A 3.7 V lithium-ion charger using LM358 is a classic DIY solution and great for learning battery charging fundamentals. Below is a clear, practical explanation + working concept.

Charging a 3.7 V Li-ion / Li-Po battery safely is very important. Over-charging above 4.2 V can damage the battery or even cause fire. In this project, we use the LM358 dual op-amp IC as a voltage comparator to automatically stop charging when the battery reaches 4.2 V.

To build an automatic dark sensor using the BD139 transistor, you’ll need

• IC 741
• BD140 Transistor
• BC547 Transistor
• 1N4007 Diode
• 3.9v Zener diode
• 10K potentiometer
• 1k ohm resistor * 6
• 220 ohm resistors
• Red led
• Green led
• Battery
• Battery holder
• Power Supply 5v

Gerber File:

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LM358 IC: The Most Popular Dual Op-Amp for DIY Electronics

If you’ve ever built a sensor circuit, audio amplifier, or Arduino project, chances are you’ve already used the LM358—even if you didn’t realize it. The LM358 is one of the most widely used operational amplifier (op-amp) ICs, especially in low-power and single-supply applications.

Let’s break down what LM358 is, why it’s so popular, and how you can use it in real projects.

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What is LM358?

The LM358 is a dual operational amplifier IC, meaning it contains two independent op-amps inside a single 8-pin package. It is designed to operate from a single power supply, which makes it ideal for battery-powered and microcontroller-based projects.

📦 Package: DIP-8 / SOIC-8
⚡ Supply: Single or dual supply
🔧 Cost: Very low (budget-friendly)

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Why LM358 Is So Popular

The LM358 stands out because of these key features:

• ✔️ Works with single supply (3V–32V)
• ✔️ Input range includes ground (0V)
• ✔️ Low power consumption
• ✔️ Cheap and easily available
• ✔️ Stable and beginner-friendly

That’s why you’ll find it in Arduino modules, sensor boards, power supplies, and audio circuits.

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LM358 Pin Configuration

Here’s the standard 8-pin LM358 pinout:

Pin	Name	Description
1	Output 1	Output of Op-Amp 1
2	Inverting Input 1	(–) input of Op-Amp 1
3	Non-Inverting Input 1	(+) input of Op-Amp 1
4	VCC–	Ground (0V)
5	Non-Inverting Input 2	(+) input of Op-Amp 2
6	Inverting Input 2	(–) input of Op-Amp 2
7	Output 2	Output of Op-Amp 2
8	VCC+	Positive supply

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Electrical Specifications (Typical)

• Supply Voltage:
  • Single: 3V to 32V
  • Dual: ±1.5V to ±16V
• Input Offset Voltage: ~2 mV
• Bandwidth: ~1 MHz
• Slew Rate: 0.3 V/µs
• Operating Temperature: –40°C to +85°C

⚠️ Note: LM358 is not rail-to-rail output, so the output won’t reach full VCC.

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Common LM358 Configurations

1. Voltage Amplifier

Used to amplify weak signals from sensors like:

• LDR
• Microphone
• Gas sensor
• IR sensor

2. Comparator

LM358 can work as a comparator for:

• Battery level indicator
• Over-voltage protection
• Threshold detection

3. Buffer (Voltage Follower)

Perfect for:

• Impedance matching
• Protecting microcontroller ADC pins

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Simple LM358 Amplifier Example

Non-Inverting Amplifier Gain Formula:$$Gain = 1 + \frac{R2}{R1}$$Gain=1+R1R2​

Example:

• R1 = 10kΩ
• R2 = 100kΩ
• Gain ≈ 11×

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LM358 vs LM741 (Quick Comparison)

Feature	LM358	LM741
Single supply	✅ Yes	❌ No
Low power	✅ Yes	❌ No
Modern usage	✅ High	❌ Outdated
Arduino friendly	✅ Yes	❌ No

👉 LM358 easily beats LM741 for modern electronics projects.

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Real-World Applications

• Arduino sensor modules
• Audio pre-amplifiers
• Battery voltage monitors
• Signal conditioning circuits
• Power supply feedback loops
• DIY electronics projects

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Limitations of LM358

No IC is perfect. Keep these in mind:

• ❌ Low slew rate (not ideal for high-speed signals)
• ❌ Not rail-to-rail output
• ❌ Not suitable for high-quality audio

For better performance, consider LMV358, TL072, or OPA2134.

BC547 Transistor

The BC547 is a general-purpose NPN transistor made from silicon. It is mainly used where small signals need to be amplified or controlled. Because of its low cost, reliability, and availability, it is one of the most popular transistors in the world.

Basic Information

Parameter	Value
Type	NPN BJT
Package	TO-92
Material	Silicon
Operation	Amplifier / Switch
Polarity	NPN

Pin Configuration (TO-92)

When the flat side faces you and the legs point downward:

Base (B) – Control terminal

• Emitter (E) – Current exits

⚠️ Pin order may vary slightly by manufacturer, so always check the datasheet.

⚙️ Working Principle

The BC547 works on the principle of current amplification.

🔹 Basic Rule

A small current at the Base controls a large current between Collector and Emitter.

🟢 Switch Mode

• Base current = OFF → No C-E current
• Base current = ON → C-E current flows

Used for:

• LED ON/OFF control
• Relay driving (small relays)
• Digital switching

🔵 Amplifier Mode

• Weak AC signal applied at base
• Strong amplified signal obtained at collector

Used in:

• Audio amplifiers
• Microphone circuits
• Sensor signal amplification

📊 Electrical Characteristics

Parameter	Typical Value
Vceo (Max Collector-Emitter Voltage)	45 V
Ic max (Collector Current)	100 mA
hFE (Current Gain)	110 – 800
Power Dissipation	500 mW
Transition Frequency (ft)	~300 MHz

🔈 Gain Classification

BC547 comes in three gain versions:

Type	Gain (hFE)
BC547A	110 – 220
BC547B	200 – 450
BC547C	420 – 800

✔ Higher gain = better amplification

🧪 Applications of BC547

✔ Audio pre-amplifiers
✔ Signal amplifiers
✔ Microcontroller interfacing
✔ Relay driver circuits
✔ Light & temperature sensors
✔ FM transmitter circuits
✔ Logic switching circuits

🔄 BC547 vs Similar Transistors

Transistor	Type	Notes
BC547	NPN	Low noise, general purpose
BC548	NPN	Lower noise than BC547
BC549	NPN	Ultra-low noise
2N3904	NPN	American equivalent
C1815	NPN	Audio application

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