light dependent 5V switch
You can make it a bit more sensitive by increasing the 10K resistor, but there is not enough current gain in one BJT to make it work in very dim light. 

If you replace the BC547 with an n-channel MOSFET you can increase the 10K to 100K or 1M and make it very sensitive. 

Alternately, connect a second BC547 to the first in a Darlington configuration (and increase the 10K).