Video audio sändare (räckvidd 3-5meter)

The circuit presented here is a simple audio/video transmitter with a range of 3 to 5 metres.

The A/V signal source for the circuit may be a VCR, a satellite receiver or a video game etc.

The circuit uses inexpensive components which are easily available in the market. A block

diagram showing the principle of its operation and complete circuit diagram are shown in

the figure. A mixer which also operates as an oscillator at VHF (H) channel 5 TV frequency is

amplitude modulated by video signal and mixed with frequency modulated audio signal.

The resultant modulated signal, which is radiated through the ant enna, contains video

carrier frequency of 175.25 Mhz and audio carrier frequency of 180.75 Mhz. The circuit

consists of transistor T1 with its resonant tuned tank circuit formed by inductor L1 and

trimmer capacitor VC1, oscillating at VHF (H) channel 5 frequency. Transistor T2 with its

tuned circuit formed using SIF coil and inbuilt capacitor forms a 5.5 Mhz oscillator. The audio

signal applied at the input to T2 results into frequency modulation of 5.5 Mhz oscillator

signal. The output of 5.5 Mhz FM stage is coupled to the mixer stage through capacitor C8

while the video signal is coupled to the emitter of T1 via capacitor C4 and variable resistor

VR1. Finally, the AM video and FM audio carrier frequencies corresponding to VHF(H) ch.5

are transmitted via the antenna. Inductor L1 can be wound on a 3mm core using 24SWG

enamelled wire by just giving 4 turns. Calibration/adjustment of the circuit is also not very

difficult. After providing 12V DC power supply to the circuit and tuning your TV set for VHF

channel 5 reception, tune trimmer VC1 until you see plain raster on TV screen. Tuning of 5.5

Mhz oscillator to exact frequency can be done using either a frequency counter or a properly

calibrated radio receiver on SW1. Adjust the core of SIF coil for the best results

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