BUILD CARD · MEDIUM
Adjustable Night-Light Threshold
Replace the fixed bias resistor with a variable resistor to choose how dark it must be before the LED turns on.
- Time
- 35 min
- Age
- 10+
- Pieces
- 13
- Board
- 10×7
- Build and check a 10×7 snap-grid circuit from a visual diagram.
- A variable resistor changes the voltage-divider threshold that switches the transistor on.
02 / PARTS CHECK
Parts list
- 2×8-snap wireconnector
- 4×2-snap wireconnector
- 1×6-snap wireconnector
- 1×500 kΩ variable resistorsensor
- 1×Photoresistorsensor
- 1×NPN transistorsemiconductor
- 1×White LEDsemiconductor
- 1×100 Ω resistorresistor
- 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
The variable resistor changes the upper half of the light-sensing divider.
Changing its setting changes the base voltage for the same amount of light.
This moves the approximate darkness threshold at which the transistor switches the LED on.
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05 / TEST
Test and expected result
- What to do
- Shade the photoresistor, then turn the variable resistor slowly through its range.
- What to expect
- The LED switching point moves, so you can choose how dark it must be before the LED turns on.
06 / SAFETY
Build smart
- Use only the 3V battery holder.
- Keep the 100 Ω LED resistor installed.
- Disconnect power before rearranging the sensor divider.
07 / TROUBLESHOOTING
If it does not work
- Start with the variable resistor near its middle setting.
- Move the control slowly while keeping the sensor under steady light.
- Recheck the transistor pins if the LED never changes.
08 / FREQUENTLY ASKED QUESTIONS
Questions about this circuit
What is the main idea behind this circuit?
A variable resistor changes the voltage-divider threshold that switches the transistor on.
What should I check first if it does not work?
Start with the variable resistor near its middle setting.
09 / RELATED BUILDS
Try a similar circuit
Continue with another build that shares the same components, ideas, or difficulty.
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