Simple TV, Video Transmitter Circuit

Have you ever wanted to build your own mini TV station?

This article is for hobbyists who want to make a simple transmitter to broadcast videos inside your house.

The transmitter uses a special coil and capacitor to pick a channel between 37 and 50 on the old fashioned TV dial UHF band, 470 to 855 MHz.

The exact channel depends on the size of the coil and capacitor.

WARNING: Building a TV transmitter may be illegal in your area.

Check the rules before you try this project.

What is a TV, Video Transmitter Circuit:

A TV and video transmitter circuit is an electronic circuit that enables the transmission of audio and video signals wirelessly from a source device such as a camera or DVD player to a television or video receiver.

This type of circuit is commonly used in applications like wireless surveillance cameras, video baby monitors or hobbyist projects where video signals need to be transmitted over short distances.

Circuit Working:

Parts List:

The heart of the TV, video transmitter circuit is a simple oscillator built around a high frequency NPN transistor such as BFR90, BFR91A, BFR92 or BFR93.

In this instance, a planar TO50 enclosure contains the selected transistor.

The transistor functions as an active component that generates a high frequency oscillation.

In the LC circuit the values of the inductor coil and capacitor C1 are the main factors that determine the oscillator’s frequency.

The resonant frequency is tuned within the correct UHF range which is 470 to 855 MHz by adjusting the values of the coil and C1.

This enables the RF signal to be transmitted on particular TV channels.

The antenna is just a normal wire about 5 cm (2 inches) long.

The RF signal is transmitted since the antenna and oscillator are directly connected.

The oscillators carrier frequency is changed using the video signal which is captured from a source such as a security TV camera or a camera with a video output.

The carrier signals amplitude is changed during the modulation process to match the video signal.

The circuit requires a voltage supply in the range of 5 to12V.

The power supply boots the oscillator allowing it to generate the oscillating RF signal.

The circuit includes a potentiometer P1 to control the modulation depth.

Modulation depth is the degree to which the carrier signals amplitude changes in response to the video signal.

Enhancing image quality requires P1 adjustment which affects modulation depth.

Once the transmitter is powered up the video source it is than connected and the circuit is manually tuned.

Finding the transmitted signal on a analog TV within the UHF band its usually in the 37th to 50th channel or the 600 to 700 MHz range which is the tuning process.

Formulas:

Below are the formulas for Simple TV, Video Transmitter Circuit:

The frequency of the television transmitter in this oscillator circuit is calculated using the formula for the resonant frequency of an LC circuit.

The resonant frequency (f) is given by:

f = 1 / 2π√LC

where:

• f is the resonant frequency
• π is a mathematical constant which is around 3.14159
• L is the inductance of the inductor Coil
• C is the capacitance of the capacitor C1.

In the context of the TV, video transmitter circuit described the values of L and C is generated from the schematic diagram.

The resonant frequency is measured by the operating frequency of the transmitter.

Also the modulation depth M is expressed as a percentage and is calculated using the formula:

M (%) = ( V_max − V_min / V_max + V_min ) × 100

where:

• M is the modulation depth in percentage
• Vmax​ is the maximum amplitude of the modulated signal
• Vmin​ is the minimum amplitude of the modulated signal.

In the television transmitter the modulation depth is adjusted using the potentiometer P1 to generate the image quality.

Adjusting P1 effectively controls the amplitude of the modulated carrier signal influencing the modulation depth.

These formulas provide a quantitative understanding of the frequency and modulation characteristics of the television transmitter making it possible to carefully adjust and improve while building.

How to Build:

• Select a high frequency NPN transistor which suits for oscillator circuit.
• If required include BFR90, BFR91A, BFR92 or BFR93.
• Place the selected transistor in a planar TO50 case.
• Refer to the schematic diagram for the values of the coil and C1.
• These components measure the frequency range of the transmitter.
• Set the coil and C1 to achieve a frequency within the required UHF band (470-855 MHz).
• Attach a wire approximately 5 cm 2 inches in length directly to the oscillator circuit to serve as the antenna.
• Connect the voltage supply 5 to12V to the transmitter.
• Keep live connections as short as possible to minimize parasitic capacitance and inductance.
• Position 100nF decoupling capacitors as close as possible to the transistor to address parasitic effects.
• Include parallel 100uF electrolytic capacitors to effectively block frequencies from 50 Hz to several hundred MHz.

Adjustment:

• Add potentiometer P1 into the circuit to control the modulation depth.
• Adjust P1 to achieve optimal image quality.
• Excessive modulation can lead to high contrast while insufficient modulation may result in alignment issues.

Tuning Procedure:

• Turn on the transmitter.
• Connect the video source to the transmitter.
• Compatible sources include security TV cameras or cameras with video output.
• Manually search for the transmitted signal on an analog TV.
• Look within the 600 to 700 MHz range or the 37th to 50th channel for optimal coverage.
• Some TVs with auto tuning capabilities may skip video signals without audio.
• If this occurs perform a manual search for comprehensive coverage.

Caution:

• Understand the legal consequences of using the UHF television band for broadcasting.
• In some nations broadcasting can be banned.
• Any legal issues arising from unlicensed broadcasting are not the writers responsibility.
• Continue with this project at your own risk while abiding by the laws in your area.

Conclusion:

Its important to remember that a TV and video transmitter circuits design are governed by the particular needs and intended use.

There are also commercial video transmitters which are built-in audio capabilities, several channels, frequency bands and higher broadcast power.

Also users should be aware of local laws pertaining to radio frequency transmission.

References

Datasheet BFR91A