user15249: I don't know how I can connect an MP503 sensor to my Arduino.
Please help.

http://eleparts.co.kr/data/design/product_file/SENSOR/gas/MP503%20(Ver1.4)%20-%20Manual.pdf

user10214: Ok, first of all I doubt you will be able to get a lot of measurements with that. Because... I know that kind of sensors and you will quickly get tired of it.
Anyway, you have to connect

pin 1 to +5V
pin 2 to ground
pin 3 to +5V
pin 4 to a resistor towards ground. Value has to be determined.

To chose the value, you have to try. Change resistor values until, in fresh air, the voltage you can sense across the resistor is approx 1V. I think it is arount 10k, but you'll have to try
Chosen the resistor, you have to simply attach the node between pin 4 and the resistor to an analog input and then read the voltage.
Now you have to calibrate the sensor. Put it in a fixed-concentration gas and measure the voltage: nCalib = analogRead(sensorPin);.
Then, when you need to measure the concentration, you just have to read the voltage again with nMeas = analogRead(sensorPin);. The factor Rs/Ro can be calculated with the formula
Rs_over_Ro = nCalib / (1024 - nCalib) * (1024 - nMeas) / nMeas;

Once you have the factor, look at the graph in figure 3 to get the concentration:

As I told you, I think you will quickly get tired of it, but this is how to use it.

15249: <p>I don't know how I can connect an MP503 sensor to my Arduino.</p>

<p>Please help.</p>

<p><a href="https://i.stack.imgur.com/LhNig.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LhNig.png" alt="enter image description here"></a>
<a href="http://eleparts.co.kr/data/design/product_file/SENSOR/gas/MP503%20(Ver1.4)%20-%20Manual.pdf" rel="nofollow noreferrer">http://eleparts.co.kr/data/design/product_file/SENSOR/gas/MP503%20(Ver1.4)%20-%20Manual.pdf</a></p>
 10214: <p>Ok, first of all I doubt you will be able to get a lot of measurements with that. Because... I know that kind of sensors and you will quickly get tired of it.</p>

<p>Anyway, you have to connect</p>

<ol>
<li>pin 1 to +5V</li>
<li>pin 2 to ground</li>
<li>pin 3 to +5V</li>
<li>pin 4 to a resistor towards ground. Value has to be determined.</li>
</ol>

<p>To chose the value, you have to try. Change resistor values until, in fresh air, the voltage you can sense across the resistor is approx 1V. I think it is arount 10k, but you'll have to try</p>

<p>Chosen the resistor, you have to simply attach the node between pin 4 and the resistor to an analog input and then read the voltage.</p>

<p>Now you have to calibrate the sensor. Put it in a fixed-concentration gas and measure the voltage: <code>nCalib = analogRead(sensorPin);</code>.</p>

<p>Then, when you need to measure the concentration, you just have to read the voltage again with <code>nMeas = analogRead(sensorPin);</code>. The factor Rs/Ro can be calculated with the formula</p>

<pre><code>Rs_over_Ro = nCalib / (1024 - nCalib) * (1024 - nMeas) / nMeas;
</code></pre>

<p>Once you have the factor, look at the graph in figure 3 to get the concentration:</p>

<p><a href="https://i.stack.imgur.com/ZQnDt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZQnDt.png" alt="figure 3: sensitivity for Mp503"></a></p>

<p>As I told you, I think you will quickly get tired of it, but this is how to use it.</p>
