Three wire remote controller - wiring to two GPIO pins

Question

My remote camera trigger has three wires (red, yellow, white). The white is shared, and the trigger can complete two different circuits by shorting it with either the red or the yellow cable. One of the circuits will tell the camera to focus, the other to fire the shutter.

I wish to sense these short circuits on a breadboard, but do not know enough about electronics to understand how to separate the common white wire. I can conceptualise two separate switches, each "pressed" by the pairing I want to use:

But if I remove the two press-down switches here, and connect the common wire to the power end of the switches, it looks to me as though I would short both pairs of wires every time I try and short either of them.

Since the clever people at Sony knew how to separate the signals (before they removed remote trigger connections from their most recent bodies :-((), I am hoping someone here will be able to tell me what I need to add to my plan!

Answer 1

You are risking damage to an expensive camera. To do this successfully requires some understanding of

• the camera's control logic,
• the maximum voltage the logic inputs can tolerate,
• the logic switching voltage thresholds,
• whether the common wire is common ground (typically the battery negative) or positive supply (battery positive).

You also risk damage by connecting the Pi to the camera and sharing its supply / ground connection.

Your best bet is to isolate the Pi from the camera using miniature relays or opto isolators. The opto-isolators are smaller and silent.

Procedure:

• Get a multimeter and switch it to DC volts. Put the black (COM) lead on the white wire and using the red lead (V), measure the voltage on red and yellow. Hopefully it is a positive voltage - probably in the range of 3 to 5 V.
• Get a couple of opto-isolator chips. These are an LED and a photo-transistor in the one package. Turning on the LED will cause it to emit light through an internal transparent insulator (good for 4 kV!) at an internal transistor. This will turn the transistor on and allow current to flow from its collector to its emitter (the pin with the arrow).
• Wire up as shown below.

^{simulate this circuit – Schematic created using CircuitLab}

I've written more on opto-isolators here.

Answer 2

You can easily do it like this:

(Note, I'm assuming you intend to hook up the remote trigger to the Pi, not the camera. DO NOT wire the camera to the Pi in the way I've drawn!)

^{simulate this circuit – Schematic created using CircuitLab}

When the switches are open, the 10k resistor pulls the signals high (logic 1). When either switch closes, the corresponding signal is pulled to ground (logic 0). Ground is the common (white) wire. This is a very standard way of wiring buttons/switches.

The best part is, your Raspberry Pi even has those pull-up resistors built in. You can enable them when you configure your GPIO pins (refer to the documentation). (They aren't 10k - internal pull-ups have a much higher value - but it doesn't matter in this case.)

Note, Vcc here is your Raspberry Pi's I/O voltage, 3.3 V.