At just 10mm x 10mm, this is quite possibly one of the smallest light sensitive switches you will ever find!
At last, a great replacement for LDR circuits.
Welcome to the future!!!
Step 1: You Will Need:
SMT 81k 0603 resistor
SMT 0603 100n Capacitor
SMT 3904 NPN Transistor (SC-70)
Step 2: Making the Circuit
I would add that I have been experienced in Electronics for 15 years, so this might not come easy to most hobbyists.
Most components are easy to solder. The phototransistor has 6 pads underneath, and only 2 are actually used in this circuit.
I have used 2 unused pads just to secure the transistor firmly to the circuit, so just etch two pads that are disconnected from the rest of the circuit. Follow the data sheet for the phototransistor if you are unsure.
Step 3: Power and Uses
Really this circuit demonstrates that in the dark, the circuit will light up the LED.
In theory you can use the circuit in parallel with another circuit. Just replace the LED with the positive and negative connections.
Experiment with power. It runs from 3v and the very maximum the phototransistor will handle is 6v.
Be careful when using USB power, add a resistor to safe guard from damaging components with too many milliamps. Check all maximum ratings from the datasheets of the components.