My FM Wireless Microphone V5 has been a very popular project with beginners and experienced constructors alike. The circuit is simple to build, works (almost) every time, and uses simple bipolar transistors in a basic AF amplifier, RF oscillator and an RF buffer. But this form of oscillator can be used or oscillators from a few kHz, through to over 600MHz. By modulating the base voltage, the transistors DC bias current changes, and with it changes the inter-electrode capacitances. This gives both AM and FM. For video we really want the AM.
So what use is 600MHz?
600MHz just happens to be in the middle of the TV band. Yes! This little circuit can be modified to become a TV transmitter with just a few minor changes. This means that your CCTV surveilance camera you bought for $10 can now be used for a few 100 metres and viewed on your TV set.
I must point out that this circuit is very crude and not designed for quality. I built it to do a job, and it did it. You should really have some form of output filtering, and a diode clap circuit at the video input. But this and the addition of sound I will leave to you. I am merely presenting you with a functional idea for further experimentation.
The circuit is very simple, it just has three transistors, but the transistor types have been changed for VHF/UHF transistors. You can use 2N2369, BSX19 or BSX20, or any small signal transistor with an "ft" of typically 400MHz or more. The transistor DC biasing is a little more tolerant to varying transistor types than the old V5 (FM) transmitter.
TR1 is a simple audio transistor, such as the BC547, BC108, or 2N2222. This transistor provides absolutely no gain, but it inverts the video modulation from the camera. Without it the video would have a negative modulation characteristic. The 4K7 pot adjusts the video level and sets the contrast you see on the telly. The 1u0 capacitor in the collector of TR1 to TR2 base can be reversed and moved to TR1 emitter if the modulation is inverted using your camera and TV system. You can alternatively remove TR1: I know nothing of the workings NTSC, or even the French version of PAL, other than the superficial info I shall give you down the page.
TR2 is the oscillator. Feedback is between collector and emitter, via the 2p7 capacitor. The 10p capacitor is a 2 - 10pf preset capacitor that sets the transmitter frequency. More about that in a minute. Modulation is applied to the base and causes the oscillator to vary in both frequency and amplitude. But this is an amateur hobby construction, not broadcasting standard. Note that in the circuit I connected the 10p cap to the +ve supply rail, but you can connect it to ground, whatever is the most convenient.
TR3 is a buffer stage, but this time it does provide a little gain. Not much, but again its main purpose is to buffer the antenna from the oscillator. In this way the transmitter has a good chance of pretending to be stable :-). The collector of this stage is taken to the antenna through a DC blocking capacitor.
You may have noticed that the V5 FM "bug" is biased to 1/2 the supply voltage, but here TR2 is biased somewhat lower. This has the effect of increasing the modulation depth. Higher frequency operation also increases the modulation depth.
Another point is that more stable biasing is used in all three stages, which makes the project more tolerant to different transistor types. This is achieved by adding emitter resistors and a fixed voltage divider for transistor bases.
The only coil (L1) in the collector of TR2 sets the operating frequency, in conjunction with the 10pf trimmer capacitor. If L1 is made to be 3 turns of 0.8mm Dia. enamelled wire on a 5mm Dia. air-cored former, (tapping = 1/2 turn) then you will cover the VHF low band, something like channels 3 to 9. The unit covers about 120MHz to 210MHz.
With L1 being 3/4 turn - that is 0.8mm Dia. enamelled wire, 24mm long, bent into a "U" loop around a 6mm drill bit, you will cover channels S11 to S32 (200MHz to 400MHz). Note that these frequencies are approximate, and vary from opinion to opinion, and depending upon which country you are in. If you are in the UK, then I believe you don't have anything lower than channel S21 (310MHz). Use the smallest coil.
I took a bit of copper-clad board, 20mm x 50mm and built a rats-nest (ugly, birdsnest, etc) for the prototype. It looks a mess, so please give me a little time before I publish a picture of it. I took a 50mm x 3mm bit of copper-clad board and used this as the +9v rail, with the 10uf connected to the middle, and the 1n0 and 10n caps either end. These three caps hold the +9v rail in place on the board. All the other components were just tacked to the board.
Keep all wire leads as short as possible. If you don't then the frequency will be too low. Long leads are your enemy, especially around TR2 collector. I began in the middle and built TR2 oscillator first, got it to jam SVT1, (E6, VHF band III) then screwed it up a couple of channels.
The camera I used was a simple colour CMOS surveilance camera, having a resolution of 628 x 582 pixels, 380 lines. The camera gives out a composite low-res PAL video signal, plus audio, and is powered from the same 9v as the transmitter. The camera came with a mains battery eliminator, but I chucked it in the bin. It got a too bit warm for my liking, like most of these cheap'n nasty things.
Since then I have plugged in my DV camcorder, with CCD sensor and 500x zoom. I works great for filming from the workshop to the video editing computer upstairs. Not as good quality as the Firewire output direct, but it is nice and portable.
You want a TV transmitter? Now you just go and build one. It is simple, easy, and you should have absolutely no problems building it. I will add to the circuit and project files when I do more work in the workshop, but at the moment I am busy with my soldering video. This project was a needed tool for the video project. It has no frills, but I have provided it as a start for you to experiment.
As a matter of interest, if your country is switching OFF the analogue TV and forcing you to buy a new "box" for each and every TV or video recorder, then perhaps you and a few neighbours can get together and each "broadcast" one channel for the benefit of all. Think about it - it is a solution to the TV profiteers - If you have 9 neighbours then you all get 10 channels and only one box :-)
There are three analogue TV systems in use today, and these are NTSC, PAL and SECAM. NTSC was the first TV system to see commercial use, but the PAL/SECAM system gives an improved performance, both in picture resolution and in the contrast possible. Of course, the Americans have stuck to their NTSC, while the rest of the world have adopted the PAL system. PAL, is just a modified and somewhat improved version of NTSC. It was developed in Germany and is used in Britain and most of Western Europe. Except for Brazil, all of the PAL systems have 625 lines.From what I understand, SECAM is the French invention designed to stop the British from watching their TV programs from the other side of the English Channel. They just had to be different! Basically, they just have two squashed pictures on the one screen.
NTSC, PAL and SECAM are of course initialisms, meaning:
|Never Twice the|
|PAL||Phase Alternating Line||Pity About the Losses|
|System Essentially Contrary|
to American Methods
|Colour Modulation||Suppressed Quadrature Modulation|
|Color Signal Freq.||3.579545MHz|
|Colour Modulation||Suppressed Quadrature Modulation|
|Color Signal Freq.||4.43361875MHz|
|Video Bandwidth (MHz)||5.0, 5.5, 6.0, 4.2|
|Sound Carrier (MHz)||5.5, 6.0, 6.6, 4.5|
NTSC is used in the United States, Canada, Mexico, Cuba, Panama, Japan, the Philippines, Puerto Rico, and South America.
PAL is used in Afghanistan, Algeria, Argentina, Australia, Austria, Bahrain, Bangladesh, Belgium, Sultanate of Brunei, Cameroon, Canary Islands, China (inc. Hong Kong), Cyprus, Denmark, Finland, Germany, Ghiana, Gibraltar, Greece, Greenland, Iceland, India, Indonesia, Ireland, Israel, Italy, Jordan, Kenya, Korea, Kuwait, Liberia, Luxembourg, Madeira, New Zealand, Nigeria, Norway, Sultanate of Oman, Pakistan, Paraguay, Portugal, Qatar, Saudi Arabia, Sierra Leone, Singapore, South Africa, Spain, Sri Lanka, Sudan, Swaziland, Tanzania, Thailand, Turkey, Uganda, UAE, UK, Uruguay, Yemen, Yugoslavia, Zambia, and Zimbabwe.
In general, if you have a 60Hz mains power frequency then you will have an NTSC TV system. If you have a 50Hz mains power system then you have PAL. If you live in South Africa remember that the TV system does not vary from day to day.
Whatever system you use, analogue TV is an analogue signal which can be amplitude modulated on an RF carrier and transmitted. All analogue television systems use "vestigial sideband modulation", which is a form of amplitude modulation. The lower sideband is incompletely suppressed. This provides a small band between the video carrier and the bottom frequency in the channel. When television was developed, the vestigial sideband was easier to accomplish than true single-sideband modulation, but there is absolutely no reason for it except for compatibility with existing technology.
Sound is a normal WideBand Frequency Modulation (WBFM) signal added to the video signal. The carrier frequency varies from country to country.
Very best regards from Harry Lythall
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