![]() This is where the chip is most efficient. If you look at the below diagram, the top of the curve is around the 6mA point. The CTR depends on the LED input current (I F) and the CTR decreases from a maximum point when the input current is both increased and decreased. According to the below diagram, taken from the data sheet, the Fairchild 4N25 is most efficient at about 6mA (top of the curve). Bear in mind this is a general guide only and you should check the data sheet for exact information. As a general guide, optocouplers are most efficient at about 10mA. ![]() The value of the CTR changes depending on the input current and the ambient temperature. The Fairchild 4N26 has a CTR of about 20%. Most optocouplers have a CTR of between 10% and 50%. It is similar to the DC current amplification ratio of a transistor and is expressed as a percentage. The CTR is the ratio of the phototransistor collector current compared to the LED emiiter forward current. The main things you need to know when choosing an optocoupler are: It will not work if you get them the wrong way-a-round. ![]() The shutter wire from the shutter release cable goes to pin 5 and ground from the shutter release goes to pin 4. You need to add a suitable resistor to the input, see below. Note that the +5 volts (VCC) from an Arduino digital pin goes to pin 1 and Arduino ground goes to pin 2. Pins 1 and 2 go to the Arduino, pins 5 and 4 go to the cable release. No current on pins 1 and 2 means current does not pass through pins 5 and 4. Basically, if you put a current through pins 1 and 2 and light the LED the photo detector transistor detects the light from the LED and allows a current to flow through pins 5 and 4. This means they can be used to allow one circuit to switch a separate circuit without having any electrical contact between the two. They work by using an LED emitter paired with a photo detector transistor. I already had a Fairchild 4N26 so this is the one I used. Due to the relatively low voltages there are many suitable optocouplers to pick from. My circuit is a 5V Arduino and a Canon 40D which has about 3.2V on the shutter release. There are many types of optocoupler and you chose one based on the requirements of your circuit. I chose to use an optocoupler, sometimes called an optoisolator. There are several ways you can connect an Arduino to a DSLR so that you can use the Arduino to activate the shutter.
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