single transistor TV transmitter

Hi,

I am making a single transistor TV transmitter on VHF using the instructions at Micro Silent TV How to build the most simplest TV transmitter?

On first look, the circuit diagram uses non-standard conventions but I decided to try nevertheless. I am using the version on the bottom of the page

**broken link removed**

It took me half an hour to assemble it on a perf board. The circuit showed working signs upon first power on using the composite output from a DVD player. The TV detected a distorted image, obviously from my transmitter. Took me another 1 hour to adjust the trimmer resistors/capacitors to get a snowy but somewhat watchable display on my TV. The display is not distorted, in fact text from the DVD player are readable. The colors are also correct. The only problem is the snow and some artifacts on the display. Moving the transmitter closer to the TV does not help - in fact the same picture remains with the transmitter as far as 10m from the TV.

I am using a 2N3904 transistor and a 1k trimmer as I do not have a 500R trimmer. All other values are exact.

My TV is made around 1998. It has VHF/UHF with PAL/SECAM/NTSC. It supports both auto scan and manual scanning using the "+" and "-" key on the remote controller. In fact, the transmitter frequency is detectable using auto-scan.

Any ideas how I can improve the quality? Or is this simply the quality of this transmitter, or is my TV just not sensitive enough?

Considering the number of parts you're using you're never going to get a high quality image out of that, transmitters are complicated for a reason =) You should be able to get simple commercial units that will transmit over short ranges without too much difficulty.

Hi Sceadwian, do you perhaps know what exactly is the limitation in this circuit? Given that it's running on 1 transistor, I know that power and stability is a problem. The frequency changes when I touch the board or move close to the circuit without even touching, so I have to tune using a plastic screwdriver with my body far away from the circuit! But putting that aside, why is the picture snowy even when the circuit is very close to the TV, and the same picture remains when the transmitter is 10m away? I mean, if the transmitting power is so low, reception would have been lost within a few meters.

I built a one transistor FM transmitter a few weeks ago using this Simple 30 meters range FM transmitter, and the result is a surprisingly stable and functional short distance transmitter, even with a cheap auto scan radio.

Would the quality improve if I try the apparently better version How to build the simplest transmitter? on the same website?

The transmitting power is probably higher than you think. The picture is noisy because the circuit is minimalistic, it's not operating in a low noise region, increased complexity (far beyond what you have) is the only solution and that means excessive cost and time to develop. You're FAR better off NOT trying to develop this any further as you'll end up with more wasted time and money in the long run over a commercial product that will perform as you want it to.

If there were truly a very simple high reliability high quality circuit that does what you want it to then we'd be using them already in our set-top boxes already. If you need quality use a commercial product, period.

Thanks for the input

Try savaging an RF converter out of an old VCR thats what i use & cost nothing.

They work great, for channel 3 or 4.... Try making an antenna for that..

Some are in the UHF band & adjustable freq.

That's right debe, I remember seeing one in a catalog a ways back now, they're used in video surveillance systems typically.