The type of chronograph I’m talking about here isn’t the time keeper type. It is instead, the type that is usually used to measure the speed of bullets, bb’s, or anything that is shot.

The “Real” chronographs that are made by the companies run 100-200$, that is about 90-190$ too much that what I would want to put into a tool for testing my projects.

So whats the next best thing to buying one? building one of course!

Speed = distance divided by time. That means that if you can find the speed it takes for the projectile to move 1 foot, then you can measure speed!

Photo diodes and LED’s word pretty well. Let me explain.

Photo diodes will increase and decrease in resistance when exposed to different levels of light. When a projectile passes through the LED-Diode, the diode will not receive as much light as when it wasn’t there.

Now that that definition is done, how can one measure resistance and then turn it into a time function? Well, say you connect these photodiodes to a microphone jack, then have another set of diodes-LEDs spaced 6″ apart connected to the other side of the jack. The resistance will show up on as a spike in the frequency of the sound wave.

Now once this is built, software it needed. “Audacity” is a great audio program that I use to analyze the time the two spikes occur.

Once the time is determined, take the 6″ that the two diodes are spaced apart from each other and divide by the time in seconds and now you have FPS!

Here are some pictures.

In this picture, you can see the two circuits, one is for the lighting of the LED’s and another for the microphone jack.

^^Now that is beautiful.

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