AutoGlow (Automatic Light When DIM)

A Perf Board circuit which uses a photoresistor and potentiometer to detect light levels. When it's dark, the transistor is activated and turns on the LED. The potentiometer adjusts the light sensitivity. Powered by a 9V battery, it shows how light input controls electrical output.

Perf Board - Used to house the components

LDR - The LED automatically turn on at night because my circuit uses an LDR to detect when it gets dark

Resistor (40KΩ, 147KΩ) - Two resistors are used in my circuits and protects my components by limiting the flow of the current

LED - The source of the light

Potentiometer (100K) - adjusts resistance by turning a knob or sliding it, which changes the current or voltage in a circuit

NPN 2N3904 - An NPN transistor is used to control or amplify current, acting as a switch or amplifier in electronic circuits.

The breadboard was used to quickly assemble and test the light-sensitive transistor circuit without any soldering. It allowed easy adjustments, such as changing resistor values, repositioning the LDR, or rewiring the transistor, to observe how the LED responses under different light conditions. The breadboard is ideal here because the circuit was firstly in the experimental phase, where I observed voltage changes and LED behaviour using the multimeter. If something didn't work, components were easily removed or swapped.Power off & gather parts

STEP-BY-STEP INSTRUCTIONS

1. Place the transistor
2. Insert the NPN, so its three legs sit in separate rows. Confirm the output (collector, base, emitter) for the specific part (as shown in diagram)
3. Wire the LED load (low-side switch)
4. From +5 V to LED resistor to LED anode to LED cathode to the transistor’s collector.
5. Connect the transistor emitter to GND.
6. Build the light sensor divider
7. For “LED turns on in the dark”: put the fixed resistor from +5 V to the divider node, and the LDR from the node to GND.
8. (If you want “on in bright light,” swap their positions but in my specific scenario this is fine.)
9. Feed the transistor base
10. Run a base resistor (= 1 kΩ resistor) from the divider node to the transistor base.

First power-on check with the multimeter (non-permanent)

Power the circuit. Measure the base voltage while changing light levels on the LDR.

Target: base ≳0.6-0.7 V should switch the NPN on so the LED lights.

Test the light response

Shade the LDR (hand over it) and shine a torch to see the LED behaviour change. Watch the base voltage rise/fall accordingly.

Lastly, I used a perf board, it involved soldering each component and connection, making the layout more permanent. This was useful after the circuit was fully tested and finalized on the breadboard.

STEP-BY-STEP INSTRUCTIONS

1. Prepare the workspace
2. Set up a clean, well-lit area.
3. Gather: perf board, soldering iron, solder wire, wire cutters/strippers, safety glasses, and all tested components from the breadboard circuit.
4. Plan the layout
5. Place the components on the perf board in the same arrangement as on the breadboard and (Tinkercad).
6. Keep leads short and group related parts close (LDR + resistor divider, transistor + LED).
7. Insert components
8. Push each component’s leads into the perf board holes.
9. Bend them slightly on the underside to hold them in place.
10. Double-check orientation of the transistor (C–B–E) and LED (anode/cathode) before soldering.
11. Solder connections
12. Heat the pad and lead together, then feed a small amount of solder.
13. Remove iron once solder flows smoothly into a cone shape.
14. Avoid solder bridges (shorts between pads).
15. Trim excess leads
16. After each joint cools, cut the protruding wire ends with side cutters.
17. Make wire links
18. Where components aren’t adjacent, use insulated jumper wires or stripped solid-core wire to create connections across the board.
19. Solder one end, then the other, keeping wires neat and flat.
20. Check continuity
21. Use a multimeter’s continuity mode to ensure connections match your breadboard circuit.
22. Verify there are no shorts between VCC and GND.
23. Connect power rails
24. Run a thicker wire (or multiple linked pads) for the +V and GND rails.
25. Solder all components needing power to these rails.
26. inspection
27. Visually inspect all joints for cold solder (dull/crumbly look) and redo if needed.
28. Confirm all polarised parts (LED, transistor) are correctly oriented.