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Remote Sensor Interfacing with ADC/Microcontroller

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crankler

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I have temperature and light sensors about 3-6 meters from the ADC/microcontroller connected by long wires. The sensor produces analog output. LM35 temperature sensor has output of 10mV/degC. It has 3 pins: Vin, GND, Vout.

I'm quite worried about the quality and accuracy of the output signal of the sensor as it reaches the ADC/microcontroller.

  • Is it okay to have 3-6meter wire for the sensor's output, supply voltage and ground?
  • Should i add circuits or filter for output and voltage supply?
  • Would it be practical/better if i place a voltage regulator on the same pcb as with each of the remote sensors or is it okay to have a long wire from the voltage regulator to the remote sensors?

Thanks...
 

Since the distance is more and the signal level is too low,, there will be loss in the signal acccounting to cable or wireloss, noise etc. Need to add circuit for the same
both options are ok but better to keep seperate regulator and power supply for sensors. i think i regulator is enough for all the sensors and 9V / 500mA or 1A should do it.
 
Should i add circuits or filter for output and voltage supply?
Make provisions for filters is a good idea. Another thing you could do is to use shielded cables. Do you know the type environment your system will be working in (e.g. home, automotive, industrial)? Is your environment going to have a lot of EMI?
 

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Need to add circuit for the same
both options are ok but better to keep seperate regulator and power supply for sensors. i think i regulator is enough for all the sensors and 9V / 500mA or 1A should do it.

I'm planning to use 7805 (5v regulator) which will supply the sensors and the microcontroller. The regulator is in the main pcb away from the sensors. The sensors are also placed in different places...Or should i place a regulator for each sensor?

Make provisions for filters is a good idea. Another thing you could do is to use shielded cables. Do you know the type environment your system will be working in (e.g. home, automotive, industrial)? Is your environment going to have a lot of EMI

I'm going to install it in a room/office, probably in the ceiling. I don't think the environment is noisy. Fluorescent lamps might be a cause of noise.
 

I had thought about doing something similar to this, temperature data logger for each room in the house to control heat vents and hopefully save some money.
The analog approach is less expensive, but prone to environmental effects. If you stay with the LM35, I would recommend shielded cable and ferrite beads on the power and ground leads. This is a DC circuit and the signal won't change rapidly. If you think your getting any EMI on the signal line, you might try a very small bypass capacitor. You could also do 3 or 4 samples from the LM35 and average them:
sample1 + sample2 + sample3 ... / number_of_samples.
This would give you some level of accuracy even in an electrically noisy environment.

At $5.70USD a pop (digikey), the DS18S20 could be justified if you don't need many sensors. This is a pure digital sensor in a TO-92 case, and would be my preferred choice. You would only need one sample from the device, has wide temp range, and .5C accuracy in the intended range. The up side... you don't need to change your circuit layout, the TO-92 would drop right in and connect to the controller via one wire. They can also be daisy chained on the same wire. The down side is the code for your controller just got exponentially more complex as you would have to addr/read/write over one wire.

Check the datasheet at maxim-ic.com

--Ooz
 
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