Build a Complete Radio Receiver Using SI4725 + LM386

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 Introduction

This project focuses on designing and building a CB (Citizen Band) radio receiver using the SI4725 digital tuner IC and the LM386 audio amplifier. It operates on the 27 MHz CB band, includes digital tuning, analog control knobs, TX switching capability, and professional-grade housing for noise suppression. The design integrates all elements: PCB layout, antenna, 3D-printed holders, and front acrylic control panel — making it both technically strong and visually impressive.

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 System Overview

    Main Features:

• SI4725 AM/FM I²C-controlled tuner

• LM386 mono audio amplifier

• Band & channel selection via potentiometers

• TX/RX switching with LED indication

• 12V input with regulated 5V & 3.3V rails

• Shielded aluminum base with heatsinks

• Custom antenna coil for 27 MHz

• 3D printed structures for antenna holding

• Acrylic glass front panel with labeled control layout

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 Core Components Breakdown

Component	Role	Details
SI4725	Radio tuner IC	Digital control via I²C, supports AM band
LM386	Audio amp	Low-voltage mono amplifier
AMS1117-3.3V	Regulator	Supplies SI4725 (VA & VIO)
LM7805	Regulator	Supplies LM386
Potentiometers	Tuning, Band Select, Volume	10-turn, 470k, 10k used
Toggle Switches	Power, RX/TX, Narrow/Wide, Antenna select
LEDs	Green (RX), Yellow (TX), Red (Power)
Antenna Coil	Custom 27 MHz, 26 turns of 0.2mm wire
3D Printed Parts	For antenna mount and internal structuring
Aluminum Frame	50mm x 100mm with dual heatsinks

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SI4725 Pin-by-Pin Explanation

Pin	Name	Description
1	GND	Common ground
2	LNA_IN	RF input from antenna
4	RFGND	Dedicated RF ground
5	VA	Analog power (3.3V)
7/8	ROUT/LOUT	Audio output (connect to pot → LM386)
9	VIO	Digital I/O power (3.3V)
10/11	SDIO/SCLK	I²C data/clock
12	RST	Reset pin (active low)
13–15	GPO1–3	GPIOs used for LED status or switching

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LM386 Pin-by-Pin Details

Pin	Name	Function
1,8	GAIN	Gain control (200x with 10µF cap)
2	IN–	Ground
3	IN+	Audio in from pot
4	GND	Common ground
5	VOUT	Speaker/audio out
6	VCC	5V regulated input
7	BYP	Noise filtering cap (10µF to ground)

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 Antenna Calculation (27 MHz)

Formula:

$$f = \frac{1}{2\pi\sqrt{LC}}$$

Let’s assume:

• f = 27 MHz

• C = 100pF (trimmer)

• Solve for L:

$$L = \frac{1}{{(2\pi \cdot 27 \times 10^6)^2 \cdot 100 \times 10^{-12}}} \approx 3.47 \mu H$$

Coil Design:

• Wire: 0.2mm enameled

• Turns: 26

• Diameter: 5cm

• Core: Air or ferrite slug

• Placement: Vertical, fixed using 3D printed support

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Potentiometer Control System

1. Channel Tuning (10-turn Pot):

• Analog input to MCU

• Voltage mapped to frequency (e.g. 0V = 26.965 MHz, 5V = 27.405 MHz)

2. Band Select (470k Pot):

• ADC input range split into bands:

  • 0–1V = Band A

  • 1–2V = Band B

  • …

3. Volume (10k Pot):

• Simple series control to LM386 input (Pin 3)

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Power Supply Layout

Source	Regulator	Supplied To
12V DC	LM7805	LM386 Audio Amp
	AMS1117-3.3V	SI4725 Tuner

Add 100µF + 0.1µF capacitors near each regulator and IC.

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 Front Panel Layout

Material: 5mm Acrylic
Control Layout:

Position	Control
Left	Volume (10k pot)
Center	Band Select (470k)
Right	Channel Tune (10-turn)
Switches	ON/OFF, RX/TX, Wide/Narrow, Antenna Select
LEDs	Power (Red), RX (Green), TX (Yellow)

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EMI Reduction and Frame Design

• Base Frame: 50mm x 100mm aluminum plate

• Heatsinks: Attached to both ends

• Mounting: PCB fixed with standoffs

• Antenna Coil Holder: 3D-printed ring structure

• Isolation: Audio + RF sections isolated on PCB

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Files & Downloads

Included Files:

•  PCB Gerber Files (2-layer)

•  STL Files for Antenna Support & Panel Mounts

•  Schematic PDF (SI4725 + LM386)

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 Conclusion

This CB radio receiver is a fantastic DIY build that blends radio engineering, analog audio, and microcontroller control. It’s functional, robust, and visually polished — perfect for hobbyists and radio experimenters.

If you're planning to use this blog article:

• Add real photos of your build where noted.

• Link the STL, schematic, and PCB files.

• Optionally embed demo videos or scope outputs.

Why SI4725 is Superior Compared to Other Radio Receiver ICs

1. Digital Tuning and Control via I²C Interface

• SI4725 is a digital radio receiver IC with an I²C control interface, enabling precise frequency tuning.

• Older ICs like TA7642 or TEA5711 use analog tuning (varactor diodes and variable capacitors), which is less accurate and prone to drift.

• The digital control allows the SI4725 to:

  • Store presets easily.

  • Have stable tuning that doesn’t drift with temperature or voltage changes.

  • Automatically scan frequencies.

  • Switch bands or modes instantly.

2. Multi-Mode Support: AM, FM, and SW

• The SI4725 supports multiple modulation types:

  • AM (Amplitude Modulation)

  • FM (Frequency Modulation)

  • SW (Shortwave reception)

• Many older ICs only support AM or FM but not both, limiting their versatility.

• This makes the SI4725 a great all-in-one solution for CB radio and other bands.

3. Better Sensitivity and Selectivity

• SI4725 uses digital signal processing (DSP) internally to enhance reception.

• It offers higher sensitivity, meaning it can detect weaker signals better than analog-only receivers.

• Improved selectivity reduces interference from adjacent channels and noise.

• This leads to clearer audio and fewer false signals.

4. Integrated Audio Processing

• The IC integrates audio processing functions such as automatic gain control (AGC), noise reduction, and demodulation.

• This reduces the external component count and simplifies circuit design.

5. Lower Power Consumption

• Operating at 3.3V with modern CMOS technology, the SI4725 consumes less power than older analog ICs.

• This benefits battery-operated or portable radios by extending operating time.

6. Compact Size and Integration

• The SI4725 integrates many functions in one IC:

  • RF front-end

  • Mixer

  • IF filter

  • Demodulator

  • DSP

• Older radios require multiple discrete components or ICs for the same functionality, increasing PCB size and complexity.

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Comparison with Specific Older ICs

Feature	SI4725	TA7642 (Analog AM Receiver)	TEA5711 (Analog AM/FM Receiver)
Frequency Tuning	Digital (I²C controlled)	Analog variable capacitor	Analog variable capacitor
Supported Modes	AM/FM/SW	AM only	AM and FM
Signal Processing	DSP-based	Analog only	Limited analog filtering
Sensitivity	High	Moderate	Moderate
Selectivity	High (digital filtering)	Low (limited analog filtering)	Moderate
Power Supply	3.3V digital CMOS	Typically 5V or higher	Typically 5V or higher
Component Count	Low (integration)	High (many external parts)	Medium
Stability	High (digital frequency lock)	Low (prone to drift)	Medium