As the door chime comprises a simple monopole whip antenna it easily gets de-tuned when the ground plane gets changed. Therefore, and also to be assured to protect the camera's remote input, I decided to use an opto-coupler which has a very high electrical isolation and a very low parasitic capacitance . A small relay could potentially be used, but as the remote is operated on 3 volts, the selection of small relays for that voltage is somewhat limited. Also, the LED drive of the remote interfaces directly to the LED of the opto coupler without the need of any external components.

It seems like the EOS1DmkII has a short-circuit current of the remote input of some 1mA or so. The LED drive is about 1.5mA (1.5V and 1kohms), and I therefore selected the widely available CNY17-3F, which has a current transfer ratio of at least 100%, i.e. no additional amplification necessary. It is also very inexpensive and can be bought here for less than SEK 8 (US$ 1) in single-piece quantities.