A Photoresistor 'Radar Gun'

This project was a very simple 'radar' gun' for use in an elementary classroom. The goal was to inspire students learning about forces and motion to be more interested in technology. To learn more about how this was used in the classroom, please visit the corresponding blog post at edjro.com

This instructable is intended to show how to build a similar 'radar gun'. In reality this is not a radar gun at all, but it can be used to accomplish the objective of providing students a simple tool to measure that a greater force causes greater motion. The basic use was to shine a light or laser on the photoresistor and measure the amount of time that light is interrupted when an object passes in between the light and photoresistor. Given the length of the object we can roughly determine the speed of the object. For example, an object that is 3 inches long and interrupts the light for 3 seconds is travelling 1 inch per second.

This not a complex project but beginners can build it! Elementary students grade 3 and above can enjoy a bit of tech mixed in with their science lessons!

Please note: The gelato container is entirely optional and I just put the breadboard, Arduino, and wiring inside there to make it easier to transport and use.

Step 1: Wiring the Photoresistor

The circuit required for the photoresistor 'radar gun' is very simple and is beginner level to wire up. This particular example requires only an Arduino Uno (or other control board), a photoresistor, a 10k Ohm resistor, a breadboard, an LED output display, and some jumper cables.

First we wire power and ground to the breadboard from the 5V and GND pins on the Arduino. Then we just need to put the resistor and photoresistor in the circuit as shown. I used jumpers so the photoresistor could stick out of the container. I didn't bother to solder it as this component was very simple and for individual use in an elementary classroom.

Next, we just need to wire in the display. I used a 7 segment display with 8 digits. I wired LOAD to digital pin #5, CLK to #6, and DIN to #7. I realize the diagram display is slightly different than the display I used but it should be simple to figure out how to wire pin #5 to the LOAD input on the display if you are using the same one I did. As you can see in one of the pictures, I went ahead and placed the entire circuit in a recycled gelato container so it would be easy and durable for transport. I drilled a small hole in the lid for the photoresistor and a larger hole in the bottom of the container for the display jumpers and the power cord. That is it for the wiring! The next step explains a bit about the code and links to our Gitlab page!

Step 2: The Code

The code is very simple for this project. Basically, we just need to keep a state of when the photoresistor has light shining on it and when it doesn't. When the light stops shining on the photoresistor we start counting and roughly determine how long the light was obstructed. Once the light comes back, we calculate the time it was obstructed and determine the approximate speed of the object. From there we display that speed (in our case it is inches per second).

Please note, I understand this is not precise. Most of this project is based on estimates and is really intended to show a very simple concept to third graders. The counting is not precise as it does not take into account the amount of time the code needs to run. Also, the object length is not exact. However, the main objective of giving students a quick and cheap way to measure greater forces and greater motion with some homemade tech was accomplished. The students absolutely loved seeing the wiring and Arduino and immediately asked more questions about the tech.

Please find the code on our Gitlab page!