1) 1 battery 9 V or 12 V
2) 1 IC NE555
3) 1 optoisolator IC 4N25 (or 4N35 or similar)
4) 2 LEDs (maybe 1 green and 1 red)
5) 1 ceramic/polyester capacitor 10 nF
6) 1 electrolytic capacitor 22 uF
7) 1 electrolytic capacitor 47 uF
8) 1 electrolytic capacitor 220 uF
9) 2 resistors 470 Ohm
10) 2 resistors 22 KOhm
11) 1 linear potentiometer 470 KOhm (or 1 Mohm for longer gaps but less precise for short gaps)
12) 1 SPST (Single Pole Single Throw) Switch
13) 1 SPDT (Single Pole Double Throw) Switch
14) 1 female mono connector as output (to be connected to exit of 4N25)
To make this circuit we need to follow the NE555 datasheet on Astable mode.
Without entering too much in details, I need to make a variable duty cycle of ON and OFF. I decided that my camera would trigger on the OFF time which I wanted to make it shortest as possible, so I looked for the right components to make a duty cycle very different from 50%. To make an example, a duty cycle close to 50% would make HIGH and LOW times of the pulses approximately equal... That way the camera trigger would stay longer on short circuit with the result of draining a lot more the batteries.
I followed the following formula to set the components values, where frequency (f) is...
f = 1.44 / [(Ra+2Rb) x C]
So, I will have a photo every Period (T) which is 1/f... T is also made by a high time (T1) and a low time (T2) T=T1+T2
The High time of the curve is given by...
While the Low time of the curve is given by...
In my case I decided to use
* Ra = 22 k in series with a potentiometer of 470 KOhm (even better a 1 MOhm would give longer timings, about the double, but less precise to find a specific second)
* Rb = 22 k (constant)
and 2 capacitors which I interchange with a second switch... I thought about this mod because I can handle the potentiometer more precisely with a x10 factor.
* C1 = 22 uF
* C2 = 220 uF
I decided to use 2 resistors of 22 K because they give me a good pulse when they start (about 1 second) and a clear bright led when camera is triggering. I also tried to put a 2 K for Rb but then the Red led just stays always ON and the Green blinks too fast I cannot even see it.
Please watch the simulation on next image to see how it works.