Project Hub /Basic Analog & Power Electronics/Discrete & Component Electronics/Build TV to FM Converter: Simple Circuit

Beginner

ANALOG / DISCRETE

< ₹500

3 – 6 Hours

Partial Kit

Build TV to FM Converter: Simple Circuit

A simple one-transistor BF494 circuit that allows you to receive FM and VHF stations on an old television set.

Originally published on ElectronicsForU

View original tutorial

Parts You'll Need

This project is sourced from another site, not written by us — this list is our best match of their components against what we stock, not a verified bill of materials. Always check the original blog (linked above) for the actual parts list before buying.

10pcs - 220 ohm 1/4 W Resistor Set Rs. 39.00
Jumper Wire 10 Pin Male To Male 20cm Rs. 45.00
2-Slot 18650 Holder with Protective Cover and On/Off Switch – Case Rs. 109.00
400 Points Mini Solderless Prototype PCB Breadboard High Quality Rs. 89.00
MB102 Adjustable Breadboard Power Supply Module (3.3V & 5V) for Arduino & Raspberry Pi Rs. 139.00
Jumper Wire 10 Pin Male To Female 20cm Rs. 45.00

Selected: Rs. 0.00

This list includes a generic resistor set as a placeholder — it is not matched to the exact value or part number this circuit needs. The correct one is specified on the original source page linked above; check it before buying.

We don't carry yet: Capacitors
Inductors / Coils / Ferrite
Header Pins / Terminals / Connectors
9V Battery + Snap
Perfboard / Veroboard / Stripboard
Semiconductor: T1

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How It Works

The Build TV to FM Converter circuit functions as a basic radio frequency oscillator and mixer stage built around a single BF494 NPN transistor (T1). Powered by a 9V battery, the circuit uses a biasing network consisting of resistors R1 (470kΩ) and R2 (470Ω) to establish the proper operating point for the transistor. The active RF mixing and tuning take place within the resonant tank circuit, which is formed by inductor L* (a custom 5-turn 23 SWG air-core coil) operating in parallel with the variable trimmer capacitor VC1 (2-22pF) along with fixed capacitors C2 (10pF) and C3 (33pF). This LC tank determines the operating frequency of the converter stage. Capacitors C1 (0.01µF), C4 (0.001µF), and C5 (0.01µF) serve as coupling and decoupling elements, routing the signals into and out of the transistor while blocking DC components and stabilizing the power rails across the 5cm x 7cm Vero board layout. By adjusting the trimmer capacitor VC1 or modifying the coil geometry, the user tunes the local oscillation frequency. This allows incoming broadcast signals to heterodyne or mix, shifting VHF television or FM radio frequencies so that an older display or receiver can pick them up. The entire assembly is connected via 2-pin connectors and controlled through a simple power switch.

Why Build This

Explore practical radio frequency (RF) design and oscillator principles using discrete analog components.

Gain hands-on experience tuning LC tank circuits with variable capacitors and custom air-core inductors.

Understand frequency conversion and signal mixing concepts relevant to undergraduate communication engineering coursework.

Real-World Application

Enables older television sets to receive FM and VHF radio broadcast stations using a simple low-cost transistor converter.

Skills You'll Learn

RF oscillator design

LC tank tuning

discrete transistor biasing

analog signal mixing

Vero board circuit assembly

Safety Precautions

Ensure correct polarity when connecting the 9V battery or external DC power supply to avoid damaging components.

Technology Tags

RF CIRCUIT

TRANSISTOR

BF494

OSCILLATOR

MIXER

FM RADIO

VHF

ANALOG ELECTRONICS

Frequently Asked Questions

What is the exact purpose of coil L* in this circuit?

Coil L* acts as the inductive element in the RF resonant tank, working alongside the capacitors to determine the operating frequency range of the converter.

Can I use a different transistor than the BF494?

The circuit is specifically designed around the characteristics of the BF494 NPN transistor; substituting other components may alter the high-frequency response and oscillator performance.

How do I tune to different stations?

You tune the circuit by carefully adjusting the 2-22pF trimmer capacitor (VC1) to shift the resonant frequency of the oscillator stage.

Gallery

This page is part of Kitkraft's Project Hub — a curated collection of electronics & robotics projects gathered from the maker community to help students and hobbyists discover and learn to build. Kitkraft does not claim authorship of the original project, and makes no representation as to the accuracy, completeness or safety of third-party source material. The summary and "How It Works" explanation above are AI-generated, rewritten with reference to the original tutorial — not a reproduction of it. Full credit and a direct link to the original source are provided above. If you're the original creator and have a concern about how your work is represented here, please contact us and we'll address it promptly.

Kitkraft Project Hub — curated from the maker community, credited at the source.

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