BUILD CARD · EASY
Phototransistor LED Control
Recreate manual Project 43 with a phototransistor controlling a protected LED.
- Time
- 15 min
- Age
- 8+
- Pieces
- 6
- Board
- 10×7
- Build and check a 10×7 snap-grid circuit from a visual diagram.
- Incident light controls phototransistor current and the brightness of a protected LED.
02 / PARTS CHECK
Parts list
- 1×7-snap wireconnector
- 1×3-snap wireconnector
- 1×White LEDsemiconductor
- 1×100 Ω resistorresistor
- 1×Phototransistorsemiconductor
- 1×3V two-cell battery holderpower
03 / ASSEMBLY
How to build the circuit
- 1
Disconnect the battery and clear the base grid before placing any part.
- 2
Match the final diagram one grid position at a time, placing connecting wires and lower layers first.
- 3
Check the direction of every LED, diode, transistor, battery, and other terminal-sensitive part.
- 4
Press every shared snap firmly, check that no wire shorts the battery, then connect power last.
04 / FIELD NOTES
How it works
Light on the phototransistor increases its current.
The series resistor protects the LED.
Covering the sensor reduces or stops the LED current.
Explore more led circuit projects →
05 / TEST
Test and expected result
- What to do
- Aim a bright light at the phototransistor, then shade it.
- What to expect
- The LED responds to the light-controlled transistor current.
06 / SAFETY
Build smart
- Use only the 3V two-cell battery holder.
- Never connect the battery terminals directly together.
- Disconnect the battery before moving any component.
07 / TROUBLESHOOTING
If it does not work
- Press every overlapping snap down firmly.
- Check the battery direction and replace weak batteries.
- Compare every component position with the final diagram.
08 / FREQUENTLY ASKED QUESTIONS
Questions about this circuit
What is the main idea behind this circuit?
Incident light controls phototransistor current and the brightness of a protected LED.
What should I check first if it does not work?
Press every overlapping snap down firmly.
09 / RELATED BUILDS
Try a similar circuit
Continue with another build that shares the same components, ideas, or difficulty.
Blue LED Switch Light
Replace the color LED in manual Project 2 with a blue LED and keep the current-limiting resistor.
LED Polarity & Current Limiting
Build a protected LED loop and learn why both polarity and a series resistor matter.
LED Reverse-Polarity Test
Reverse the protected LED and observe that polarity controls whether it can light.