How bright is it in here, in four LEDs

Four LEDs that light one after another as a room gets brighter. A photoresistor and one quad op amp chip, no code.

Free build sheet

The parts, every wire with its length and bend points, a bend guide, both sides of the board and checkboxes to track your progress. Print it or keep it open at the bench.

Open the build sheet

What you need

18 parts on a perfboard 18 × 32 holes, plus hookup wire.

Qty	Part	Where to buy
1	Ceramic Cap100nF C1	Buy Ceramic Cap on Amazon
1	Power Input (2-pin) J1
1	Screw Terminal 2P (5.08mm) J2
4	LEDred 5mm LED1, LED2, LED3, LED4	Buy LED on Amazon
5	Resistor (axial)10k R1, R2, R3, R4, R5	Buy Resistor (axial) on Amazon
1	Resistor (axial)22k R6	Buy Resistor (axial) on Amazon
4	Resistor (axial)330 R7, R8, R9, R10	Buy Resistor (axial) on Amazon
1	DIP-14 U1

Check every pin against the datasheet of the exact part in your hands before wiring.

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Cut list

21 wires, 787 mm (31.0 in) in total, in the order the build sheet cuts them. Wire numbers match the build sheet. Each length includes 5 mm per end to go through the hole.

Wire	Colour and net	Route	Length	Bends
6	BlackOUT4 M23 → M16	M23 → M16	28mm	Straight
18	BlackGND G8 → F14	G8 → F14	28mm	20mm (90°)
12	BlackSENSE P5 → H19	P5 → H19	66mm	8mm (90°), 43mm (90°)
17	BlueGND E8 → E4	E8 → E4	20mm	Straight
5	BlueOUT2 H22 → E16	H22 → E16	33mm	13mm (90°)
11	BlueSENSE P3 → J19	P3 → J19	66mm	8mm (90°), 48mm (90°)
16	GreenVCC L3 → C3	L3 → C3	33mm	Straight
4	GreenT3 J13 → K19	J13 → K19	38mm	13mm (90°), 28mm (90°)
10	GreenSENSE Q4 → H15	Q4 → H15	61mm	33mm (90°)
1	RedOUT1 F23 → F19	F23 → F19	20mm	Straight
13	RedVCC N13 → I15	N13 → I15	28mm	18mm (90°)
19	RedGND E15 → I19	E15 → I19	30mm	15mm (90°)
7	RedT2 H7 → G15	H7 → G15	33mm	8mm (90°)
15	WhiteVCC N11 → K4	N11 → K4	35mm	13mm (90°)
3	WhiteT1 E12 → G19	E12 → G19	38mm	8mm (90°), 25mm (90°)
9	WhiteSENSE O4 → J15	O4 → J15	51mm	33mm (90°)
21	WhiteGND D14 → D30	D14 → D30	51mm	Straight
2	YellowOUT3 L23 → L19	L23 → L19	20mm	Straight
8	YellowT4 L7 → K15	L7 → K15	33mm	8mm (90°)
14	YellowVCC O12 → F13	O12 → F13	35mm	8mm (90°)
20	YellowGND F7 → Q8	F7 → Q8	40mm	33mm (90°)

About this build

A light meter that answers one question: how bright is it in here? A photoresistor off the board on a screw terminal sits in a divider with a 10k resistor, and that one voltage feeds all four comparators inside an LM324 quad op amp.

A five resistor ladder across the 5V rail, a 22k at the top and four 10k below it, sets thresholds at 0.81V, 1.61V, 2.42V and 3.23V. As the room gets brighter, the LEDs light one after another. The 22k at the top is there for a reason: on 5V the LM324 inputs only work up to about 3.5V, and the bigger top resistor keeps every threshold under that.

Each LED gets its own 330 ohm resistor, so a lit LED keeps the same brightness whether it's alone or the whole bar is on. There's no photoresistor in the PerfBoard catalog, which is why the sensor comes in from off the board.

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