This is a homemade chronograph and this device is used to measure muzzle speed which basically
is the speed of bullets.
If you remember, a few weeks ago I’ve made a coil gun, which by the way was very powerful
and I recommend you to watch that video.
I’ll place a card somewhere here and the link in the video description.
Anyway, if I want to know the speed of that metal projectile, I need such project.
Basically, this device will measure the time between the input and output of the bullet
and will give us the speed in meters per seconds.
Is a very easy to make project, but I hope you will learn something new.
I will show you what we need to make it, the schematic, the code, how to assemble everything
and then we will test it out and see if it works.
So guys, let’s get started.
What’s up my friends, welcome back.
Here I have a laser module and these here are light sensors, so called LDRs or light
dependent resistors.
I mount this on my breadboard with a pulldown resistor in a voltage divider configuration.
As you can see on the oscilloscope, when the laser light touches the detector, it will
change its value and so the voltage output will also change.
So every time I cut the pass of the light the output changes so we can detect that voltage
change with an Arduino for example.
Instead of a voltage divider we can use these modules that already have a digital output,
high or low.
My first idea was like this.
We would have a pair of laser and LDR sensor at the input and another pair at the output.
When the bullet passes, it will cut the pass of the light so we can start a counter.
When the bullet reaches the output, we stop the counter and calculate the speed.
Sounds easy enough, right?
But the laser light is very narrow so the bullet might not pass exactly in front of
the light.
That’s why I wanted to use 2 pieces of mirror face to face.
The laser would start at the bottom corner and it would go back and forth creating some
sort of light wall.
In this way we make sure that the bullet will be detected on any point.
For that I’ve 3D printed this support that will carry the mirrors, the laser module and
the light detector on top.
But this method doesn’t work because after some tests, I realized that the laser power
was lost on each refraction and the angle was too big.
It looks like the mirror is absorbing the light too much.
So I had to try another option.
I’ve soldered a few LDRs on a small PCB in straight line, side by side.
I’ve connected them in series.
Then I connect them to the module before.
Have in mind that this has a small comparator and we can set the threshold using the potentiometer.
I glue this series of resistors on the 3D printed support I’ve made for this project.
I also glue the comparator module and add 3 wires to it for supply and digital output.
Then I take the laser module and I remove the focusing lens.
That will create a bigger line of light and like that I would be able to cover all the
LDRs.
I glue that laser in front of the LDRs on the other side of the 3D printed support.
As you can see, now I could clearly create a shadow over the light sensors.
So, turn on the laser and the LDR module.
Then rotate the potentiometer till you see the output LED will turn ON, that’s the
perfect point.
Now as you can see, I can detect any obstacle.
Even this screwdriver is detected, and this is thinner than the bullet I use with my coilgun,
so now we are good to go.
We have to make two parts like this one.
By the way, print the support with PLA material, 20% infill and 2 perimeters.
Also print the rest of the parts for the case.
Ok, now we have both the input and output and we place them together on another 3D printed
support, they should go something like this.
Inside this case we will have the electronics.
I want to make this project work based on batteries.
For that I will use a battery like this one together with a charging module with a USB
connector like this one as I always do.
We also need an Arduino, a buzzer and some sort of display.
I will use this kind of seven segment display for this since we only need to show numbers
for the speed.
Is very easy to use with a serial communication.
I would also add a push button in order to have some sort of control and that’s it.
That’s all we need.
This is the schematic for the project.
Just the sensors connected to the Arduino, the battery for supply and the display to
show the speed value.
I first solder the charger to the battery.
Then, between the battery and the circuit I add a switch so we could turn the circuit
on and off.
This switch will power the Arduino which is connected to the sensors, the buzzer and the
display.
Now all connections are made with thin wires so before we add this all inside the 3D printed
case, we should give it a test.
Download the code from below and upload it to the Arduino.
The display will show a 0 value.
But now I pass my hand between the sensors and we get speed value which by the way is
in meters per seconds.
The value will change each time we pass something between the sensors.
Also, it will stay like that till we press the push button in order to reset the speed
meter.
And that’s basically how it works.
If we take a look at the code we can see that we have the sensor pins defined on pin 8 and
9.
Using interruptions we detect when the first sensor is triggered and we save the time value.
Then we detect the second sensor and we equal the time to the difference between the actual
time and the previous time.
This will be in microseconds so we divide it by 1 million so we get seconds.
Then we divide the distance between the sensors by that time value.
By the way you should manually measure the distance between the sensors with a rouler.
Then get here in the code and change that value according to your distance.
We get the speed and we divide it in digits and print those to the seven segment display.
That’s it for the code.
Now I can close the case.
I glue the USB charger in front of this hole of the 3D printed case so we could recharge
the battery.
I also glue the battery and the Arduino inside of the case.
The display and the push button will go on this other 3D printed part.
Then, this part will get snaped in place over the case and that’s it.
The main switch can be placed on the side of the case.
I add the 3D printed leg on top and screw it in place.
At the same time I screw in place the two supports with the sensors.
Now we have a working bullet speed meter.
Make sure you adjust the sensors again using the potentiometers.
Let’s test it with my coil gun from the other project.
I power it on and charge the capacitors above 400V.
Make sure the meter is reset and I fire the bullet.
Amazing, I get a speed of 128 m per seconds.
I reset the meter and fire it again but I charge the capacitors a bit more.
This time I get 139m/s and that’s amazing, because have in mind the average speed of
firearms bullets can go between 120m/s for old muskets up to 1200m/s for now days rifles.
So the speed of my coil gun is far from those but even so, is quite dangerous and it could
harm you so be careful if you make that same project.
Let’s make another test.
So guys, this was a simple project.
There are multiple ways to make this.
You have mi circuit, the part list, the 3D printed files and my code below in the description
in case that you want to make the same project.
I hope that you have learned something new and if so, consider giving me a LIKE or comment below
Thanks again and see you later guys.