This LED turns red when the room gets hot

An RGB LED that shows the room temperature: red when it's hot, blue when it's cold, green in between. Two knobs set the limits.

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

13 parts on a perfboard 15 × 29 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
2	Potentiometer Q1, Q2
2	N-ch MOSFET (TO-92)2N7000 Q3, Q4	Buy N-ch MOSFET (TO-92) on Amazon
1	RGB LED (common anode) Q5	Buy RGB LED (common anode) on Amazon
1	Resistor (axial)10k R1	Buy Resistor (axial) on Amazon
2	Resistor (axial)100k R2, R3	Buy Resistor (axial) on Amazon
1	Resistor (axial)330 R4	Buy Resistor (axial) on Amazon
1	DIP-8 U1

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

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Cut list

23 wires, 543 mm (21.4 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	BlackHOT_OUT F18 → F20	F18 → F20	15mm	Straight
12	BlackGND L11 → K9	L11 → K9	18mm	10mm (90°)
18	BlackVCC D14 → D11	D14 → D11	18mm	Straight
17	BlueGND H15 → H19	H15 → H19	20mm	Straight
5	BlueSENSE M6 → K11	M6 → K11	28mm	10mm (90°)
23	BlueVCC K3 → C3	K3 → C3	30mm	Straight
11	BlueCOLD_OUT M18 → H12	M18 → H12	38mm	20mm (90°)
22	GreenVCC J5 → J4	J5 → J4	13mm	Straight
16	GreenGND G14 → F12	G14 → F12	18mm	10mm (90°)
10	GreenCOLD_OUT F24 → I26	F24 → I26	21mm	8mm (45°), 11mm (45°)
4	GreenSENSE L7 → H8	L7 → H8	23mm	15mm (90°)
1	RedGREEN H25 → H26	H25 → H26	13mm	Straight
13	RedGND J8 → G6	J8 → G6	23mm	13mm (90°)
19	RedVCC F14 → J13	F14 → J13	23mm	8mm (90°)
7	RedHOT_OUT H20 → F26	H20 → F26	30mm	10mm (90°), 15mm (90°)
3	WhiteSENSE N7 → N4	N7 → N4	18mm	Straight
15	WhiteGND N11 → H13	N11 → H13	30mm	10mm (90°)
21	WhiteVCC F11 → J7	F11 → J7	30mm	15mm (90°)
9	WhiteCOLD_OUT K18 → H24	K18 → H24	33mm	20mm (90°)
14	YellowGND H5 → D5	H5 → D5	20mm	Straight
2	YellowLED_V J26 → G28	J26 → G28	23mm	10mm (90°)
20	YellowVCC J15 → J20	J15 → J20	23mm	Straight
8	YellowHOT_OUT D18 → F9	D18 → F9	38mm	28mm (90°)

About this build

A three colour room thermometer with no microcontroller. A 10k NTC thermistor forms a divider with a 10k resistor, and that one voltage goes to both halves of an LM393 dual comparator. One half watches for the hot point, the other for the cold point, and a trim pot sets each one.

The LM393 outputs are open collector, so they can only pull down. Red sinks through the hot comparator, blue sinks through the cold one. Green runs through two 2N7000 MOSFETs in series, with their gates on those same two outputs, so green only lights when neither comparator is pulling down.

The thermistor isn't on the board. The PerfBoard catalog has no thermistor, and a temperature sensor wants to sit in the air you're measuring anyway, so it comes in on a 2 pin screw terminal.

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