Introduction: Measure the Speed of Nerf Darts

About: My name is Britt Michelsen. I am a chemical engineer from Germany especially interested in computational fluid dynamics. To balance all the theoretical work, I like to make stuff in my free time

In this instructable I am going to show you a cheap way on how to measure the speed of Nerf darts. Even though I own a chronometer using it annoys me most of the time. To get a good results and not a bunch of errors, you need the lighting to be perfect. Which quite often isn't the case. Hence I wanted to build a cheaper chronometer, that is independent from the light source and preferably integrated into a Nerf barrel.

Honestly I was cursing quite a few times during the construction of this barrel, since the space inside of it is kind of restricted. If you are looking for an easier method, just use a pvc tube and glue the components to the outside of it.
I ended up loving the barrel, since it makes sure that the Nerf darts fly straight through it ever time.

Step 1: Stuff You Need

  • Atmega328P
  • SAA 1064 7-segment driver
  • IC7805
  • 2 x NPN transistors (z.B. BC547)
  • 2 x IR LEDs (Osram LD 274-3)
  • 2 x photo transistors (SFH 3100 F)
  • 16 MHz quartz
  • 1 x 4.7 uF Capacitor
  • 1 x 1 uF Capacitor
  • 1 x 2.7 nF Capacitor
  • 2 x 22 pF Capacitors
  • 1 x 22 kOhm Resistor
  • 3 x 10 kOhm Resistors
  • 2 x 220 Ohm Resistors
  • 3 x 7-segment displays
  • wire
  • perfboard
  • Nerf barrel
  • modelling clay
  • drill
  • sanding paper
  • paint
  • JB weld

Step 2: Attaching the Leds

Start by marking the points, where the holes for the LEDs and the photo transistors are going to be. Try placing them as far apart as possible (to get a better result), but make sure that the Nerf can still be attached. It reaches quite far into the barrel.

Once you have marked the hole and are sure, that it won't interfere with the function of the barrel, drill the holes. You will have to drill two holes for the photo transistors (you will see what I mean, when you take a closer look at it).

Attach wires as shown in the second picture and use a third hand to hold the LEDs in place. It is important, that they don't reach into the barrel, else they will slow down or even stop the Nerf dart. Once you found the right position use JB Weld or an other type of strong two component glue, to hold them in place.

Step 3: Modifying the Barrel

The space in the barrel is very restricted. In order to hide the electronics, fill the four holes in the front and the ones underneath the rails, as shown in the first picture. I used modelling clay to do so.
Use sanding paper to even the surface. Carefully drill a hole for the segment display. I used diamond files to get them to fit perfectly.
Depending on the size of your battery compartment you might have to mill some of the plastic away, as shown in the third picture. Furthermore you will have to remove parts of the middle rail to fit the wires and drill a hole for the button (as shown in the sixth picture).

If you want you can paint the barrel, but that's up to you.

Step 4: Electronics

The dart speed is measured using two light barriers which are located at the front and the end of the barrel. The light barriers consist of a infrared LED (Osram LD 274-3) and a photo transistor (Q3 and Q4) (SFH 3100 F) each. The LEDs are irradiating the photo transistors across the barrel (see picture) and allow a current between the 5V VCC and ground respectively. This leads to a measurable signal in A1 and A0. The photo transistor also contain a daylight filter which keeps the ambient light from interfering with the measurements.

When a dart passes the light barrier blocking the IR-LED the current drops instantly to zero. At this time the micro controller starts counting the microseconds until the second LED is blocked. As we know the distance between the LEDs we can calculate the dart speed.
The value is send to a LED driver chip (SAA 1064) to display it on the seven segment display located on the side of the barrel.

I used an Arduino UNO to upload the following code to the Atmega:
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