Here is a simple Optoisolator Circuit (also known as an Optocoupler) that I use for my Nikon D750 camera, for an external remote Digital Shutter Trigger on my Camera’s Power Slider.
It is highly recommended that you isolate your Camera's Trigger Control Circuit from any external voltage, even an Arduino!!! The only way to efficiently and effectively do this is via an Optical Isolation Circuit!!!
R1 and R2 are Current Limiting Resistors in this circuit and are designed for the 4N25 Phototransistor Optocoupler, driven from a Digital Logic CMOS Buffer IC, powered by a 12v Power Supply (I use a 12v Batt. for remote shoots to power my camera slider and the logic that controls the slider).
According to the 4N25’s Data Sheet the ideal configuration is to have 1.18v, @ 6mA, across the LED inside the 4N25 Optosolator, thus with my 12v Vcc I am using a 1.6k Resistor that gets me very close to the calculated value for these optimum 4N25 values. I would also suggest you use the D1 and D2 diodes in this circuit to protect the two 4N25 in case you reverse the connections on pins 1 and 2, else you will burn up the 4N25.
If you use something like the Arduino with a different voltage applied to pin 1, you will need to calculate the value of R1 and R2 to achieve the same LED voltage drop and current. PS: It’s ok to use a resistor that would allow a current in the range of 6mA to 12mA, you get outside these numbers and the performance my very, especially something below 5mA.
Optoisolator are somewhat temperature sensitive, parameters for this Optosolator start to be effected when the ambient/case Temp gets outside the 20 to 25 degrees Celsius range. However, this particular circuit will be fairly stable over a larger range of temperature, refer to the 4N25’s Data Sheet for extreme temp environments.
The 4N26, 4N27, 4N28, 4N35, 4N36, and the 4N37 will also work in this circuit, and in general there are a number of other Optoisolators that would work.
Polarity is important in this circuit, if you reverse a connection or make a wrong connection, the circuit will not work… at best, at worse you will burn something up, possibly the camera’s Trigger circuitry!!!, so be vigilant.
Implement this circuit at your own risk. If you do not understand what is happening with this circuit you would be well served to seek proper assistance. The Phototransistor Optocoupler is one of the least understood discreet electronic devices in today’s world of electronics (IMO), and details about the “How to” are beyond the scope of this instructable.
Note: MY Auto focus setting in the D750 Menu "f4" Assign AE-L/AF-L button (see picture)
Recommend setting “AE-ON”, this will decouple the autofocus function from your shutter release button (half-press) and assign the auto focus function to the AE-L / AF-L button on the back of the camera. Once you do this, your camera will no longer auto-focus by half-pressing the shutter and will only focus when you depressing the rear AE-L / AF-L button. (pg 361-362 in the D750 User’s Manual).
The Nikon D750 and many (not all) of the Nikon Cameras require that both the "Focus Lead" and the "Trigger Lead" be connected to the "Common Lead" for the trigger to Fire. Also be aware that some of the other Camera Brands have a type of "Voltage Divider" that is used for their respective Camera Trigger to Fire.
Here is the actual Ckt that I use: some buffers to create a slight delay of the Trigger Signal. The 555 is triggered with a negative pulse (from other control logic) creates a 3/4 Sec Pulse that ensures that the Shutter will fire.
The Camera Cable is a Nikon MC-DC2 plug on one end (plugs into the Camera) and a 2.5mm Plug on the other end (connects to the output from the Optocouplers).