Intervalometer schematic

INGREDIENTS

 

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)

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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...

 

T1= 0.69(Ra+Rb)xC

 

While the Low time of the curve is given by...

 

T2= 0.69(RbxC)

 

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.

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Taken on May 29, 2010