applies.

Remember LM35 give only 10 mV per degree.

If its a physical problem, what changes should i make to handle this ?
The important cable is GND cable.
The current drawn by the LM35 is extremely low so it does not intervene.
But if you use the GND cable to control a relay, current will be much higher and because of the Ohms law there will be a voltage across the GND cable..
Consequently Arduino show  more degrees.

You should not use GND
cable that connects  LM35 GND to Arduino board GND  for another component.

With only 10mV per degree it is prohibited. I hate to prohibit, but here  it is mandatory.

Another possible problem:
Like any CMOS IC  LM35 does not support capacitive cables. Long cable are capacitive.
If it  is the case a serial resistor must be added to the output of LM35 series to protect it from  cable capacity.

See the datasheet, preferably datasheet from National Semiconductor.
Datasheet recommends  2 kohms, I obtained an optimum with 4.7 kohms.

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