光线变化报警器, a Arduino project

This light change alarm detects sudden shifts in ambient brightness and triggers an alert. Built around an LM358 operational amplifier and a photoresistor, the circuit continuously monitors light levels and compares them against a user-adjustable threshold using analog electronics—no microcontroller code required.

The guide provides a complete wiring diagram, parts list, and step-by-step assembly instructions for connecting the photoresistor input stage, the LM358 amplifier, the comparison circuit, and the LED and buzzer outputs. Once powered via USB-C, the system can be calibrated with the potentiometer to trigger alarms at desired light thresholds.

Wiring diagram

Gather all the parts

Qty	Component
1	LM358 Dual Operational Amplifier DIP-8 or SOIC-8 Dual op-amp in 8-pin DIP (or breakout module exposing the same pinout via headers). Operates from a 3–32 V single supply (or ±1.5 V to ±16 V dual supply); output swings near GND on the low side and to about VCC−1.5 V on the high side. The breakout-module form factor shares the same pinout as the bare LM358 DIP-8.
1	LM358 Dual Operational Amplifier DIP-8 Dual op-amp in 8-pin DIP (or breakout module exposing the same pinout via headers). Operates from a 3–32 V single supply (or ±1.5 V to ±16 V dual supply); output swings near GND on the low side and to about VCC−1.5 V on the high side. The breakout-module form factor shares the same pinout as the bare LM358 DIP-8.
1	LM358 Dual Operational Amplifier 5 V sink breakout with CC resistors Dual op-amp in 8-pin DIP (or breakout module exposing the same pinout via headers). Operates from a 3–32 V single supply (or ±1.5 V to ±16 V dual supply); output swings near GND on the low side and to about VCC−1.5 V on the high side. The breakout-module form factor shares the same pinout as the bare LM358 DIP-8.
1	Photo Cell (CdS Photoresistor) GL5528-class CdS LDR CdS light-dependent resistor. Resistance decreases as light increases. Adafruit product 161 currently specifies approximately 1 kΩ in light and 10 kΩ in dark. Use either symmetric lead in a voltage divider and read the divider with an analog input.
1	LED Red 5 mm LED + 1 kΩ series resistor Standard 3mm/5mm through-hole LED. A current-limiting series resistor is added automatically.
1	Buzzer 5 V active, ≤10 mA Piezo buzzer for sound output
1	10kΩ Potentiometer 10 kΩ linear A 3-terminal passive resistive voltage divider with a total resistance of 10kΩ. One end connects to 5V, the other to GND, and the wiper outputs a variable voltage between 0V and 5V.

Assemble it in 5 steps

1. 准备 USB-C 5 V 电源

使用明确标注为“USB-C 5 V sink / 带 CC 电阻”的电源输出小板。将其 VBUS 接为全电路 +5 V 母线，GND 接为全电路公共地。不要把 Type-C 母座的 CC 引脚悬空或自行短接；应使用带两个 5.1 kΩ CC 下拉电阻的成品小板。

先不插入 USB-C 电源，用万用表确认 VBUS 与 GND 没有短路。
所有器件的地必须接到同一条 GND 母线。
仅接 5 V USB 电源；不要接 9 V/12 V 快充触发板输出。

2. 搭建光敏输入与 LM358 放大器

将 ldr_1 的一端接 +5 V，另一端作为 SENSE 节点。SENSE 接 LM358 的 3 脚（IN+ A）。在 SENSE 与 GND 间接 100 kΩ 电阻。LM358 的 2 脚（IN− A）经 10 kΩ 电阻接 GND；再从 1 脚（OUT A）经 100 kΩ 电阻反馈到 2 脚，形成约 11 倍非反相增益。LM358 的 8 脚接 +5 V、4 脚接 GND。

DIP-8 芯片缺口朝上时，从左上角起逆时针数脚；LM358 的 A 通道是 1、2、3 脚。
在 LM358 的 8 脚与 4 脚之间、靠近芯片处并联 100 nF 陶瓷去耦电容。
不要把 LM358 的 1、2、3 脚与 LM393 的相同脚号混淆。

3. 接好 LM393 阈值比较器

将 LM358 的 1 脚输出接 LM393 的 3 脚（IN+ A）。threshold_pot_1 的两端分别接 +5 V 与 GND，中心滑动端接 LM393 的 2 脚（IN− A）。LM393 的 8 脚接 +5 V、4 脚接 GND。未使用的第二比较器：5 脚和 6 脚都接 GND，7 脚悬空。LM393 的 1 脚是开集电极报警输出 ALARM。

顺时针或逆时针转动阈值电位器会改变触发亮度；最终以实际测试方向为准。
LM393 的 8 脚与 4 脚之间也要就近放置一个 100 nF 去耦电容。
LM393 的 1 脚只能拉低，不能像推挽输出那样直接提供正电流。

4. 连接 LED 与有源蜂鸣器

alarm_led_1 的阳极必须先串联 1 kΩ 电阻后接 +5 V，阴极接 ALARM（LM393 1 脚）。buzzer_1 必须选 5 V、工作电流不超过 10 mA 的有源蜂鸣器：其“+”脚接 +5 V，“−”脚接同一 ALARM 节点。这样 LM393 在触发时拉低 ALARM，LED 和蜂鸣器同时工作。

LED 长脚通常为阳极；若不亮，先断电再反接 LED。
蜂鸣器外壳或引脚上的“+”标记必须接 +5 V。
蜂鸣器额定电流若超过 10 mA，不能直接接 LM393；必须增加 NPN/MOSFET 驱动级。
LED 必须保留 1 kΩ 串联电阻，不能直接接 5 V。

5. 上电与校准

再次检查 +5 V/GND 极性和芯片方向后插入 USB-C 电源。让传感器处于希望触发的环境亮度，缓慢旋转 threshold_pot_1，直到 LED 和蜂鸣器刚好切换；再略微回调以留出余量。用手遮住或照亮 ldr_1 验证报警方向。

本设计默认“光线增强时报警”，因为 LDR 接在分压器上臂；若要“变暗时报警”，将 LDR 与 100 kΩ 分压电阻的位置对调，且重新校准。
调试时避免让 USB-C 金属外壳或裸线碰到面包板导线，防止短路。

Review all connections

1. Connections between "usb_c_5v_1" and "Arduino"

Function	usb_c_5v_1	Arduino
`power`	VCC	5V
`ground`	GND	GND
`power`	IN+_A	5V
`ground`	IN-_A	GND
`analog`	OUT_A → USB-C breakout unused analog pin—do not connect on physical board	EXT

2. Connections between "ldr_1" and "Arduino"

Function	ldr_1	Arduino
`power`	Terminal 1	5V
`passive`	Terminal 2 → LM358 Dual Operational Amplifier IN+_A	EXT

3. Connections between "lm358_1" and "Arduino"

Function	lm358_1	Arduino
`power`	VCC	5V
`ground`	GND	GND
`analog`	IN-_A → 10 kΩ gain resistor to GND and 100 kΩ feedback resistor to OUT_A	EXT
`analog`	OUT_A → LM358 Dual Operational Amplifier IN+_A	EXT
`ground`	IN+_B	GND
`ground`	IN-_B	GND

4. Connections between "lm393_1" and "Arduino"

Function	lm393_1	Arduino
`power`	VCC	5V
`ground`	GND	GND
`analog`	IN-_A → 10kΩ Potentiometer Wiper (middle)	EXT
`ground`	IN+_B	GND
`ground`	IN-_B	GND
`analog`	OUT_A → Buzzer SIGNAL	EXT

5. Connections between "threshold_pot_1" and "Arduino"

Function	threshold_pot_1	Arduino
`power`	End1 (VCC side)	5V
`ground`	End2 (GND side)	GND

6. Connections between "alarm_led_1" and "Arduino"

Function	alarm_led_1	Arduino
`power`	ANODE	5V
`ground`	GND → LM358 Dual Operational Amplifier OUT_A	EXT

7. Connections between "buzzer_1" and "Arduino"

Function	buzzer_1	Arduino
`ground`	GND	GND

Deploy the firmware

firmware.cppOpen in Schematik

// This project is an entirely analog alarm circuit.
// LM358 and LM393 perform sensing and comparison in hardware.
// No microcontroller I/O or firmware is used.
void setup() {}
void loop() {}