I've always been fascinated by those fantastic images of drops of water splashing on a plate, balloons being pricked and playing cards being sliced by a speeding bullet. Being an avid amateur photographer, I always wondered how those images were taken..to click the camera exactly at the instant of impact is almost humanly impossible...thats right..I said almost impossible. Have a look at the image below..I took it without any fancy equipment. Just a plain old autofocus consumer camera and a hell of a lot of patience ( I know, thats the part that i hate too- it took half an hour of squatting in my front yard holding a girly umbrella in the rain) I have never been able to replicate that kind of success after that day. It just happened to be a one in a million shot, and I must say; it turned out pretty darn good :)
http://flickr.com/photos/shakirfm/2304071488/in/set-72157594336999688
But its not nice to leave photography to silly things such as luck..I wanted to take that kind of shot every single time i wanted to...the answer was of course a STROBE!!
The concept is extremely simple- keep the shutter of the camera open, have a sensor detect the moment of impact (or interest) which will trigger a flash. Do this in a dark room and you have an image of the event that occurred only at the instant the flash was fired. Simple as that!
I decided to build a simple 2 stage amplifier circuit which will trigger an external camera flash using an SCR (Silicon Controlled Rectifier)
Components:
R1=R5- 10k Ohm, R2=R6-1k Ohm,R3=R7-5k Ohm,R4=R8-100 Ohm
Q1,Q2= BC 547
M-Condenser MIC
C1=C2=C3=220 uF
ULN 2003 Darlington driver IC
4N 25 opto isolator IC
SCR -TYN 612M
PS: Fixing the Q point for these transistors for maximum amplification proved to be tricky business..The general rule of thumb I followed was to keep the base-emitter bias about 2.5 V ( which is determined by R1 and R3) and keep a fairly low resistance at the output side ( 1K Ohm seemed to work just fine) I would suggest playing around with the values on a bread board before soldering anything..
Since I don't have an oscilloscope ( :,( ) I used my analog multimeter to check the output of the circuit [blowing into the mic and watching the deflection and then adjusting the bias for max deflection ]
The sole purpose of this circuit is to detect a short sharp sound..no fidelity is required. Large coupling capacitors of around 220uF work just fine.
The ULN 2003 is a darlington driver IC.. It is very sensitive to even the teeniest input signal so it was an ideal thing to use here.
The opto isolator (4N 25) was thrown in to isolate the high voltage ( as much as a few hundred volts, depending on the flash being used) side of the flash hot shoe from the rest of the circuit. The opto is used to drive the gate of the SCR. A separate battery is used for this part of the circuit ( note the different ground symbols on either side of the opto isolator)
All external camera flashes have 2 contacts poking out of the body. One of them is +ve and the other -ve (connect the appropriate points to the SCR ) and the potential difference between these 2 points can be in excess of a few hundred volts....so unless you derive some sort of perverse pleasure from jolting yourself with high voltage DC, do not touch these points with your bare skin :)
Here are a few photographs of the setup
The tin can has a microphone taped under it. When tapped lightly, it triggers the flash
The white tube houses the MIC
The amplifier for the mic
A rubberband which was shot onto the platform frozen in time at the moment of impact
The reason I taped the mic onto a tin can like that was to do this:
Sadly, just not as striking as the one in a million shot I showed at the beginning. My consumer grade autofocus camera (olympus stylus u410- 4 Mpxl ) just does not seem to be able to focus in this kind of light... I think i had better luck with the balloons though.....
This one turned out the best in my opinion...
THings didn't always work the way they should have though :)
The balloon bumped into the mic instead of rupturing...
Too early....
Too late.
Hazy air pop
Before...
After.
One more hazy shot
I intend to connect a microcontroller between the flash and the microphone next.. Getting precise time delays is very straight forward using a microcontroller and it will also allow me to interface a keypad and display where I can specify a delay of my choice.
