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Emergency!

dirtdiver

Hello everyone, I'v been given a school project:

  

       calculate the gravity of earth using two sensors to estimate the time needed for a free falling object to go from sensor1 to sensor 2.

Now, I'm gonna use 2 photoresistors and 2 lasers ,can anyone suggest how to measure that time?

Can that value be displayed on a pin when I debbug it?

 

I need to make that project fast so any advice is appreciated!

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  • Photoresistors have relatively slow response and lasers are very much overkill. An LED or flashlight along with a photodiode or perhaps a CdS cell (if it is fast enough) will be fine.

     

    The Arduino has a function that will give you the current time in miliseconds or microseconds. Look at: http://www.arduino.cc/en/Reference/HomePage

     

    pseudo code:

    1. Wait for sensor 1 to detect falling object.

    2. record time t1=millis(); Or you can use micros().

    3. Wait for sensor 2 to detect falling object.

    4. record time t2=millis();

     

    5. dt=t2-t1.

     

    You can't display anything on a pin; you can control the signals going to a pin. Just use the Serial port to send the dt value back to the console.

     

    6. Serial.println(dt).

     

    This will give you the average velocity between the two sensors. You'll need to know how far the object has fallen before it gets to sensor #1. In addition, the farther you make the separation between sensor #1 and #2, the better your accuracy will be.

     

    A better experiment is to simply build a pendulum and measure it's period with a single sensor. The experiment will be smaller and probably give you a more accurate estimate of g.

     

    Not sure how this is an emergency, by the way.

     

    -Scott

Reply

  • Photoresistors have relatively slow response and lasers are very much overkill. An LED or flashlight along with a photodiode or perhaps a CdS cell (if it is fast enough) will be fine.

     

    The Arduino has a function that will give you the current time in miliseconds or microseconds. Look at: http://www.arduino.cc/en/Reference/HomePage

     

    pseudo code:

    1. Wait for sensor 1 to detect falling object.

    2. record time t1=millis(); Or you can use micros().

    3. Wait for sensor 2 to detect falling object.

    4. record time t2=millis();

     

    5. dt=t2-t1.

     

    You can't display anything on a pin; you can control the signals going to a pin. Just use the Serial port to send the dt value back to the console.

     

    6. Serial.println(dt).

     

    This will give you the average velocity between the two sensors. You'll need to know how far the object has fallen before it gets to sensor #1. In addition, the farther you make the separation between sensor #1 and #2, the better your accuracy will be.

     

    A better experiment is to simply build a pendulum and measure it's period with a single sensor. The experiment will be smaller and probably give you a more accurate estimate of g.

     

    Not sure how this is an emergency, by the way.

     

    -Scott

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