Phototransistor Carrier

The LK7290 is a Phototransistor; a transistor which is sensitive to light.  It is a little more complicated to use than the LDR because it is polarised (i.e. you need to connect it the right way around) and it does not conduct as much current as an LDR.  However, it generally works in a similar way to an LDR (its effective resistance does change to light) but it is rarely a drop-in replacement and the value of associated components (usually a resistor) will need to be adjusted and a 50K resistor (LK6231) may also be required.