Recently I've made a very simple remote trigger for Xiaomi Yi camera on my drone so I can take pictures using my RC transmitter.
I have Pixhack (a Pixhawk clone) flight controller which has not only servo outputs, but also "relay pins" which basically output 3.3v. Below is short version of what I've done. I decided to write this post after I already built the thing, so don't have nice step-by-step pictures.
* Xiaomi Yi Bluetooth shutter
* CPC1016N opto relay http://lib.chipdip.ru/318/DOC000318377.pdf . Maybe you can use something else, but I just picked the one which was in closest DIY shop.
* 100 Ohm resistor. The idea here is that you have 3.3v input, 1.2v loss on opto relay LED, but you want current above opto trigger and below its max current. So (3.3v-1.2v)/100Ohm = 20mA looks ok.
* two-wire servo cable (connect top and bottom pins)
* two small wires (~1-2 cm)
* pair bluetooth shutter with camera
* remove protected rubber case
* remove rubber push button, this wil open a small membrane-like trigger below it
* remove membrane from the trigger, this will open two contacts and the camera will trigger if you short-circuit them
* solder two small wires to those contacts
* solder opto relay LOAD pins to it (polarity doesn't matter)
* solder resistor to one of opto lelay INPUT pins
* solder servo cable wires to resitor and to the other opto relay pin
* connect to pixhawk relay pin, set RC channel action to "Relay On/Off"
* verify that it works, if not, you might need to reverse servo cable wires
* fill in Bluettoth shutter part where you did solreding with epoxy glue and sink wires, resistor and opto relay there
An alternative setup in Mission Planner is to assign camera shutter to that relay and use "Camera trigger" action instead of "Relay On/Off".
I've attached schematics and how the end result looks like.