DSLR Hotshoe wiring to Arduino

Question

I want to wire the hotshoe of my dslr (Nikon D5100) to my Arduino board in order to react to the shutter release of the camera.

I measured a 5 to 8 volts signal between the trigger and the ground lugs when I push the shutter release. So my question is, if I wire the trigger lug to a digital input of the Arduino, where should I wire the ground lug to ? And subsequent question, do you think I should fear for my camera with the current/voltage involved?

I am fairly new to electronics, so I hope my question doesn't sound too stupid.

Answer 1

While the suggested optocoupler is certainly a safe thing to try you may find it doesn't work. Traditionally SLR hot shoes were a simple switch to fire the flash and while modern DSLR systems no longer use the high voltages that some older flashes presented to the camera many use a transistor to pull the line to ground.

Any voltage present on the pin may be via a high-value pull-up or possibly some very low current leakage and not capable of delivering the 5mA odd required to drive an optocoupler. That may explain your somewhat variable readings which I'd expect to be stable if the line was driven hard.

Here's an example of the circuit used within the Camera Axe camera / flash triggering system that may be indicative of what you'd find within the camera:

I didn't have a D5100 available for testing but measuring a family member's Nikon D7000 and my Canon 5D Mark III no voltage was present on the hot shoe. In both cases measuring the flash while detached from the camera but powered up gave a reading of around 5V so it appeared both systems used a pull-up within the flash. I'd suggest a circuit such as the following that I just tried with both:

^{simulate this circuit – Schematic created using CircuitLab}

R2 could be omitted if you use the internal pull-up on the input line while the diodes just give a little extra protection against ESD and over-voltage situations rather than relying solely on the clamps within the AVR. Here's an example of the measured signal for a 1/10 second exposure using that circuit:

Answer 2

A simple optocoupler would be the best bet, to protect both sides. The led side would be connected to the trigger and ground of the hot-shoe, while the other side is connected to the arduino.

The Resistor on the arduino side is a weak pullup to the arduino power supply. You could skip it if you enable the internal pullup instead, in which case you do not wire the vcc/resistor part.

In this case, when the hotshoe is triggered, the optocoupler enables it's transistor, and the arduino would register a change from logic High, to logic Low.

Answer 3

I've been using @PeterJ answer for a time-lapse camera controller, and it works perfectly. However, some applications (like mine), are battery-powered and require very low power on standby. If this is combined with a camera that is powered off during standby by a low-side switch, the circuit suggested by PeterJ leads to a current loss of about 0.1mA. This is because the camera works with 7.4 volts and the controller with 3.3V. And when the camera is powered off (by switching ground) the central contact of the hotshoe pin stays at 7.4V (at least that's the behaviour of an Olympus E-M5 camera). Leading to a current flowing through the diode clamp.

In this case, an alternative circuit would be something like this

It still protects from high voltages but stops any current flowing in such case. If anyone thinks this circuit has a weaknesses I'm not seeing compared to PeterJ suggestion, please comment.