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This is intended as a very basic guide to optos - so all you pros out there can save the critiques for pages that deserve it ;-)

There are many different types of optos on the market, with specifications to suit all sorts of different needs. We are using Super-El Cheapo optos for our cameras because that's all we require. For this page I'll stick to the PS-2501 family of single, dual, and quad optos. The number of circuits in the chip is given in the part number by an '-n' at the end of the number - with 'n' being a number 1,2, or 4. For example: a quad opto would be a PS-2501-4.
Here are the PS-2501-1 and PS-2501-2 pinout diagrams:

Note: in the above image, the dual arrows between the input LED's and output transistors simply represents light.

There are many, many other optos that can be used in place of the 2501 for triggering a camera.
Check data sheets of the specific part for pinout and In/Out specs.

An opto - for trail-camera purposes - can be though of as a solid-state, light-activated relay. When the input LED (pins 1 & 2 of the PS-2501-1) is turned ON, it illuminates the output transistor (pins 3 & 4 of the PS-250101), which is light sensitive, so the output transistor turns ON. The output transistor takes the place of the mechanical switch found inside a relay. Some key pieces of information to remember are:

1) The opto's LED input (pins 1 & 2 of the 2501-1) is polarity sensitive.
The triangle symbol (Anode) of the LED must be connected to the most Positive side of the circuit.

2) The input LED MUST have a current limiting resistor!!!! The LED will burn out if a resistor is not used.
A good general-purpose value is 1K. Higher or lower values might be needed for your specific circuit.

3) The output (pins 3 & 4 of the 2501-1) is also polarity sensitive. The small arrow (emitter - pin 3 of the PS-2501-1) should typically be connected to the most negative side of the circuit to be triggered. For cameras, the "common" usually goes to the emitter (pin3 of the 2501-1), with "shutter" and "refresh" going to their respective collectors (pin4 of the 2501-1). Note that this is not true for all camera models!! - Test your cam.

4) For driving any substantial load, the output transistor should also have a current limiting resistor. Maximum transistor current is approx 50mA. For trail-camera use, a resistor is not required here.

You might want to test an opto just to make sure it's working right - or you might find yourself curious to see this mysterious bit of plastic actually DO something. Either way, here's a simple opto test circuit:

Testing a PS-2501-1. Check data above for the pin #'s on the PS-2501-2

Connect pins 2 & 3 to ground. Connect pin 4 to the Cathode (negative side) of a test LED. Tie the Anode (positive side of test LED) to a 1K resistor, and tie the other lead of that resistor to the positive supply.

Now connect a 1K resistor to pin 1 and leave the other end of the resistor unconnected. Using a piece of jumper wire, jump the unconnected end of the resistor to the positive supply. When jumpered, the TestLED should light up.

You can test the second opto of a PS-2501-2 in the same way. The two opto circuits (or 'channels') are completely isolated, and you can use either one individually or at the same time.