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The Photoresistor 'Radar Gun'

This post goes along with the previous post Learning About Forces and Motion with Ramps. Rosanna and I thought it would be neat to integrate technology into the lesson about motion just to get the kids thinking about it. I came up with the idea to create a sort of 'radar gun' that the students could use to measure the approximate speed of their tennis balls as they rolled down the ramp. The idea is simple...the steeper the ramp, the faster the ball will roll due to the force of gravity (this is 3rd grade but of course the lesson could be more complex for higher grade levels).

So what is the photoresistor 'radar gun'? is not really a radar gun at all but it is very simple and very cheap (photoresistors are quite cheap) and could work with a bare minimum amount of coding. For starters, a photoresistor is a resistor that will have a variable resistance based on how much light is shining on it. More light = less resistance. For an quick introduction to photoresistors and how they work, Wikipedia is a pretty good source. Typically, photoresistors are used in light sensitive circuits such as street lights that automatically turn on when it gets dark enough. For our case we are going to use it a little differently. Essentially, we are going to shine a light (led flashlight or laser) onto our photoresistor across the ramp. Then, when the tennis ball rolls past it will obstruct the light, causing the photoresistor have a much higher resistance. We can measure the amount of time the photoresistor has that higher resistance and based on that, estimate how fast the ball was travelling (we know the size of the tennis ball).

I used an Arduino Uno 2 (more about Arduino later but basically it is a simple computer with inputs and outputs you can control) and a 7 segment LED display (both shown in the picture) to construct the radar gun. I used a breadboard to connect the photoresistor, Uno and display together and encased the whole thing in a recycled gelato container (I love using home recycling for simple saves a lot of money and usually works pretty well). I drilled small holes in both ends of the gelato container. On one end, the photoresistor itself is just outside the container and on the other end all the wires (5 small jumpers for the LED display and one power cord). This housing made it easier for Rosanna to transport the ‘radar gun’ to and from school and also protected the electrical connections(which I didn’t bother to solder this time).

Using the photoresistor electronics inside the gelato container, it was easy for Rosanna to go around her classroom and hold the gelato container on one side of the students track, while another student held a flashlight trained on the photoresistor. As the ball rolled past, the students could see the speed displayed on the LED display (in inches per second). Although this technique is not an incredibly precise measurement of speed, it is close enough for the students to measure the change in motion.

It was amazing to see how the students reacted to this very simple addition of technology and electronics to an otherwise familiar experiment. It only took me a couple of hours to pull everything together for the ‘radar gun’ but the students were very interested in it. Even though it was not the main lesson for the day, the students were intrigued and asked questions about what it is, how it works, and how hard it was to build. Adding that element of intrigue and curiosity to science was our goal exactly and we will continue to work to inspire students to pursue STEM interests.

Check out the detailed instructions on Instructables for how to build the photoresistor ‘radar gun’ and let us know if you built it for your classroom! If you have problems getting it to work, please contact us for help! Depending on your environment you may need some small adjustments to the code for ambient lighting conditions. However, it should always work if the lights are off in your classroom when you perform the measurements.

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