Switching a 5V power relay with optocoupler
I have looked at the schematics of a few 5V power relays and noticed different designs. Some use just a transistor to draw the higher current to switch the coil and some use optocoupler (surge protection) with a phototransistor.
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  Why do I need an extra transistor to switch the relay?



Some optocouplers cannot provide with the current requried to operate the relay. An additional (power) transistor is needed to control the relay.  
Some cheap optocouplers have bad (current ransfer ratio) CTR or a too wide spread in CTR. Using the optocoupler to control the next (power) transistor solves this.