At just 8mm, this is my smallest circuit I have created and 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:Bare PCB
SMT 81k 0402 resistor
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.
I would advise using a magnification aid, it may make this so much easier for you.
Start by etching a circuit that matches my photo. It's only one layer, and you can draw onto the copper surface before etching it out with the scalpel blade.
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.
Orientations of all devices are included with this photo.
Step 3: Power and usesReally 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 milliwatts. Check all maximum ratings from the datasheets of the components.