Simple TV, Video Transmitter Circuit

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

This guide 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 Description

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 case the chosen transistor is housed in a planar TO50 case.

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

The frequency of the oscillator is primarily determined by the values of the inductor coil and capacitor C1 in the LC circuit.

By adjusting the values of the coil and C1 the resonant frequency is set within the desired UHF band 470 to 855 MHz allowing the transmission of the RF signal on specific TV channels.

A simple wire approximately 5 cm 2 inches in length serves as the antenna.

The antenna is directly connected to the oscillator enabling the transmission of the RF signal.

The video signal, obtained from a source like a security TV camera or camera with video output, is used to modulate the carrier frequency generated by the oscillator.

The modulation process involves varying the amplitude of the carrier signal in accordance with the video signal.

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

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

Potentiometer P1 is included in the circuit to control the modulation depth.

Modulation depth refers to the extent of variation in the amplitude of the carrier signal in response to the video signal.

Adjusting P1 influences the modulation depth and it is crucial for optimizing image quality.

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

The tuning involves searching for the transmitted signal on an analog TV within the UHF band typically in the 600 to 700 MHz range or the 37th to 50th channel.

Formulas:

The frequency of the television transmitter in this oscillator circuit can be determined 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 (approximately 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 can be obtained from the schematic diagram.

The resonant frequency determines the operating frequency of the transmitter.

Additionally, the modulation depth M can be 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 optimize 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 allowing for precise tuning and optimization during the construction process.

Construction:

• Choose a high frequency NPN transistor suitable for the oscillator circuit.
• Options 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 determine the frequency range of the transmitter.
• Set the coil and C1 to achieve a frequency within the desired 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:

• Integrate potentiometer P1 into the circuit to control the modulation depth.
• Calibrate P1 to achieve optimal image quality.
• Excessive modulation can lead to high contrast while insufficient modulation may result in synchronization 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:

• Be aware of the legal implications of broadcasting on the UHF television band.
• Broadcasting may be prohibited in certain countries.
• The author does not assume responsibility for any legal penalties resulting from unauthorized broadcasting.
• Engage in this project at your own risk adhering to legal regulations in your jurisdiction.

Conclusion:

It is important to note that the design of a TV and video transmitter circuit can vary based on the specific requirements and intended application.

Commercial video transmitters are also available and may include additional features such as multiple channels and frequency bands higher transmission power and built in audio capabilities.

Users should also be aware of legal regulations related to radio frequency transmission in their region.

References

Datasheet BFR91A