Step 1: The Theory
A large capacitor, or several would be used to store the charge.
A relay used as a switch.
Timing circuit(could be based on the main capacitor voltage)
Step 2: Stuff to Get
Secondly, a big capacitor is needed. I am going to be using a 40V 10mF capacitor. The voltage of you capacitor will need to be big enough to power all your LEDs in series, as you cant use them in parallel, without some sort of current limiting circuit on each LED.
Finally you need a timing/discharge circuit. I thought of using a relay with a resistor, but this system would be much to unpredictable. So I recommend a 555 as the timer, because it is very easy to vary the delay.
Image is missing the timing/discharge components.
Step 3: The First Thought
Here is my first attempt at designing a timing delay circuit.
It has several flaws.
-LEDs would be on when charging
-unpredictable time delay due to varying coil current.
-relay would be partialy closed for a while which could cause a spark
-relay is mechanical so could malfunction.
Step 4: A Better Way
In the "Power Control Delay Circuits" section I found what I was looking for.
A fairly simple circuit. Variable time delay can be achieved by using a pot instead of R1.
If you're feeling fancy you could replace it with a bunch or resistors and a rotary switch.
The bellow diagram is missing the reset pin connection. It should be connected to the supply voltage.
I found that the diode can be missed out. I used a smd tantalum 10uf capacitor cause I had a few laying around. The potentiometer is 1Mohm.
Step 5: Look Ma, No Relay
Some of us don't like to have mechanical components in our designs because there are reasons they have become obsolete.
Bellow is a schematic which would replace the relay with a FET.
I don't have any P-channel FETs so I made a circuit to drive an N-channel FET the same way a P-channel would behave.
The resistor values have to be adjusted for your charged voltage, but I am planing on using about 16V(safe limit for the 555)
Also you don't want too little resistance or the cap will drain faster, and you don't want too much because that will cause the FET to be in the semiconducting region. I recommend something in the range of 300-1000 for R2 and 2-10k for R1.
In the pictures you see the fet connected to the timer. Notice that the fet doesn't need a heat sink, as it will not be constantly conducting. The wires coming off to the right are the supply for the timer and the black one on the left is the source pin of the fet, this will be connected to the negative terminal on the capacitor.
Step 6: Enclosure
This is a bad solution and doesn't work very well/hard to implement. If you have any ideas do comment.
A water bottle.
Basically I am using a water bottle inside a water bottle. The cavity in between the two would be filled with oil or other lubricant.
Alternatively a several large ball bearings can be used to hold up the inner shell.
As you see in the picture, the inner shell is partially filled with "something heavy". This could be the capacitor and control circuit set in resin or hot glue. This bit makes sure that the led aren't facing down.
Ideally I would like to have a clear plastic ball inside another clear plastic ball. But then charging gets complicated, and getting a ball inside a ball means cutting them up.
Step 7: Mounting the LEDs.
I had some thin rod left over from a uni project which I decided to use to hold the LEDs in place. I just bent the rod around the heat sinks that the LEDs are mounted too. They rattle a bit but are not gonna just drop out.
Basically I put the rod through the dips in the corners of the heat sinks. This also helps spread the heat around, so I recommend solder up the LEDs before putting them together.
Step 8: The Finished Product
The red and black wires are for charging the capacitor.
I will be using 4x2 lion cells i pulled from a laptop battery.
That makes about 16.8V at full charge.
That's about as high as I am prepared to go with my 555.
This circuit would work with anything down to 10V but would give less of an effect.
You could charge the cap with 3x6V Lead Acid batteries in series. this would give about 18V so I think a diode in series would bring the voltage down to a safe level.
Step 9: Other Thoughts
I think that charging the capacitor with a constant current charger.
Maybe use more capacitance for a brighter flash.