I'm making the intervalometer here ATtiny85-Powered Programmable Intervalometer for Canon DSLR - Hackster.io, and had a couple questions. I already bought a cable for my camera and plan to make a simple circuit with buttons to make sure the pin out is the same. I'm pretty new to this, and know a lot more about photography than circuits, but wouldn't I want to keep the camera and Attiny grounds separate? I was also wondering if there were any other precautions I could take to protect the camera.
Please post the specifications for the interface of the Camera.
It should be pretty safe, the attiny85 is running at only around 3V.
But if you are really concerned, you could replace the npn transistor in that circuit with an opto-isolator. Then, the grounds would not need to be connected.
With an opto-isolator, you would need to reduce that 1M resistor down to 1K or less, for example 220R would be fine.
PS. The circuit is missing an important component without which the attiny might act bat-sh*t-crazy. It needs a 0.1uF ceramic cap between the Vcc and GND pins of the attiny chip.
Another more important component is a pull-up 4k7 ohm on PIN2 (PB3). Otherwise the Attiny85 doesn't run (sometimes).
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I'm honestly not sure where to get that. I just tried google and only found lists of compatible remote triggers. I have a Canon 5d Mark 3. It's just three pins like the one used int the guide, but the connector is different.
That's a good point. The attiny85 battery is lower voltage than the camera. I actually thought that the transistor was enough that I should isolate the grounds though, so good to know it's not. I have some optocouplers left over from another project, so I'll look around for those.
I don't have this on any of mine, what's it for?
I do have a pull up on pin 1 (PB5), which is the active low reset.
If I'm not using it for i/o of some type.
I'd have probably skipped the optics and driven a mosfet directly to switch it to ground..
I'm new to this and don't fully understand the difference between using a mosfet and a transistor. I've actually only used mosfets when putting things together myself simply because that's what I learned with.
As for the pins, I am with you on that. I've only ever pulled up the reset pin for stability. I couldn't find anything in a quick google search about it either.
Edit: Doing some more research on BJT vs MOSFET, now I know enough about them to know what I said earlier didn't make much sense. I'm thinking BJT may be the best option here due to the times I'd want to take many photos in quick succession. Now to do some research on optocouplers to see if they'd meet my needs.
A mosfet is switched by voltage not current... I drive a IRLZ44 directly off my 3.5V Pi and it directly dives a 24V solenoid. Output pin to Gate, drain to solenoid and source to ground... When the signal goes high the fet completes the ground to the solenoid.
What ever you understand is the best, since you have to fix it when it fails ... 
Good luck
I had, sometimes, some problems to run programs on chip Attiny85 PDIP.
Sometimes starts, sometimes not.
When I saw the schematic of Digispark Attiny85 board I didn't found resistance on PB5 (RST) but on PB3 (pin2, resistance R3). I tried to put 4k7 as pull-up and everything began to work perfectly.
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I don't think BJT is better for you.
You can use it as a simply transistor and you can switch on with TTL voltage.
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Since I'm doing photography where timing is important I was leaning towards BJT, but the timing on a Mosfet is still quick enough. Interesting about pin 2. Nothing I've made with an Attiny85 is meant to run for long times, so I may have just not noticed.
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