The heart of this circuit transmitter ‘is the integrated circuit LM1889N of National Semiconductor, that consists of a Modulator of Video for TV in an involucres of 18 pins DIL.
This integrated circuit ‘used in videocassettes and videogames, exactly to process the image information and sound, so that they can be played in a channel free from the strip of VHF.
As it is component that ‘used in commercial equipments, besides the reliability, we have a certain obtaining easiness. The readers, with luck, until they can find this available component in an old videogame that it is out of use, and you take advantage of it to set up your station communitarian. count all the necessary stages to the processing of the video signals and audio of a transmitter of signs of TV.
The coil L1 together with the capacitor in parallel, it generates the sign of 4,5 MHz that, modulated with the sound, it should be separate from the bearer of video of this frequency. Like this, the adjustment that should do in this coil it simply consists in you take it to 4,5 MHz, in way we obtain it sound.
The audio modulation ‘done by a varicap in a very simple way, so that the intensity of the audio sign obtained in most of the exits of the sound table it should provide a good reproduction.
The video sign, that obtained ‘of the video exit of any camera, videocassette, DVD, applied in the pin 13 of the circuit integrated after going by an amplification stage with two transistors.
This sign will modulate in width the video bearer whose frequency certain ‘for L2 and the capacitor in parallel.
We should adjust the frequency of this oscillator for the channel free from the strip of VHF, usually a channel baixo(2 to 6) in that the broadcasting station will operate.
The exit of RF, obtained in the pin 11 mischievous ‘to an amplifying stage with a transistor and of this for the antenna expresses or a telescopic antenna, in case the transmission is of short reach. Observe that in the modulation of audio of this circuit, the sign generates two subportator being below a 4,5 MHz above the frequency of the channel and other 4,5 MHz.
As one of them not eliminated ‘, she can cause interferences in the adjacent channel. That means that you should choose a free channel in your place, but that doesn’t have adjacent channels operating.
The power supply of the circuit can be made with tensions of 12 (7812) or 15(7815) Volts of a source with at least 1A and excellent filter. A deficient filter in this type of so much circuit can provoke snores in the sound, as undulations in the image.
Suggestion of power supply for circuit using the ic 7812
Schematic for assembly of the tv transmitter with lm1889n
Assembly of the circuit of the TV transmitter
The healthy resistors of 1/8W or larger. The power source so much can be the one of 12V/1A, as it can be made a modification to operate with 15V, being enough for that to change the transformer and the integrated circuit. The circuit plate printed for the assembly ‘shown below in the illustration. The coils, as always, are the elements more critics of the project. L1 ‘formed by 40 you exhale of thread enameled fine (30 to 34) in a form of 5 diameter mm and 18 height mm, with nucleus of adjustable ferrite. The form for this coil can be obtained in old radios and televisions. The coil L2 that determines the frequency of the channel can have from 2 to 6 you exhale of thread 18 to 22, in used similar form her for L1. For 2 or 3 you exhale we will have the operation in the high channels of VHF, between the 7 and the 13, and for 4 to 6 you breathe the operation it will be among the channels 2 and 6. For the entrance of audio and video important ‘to use armored threads and appropriate connectors. For the antenna to use a connector for cable of 75 Ohms. The transmission section should not be in the same box that the source. The transmitter should be shielded for larger operation stability and to avoid the reception of noises that affect the transmission. The trim pot P1 has for purpose to adjust to component DC of the video sign, important to obtain the maximum revenue in the transmission.
Adjust alignment of the video, audio, the coils, antenna and frequency of the operation
To test and to adjust the transmitter, call in your entrance the exits of audio and video of a videocassette with any ribbon of good quality. Use appropriate cables. Tie the transmitter and, in that she intend to do the operation. The antenna of the transmitter can be a piece of some centimeters of covered common thread. Adjust the coil initially L1 so that the sign of maxim image intensity is captured. Adjustment the way trimpot to obtain the best image. Then, adjust slowly the coil L2 until obtaining the sound sign. Obtaining the sound sign, retouch the tuning of way L1 to obtain the maxim transmission. If it has difficulties in obtaining the tuning in the wanted channel, alter the value of C7 or I number it of you exhale of L1. If it has difficulties in tuning in the sound, alter C6 or then the number of you exhale of L2.
LIST OF MATERIAL
Power supply
- CI1 – 7812 or 7815
- D1 D4 1n4004 or equivalent
- C1 C4 – 10NF
- C5 – 2200µF TO 4700µF / 50 V
- C6 – 100NF
- C7 – 100µF 35V
- C8 – 100NF
- L1 – Shock of RF 100µH / 1 THE
- T1 – Transformer, primary in agreement with the net (110/220) and secondary from 18 to 24V / 1A
- Several – printed circuit board, threads, box, etc.
Semiconductors:
- Cl1 – LM1889- Integrated Circuit – Modulator of Video
- Q1, Q2, Q4 – BC547 or equivalent
- Q3 – BD135 or equivalent, example 2n2218 (different Pinage)
- D1 – BB809 VARICAP or equivalent
Resistors 1/8W, 5%:
- R1, R15 – 82 Ohms-
- R2 – 120 K Ohms
- R3 – 27K Ohms
- R4 – 1,2 K Ohms
- R5, R13, R14 – 270
- R6 – 470 Ohms
- R7 – 56k Ohms
- R8 – 68k Ohms
- R9 – 220 K Ohms
- R10 – 2,7 K Ohms
- R11, R17 – 1 K Ohms
- R12, R16. R19 – 10 K Ohms
- R18 – 47 Ohms x 1W
- R20 – 22 K Ohms
- P1 – 10 K Ohms trimpot
Capacitors:
- C1 – 4,7µF/1 6V – ELECTROLYTIC
- C2, C3 – 10µF/1 6V – ELECTROLYTIC
- C4, C8 – 1 nF – ceramic
- C5 – 120 pF – ceramic
- C6 – 47 pF – ceramic
- C7 – 56 pF or 47 pF – ceramic
Several:
- L1, L2 – Coils – L1 – 0.3µH and L2 – 7 to 13µH.
- XRF – it Collides of 100 µH – to see text.
- Several -printed board circuit , entrance Jacques and output of signs, box for assembly, power source, forms for the coils, enameled threads, etc.
