High speed photography makes use of a very fast flash triggered at the right time to freeze motion, in this project I will design a simple circuit that will detect the vibration of a bullet hitting an object and trigger a camera flash in order to photograph the bullet in flight.
Step 1: Parts Needed
NPN 2N2222 transistor
1kΩ resistor 47kΩ resistor
Yongnuo YN560II camera flash (other flashes may require different resistor values)
Camera with manual controls (I use a Sony NEX 5n)
.22LR firearm, slingshot, bow & arrow or air rifle/cannon etc. (higher velocity rounds will give excessive blur)
Targets to shoot!
Step 2: Schematic
This simple circuit uses a piezo transducer to convert the vibrations from bullet impact into an electrical current, this current then closes a transistor completing the 4.12 volt circuit from the flash, thus firing it. This circuit was found to be too insensitive to detect the very first initial vibrations from bullet impact and thus the flash fired too slowly to capture the bullet in frame, however it did capture the object breaking apart from bullet impact. Next we will look at improving the sensitivity.
Step 3: Schematic 2
In order to add variable sensitivity to the circuit current was tapped from the flash itself, this was routed into the base of the transistor through a 47kΩ resistor and a 100kΩ potentiometer. Please note that the transistor was also flipped around so the collector and emitter wires have swapped places, this allowed the use of lower resistance values in the tap. With a small amount of current routed into the base of the transistor some current is now passing through the collector and emitter wires, but not quite enough for the flash to fire, any added current from vibration of the piezo transducer will now tip the transistor over the edge and allow enough current through for the flash to fire. This can be dialled right up to be sensitive to sound.
Step 4: Construction
No board was used in the construction, rather the wires were just soldered together. A socket was used to easily connect the flash through either its pc port or hot shoe adaptor.
To keep everything clean it was mounted in polycarbonate sheet, this isn't required but prevented damage to the circuit from flying debris and an easy cleanup.
Step 5: In Use and Pictures
The polycarbonate plate will now detect when the bullet hits any object placed on top. To use the camera was set to have a 10 second timer followed by a 2 second exposure, this 2 seconds provides the window if time to take the shot at the object, the sensor detects the vibration and fires the flash exposing that brief flash duration to the camera capturing a picture of the bullet impact.
Example 1: A drink can with a subsonic .22LR bullet just exiting.
Example 2: An apple after being hit with a high velocity .22LR
Example 3: The sensor was dialled up to be sound sensitive to capture the bullet just impacting a piece of chalk.