High Speed Photography

High speed photography can be accomplished by making the room dark, taking a long exposure with a normal camera, then triggering a flash right when the action to be photographed happens. Because the room is dark no light will be recorded unless the flash is on. The length of the flash determines how fast an object can be captured without motion blur.

To experiment I measured the time a flash lasts using a photocell and oscilloscope. The internal flash on my Nikon D70 could go as short as about 200uS while the flash from a disposable camera lasted about 1mS. I also took pictures of a spinning fan blade. The longer exposure length of the external flash is visible in the motion blur of the fan.

Different photos of a fan

While the internal flash would be capable of taking “faster photos” it would require the camera to be triggered very precisely to capture the exact moment. I read that the IR remote for the D70 can suffer from several milliseconds of lag, which would make it very hard to capture an event. Additionally the flash is directly above the lens and can not be moved, which might not be ideal. For these reasons I decided to go with an external flash based off a disposable camera.

Flash circuit as it came from the camera

Inside the disposable camera (CVS branded in my case) is a circuit to boost the 1.5V from the AA battery to about 300v and a trigger mechanism to provide excitation voltage to the flash tube. There is a switch on the camera to enable/disable the flash, which I shorted out because I always want to enable the flash. The trigger is two thin pieces of copper placed directly over the lens. When the shutter is opened it bumps into the pieces of copper and makes them touch, this triggers the flash.

A flash tube is triggered by a high enough voltage source applied across it to ionize the gas inside it (usually Xenon). This ionized gas is conductive which allows the energy stored in the capacitor (which is also relatively high voltage at around 300v) to be dumped through the flash tube. This rush of energy knocks electrons out of their orbital states, which results in a burst of energy (light).

Flash board modified to be triggered

From inspecting the PCB traces I think when the trigger switch is shorted the inrush of current from the 1.5v supply is fed through a small transformer. This boosts the voltage (only on the initial change form 0v to 1.5v) up to some high voltage. This high voltage is applied to the flash tube which causes the arc over so the energy stored in the main capacitor can be discharged. I don’t measure this trigger voltage so I attached a relay (rated at 250V AC) to the trigger. This would enable me to trigger the flash completely isolated from the high voltage. I also installed a 12k resistor across the terminals of the flash capacitor through a relay. When power was not applied to the relay the resistor would discharge the capacitor in 30s and when power was applied the resistor would be disconnected from the circuit. I also removed the AA battery and connected wires to provide the 1.5V from a separate supply.

Box to house flash

I built the system into a box and using some of the circuitry from my Foot Keyed CW Transmitter created a force based trigger. When enough force was applied to the force sensitive resistor its resistance would change enough so the voltage across it would go above the threshold on an opamp comparator set by a potentiometer. This was then run through a transistor to supply enough current to drive the relay. Dropping or breaking something on the resistor pad provided enough force to trigger the flash.

Breaking an egg with a knife

Breaking an egg with a spoon