This is the component list. The Bluetooth part is optional if you want to see the read on your smartphone. Get all the parts from the links below, then mount the schematic and upload the code from the next chapter.

• Arduino NANO
• Boost converter
• SH1106 0.96" display
• IR switch
• HC05 Bluetooth module
• 3.7V Battery
• USB charger board
• 100R Resistor
• 4.7K Resistor

In this project we will use the basics of an IR senosr to measure the speed between the detectors. The idea is to dectect or not the infrared light. This IR sensor works exactly like a commune BJT transistor but instead of controlling the current from the collector to the emitter by applying a small current to the base we control the current applying infrared light. The base of this transistor is exposed to the light and when the infrared wave touches the base, the circuit is open and no current flow through the transistor. But when no light is exposed current could flow. So basically, this is a switch activated by light. So, the detection circuit is something like this below.

The output will be the emitter of the transistor. We add a pulldown resistor to the output of 4.7K ohms. In this way when the circuit is open the output will be ground and when the circuit is closed the output is a high value, in this case 5 volts because that’s the basic voltage of the Arduino. Actually, the output is a voltage divider between the resistance of the transistor and the pulldown. So, the pulldown should have a high value, in this case 4.7K ohms, so when the transistor is conducting, the voltage drop between the collector and emitter should be very small and all the voltage should drop on the pulldown resistor. Next, we should add a small resistor, around 100 ohms to the positive pin of the infrared LED in order to limit the current and not burn it. So just like that we've detected IR light. Now when the bullet passes in front of the light, the output of the IR switch changes and we can detect that and measure the time between the sensors with the Arduino and like that, the the bullet speed. 

Make connections as above. The IR LED and the IR detector (phototransistor) must eb placed face to face so the IR light could reach the phototransistor base. Then download the code from below and test it out. 

For the code you will need the u8g2lib library so download it and install it to Arduino IDE. Compile the code for Arduino NANO and upload it. Get the other files for the Android APK or other information from below and the 3D files from this Thingiverse link.