If your smartphone/tablet is not able to drive the opto-coupler based cable, the layout shown below is very likely a working alternative as a cable for use with DSLR Remote to remotely control your camera:
Mainly it consists of two transistors, on one side via resistors connected with opposing polarities to the tip and center ring of a 3.5 mm stereo headphone jack and on the other side to a cable matching the built-in remote control socket on your camera.
What you need:
- 2 transistors BC 547
- 4 resistors 1.5 kΩ
- 1 stereo audio jack, 3.5 mm
- 1 cable with jack matching the remote control socket on your camera (here: stereo audio jack, 2.5 mm)
- a piece of strip grid board
- duct tape or better heat-shrink tubing
- side cutter or equivalent
- soldering iron and solder tin
From the strip grid board cut a piece like shown in the picture. The conducting paths must be vertically oriented (see next picture). On the right hand side the principal arrangement of the two transistors and the four resistors is shown.
Start now with soldering the two transistors (note the orientation: left hand transistor with the straight side "down", right hand transistor with the straight side "up").
Bend one connecting wire of each resistor in a way, so that they can be placed with a one hole gap. First arrange the two "lower" resistors and solder them.
Do the same with the two "upper" resistor. In doing so, connect the two "upper" solder joints with an excess of solder tin as shown on the right hand side of the picture.
Solder the two remaining connectors of the jack to the two inner conducting paths. The assignment of the contacts and the camera-side terminals are shown in the picture.
To finish, solder the ground wire of the cable to the connected inner conducting paths and the signal lines to one outer conducting path each. If you are unsure about the assignment of the signal lines: there's a 50-50-chance for you to connect them in the correct orientation. So next thing to do is to check the proper function: start DSLR Remote and plug the 3.5 mm jack to the audio connector of your smartphone/tablet. Plug the 2.5 mm jack to the matching connector of your camera and switch on the camera. If not already done choose the remote type Cable in the settings of DSLR Remote and move to the remote frame. You should find two buttons here: S for Shutter and F for Focus. If you press the button F now your camera should focus. If it does you did connect the wires in the correct way. Now check the shutter function by pressing the button S to make shure this works as well.
If your camera takes a shot when F is pressed and just focus, respectivelly, if S is pressed you switched the wires und have to solder them the other way around.
As a last step, you should cover the whole with duct tape or heat-shrink tubing for insulation.
Ready to use!