Hi everybody,
I hope somebody can help me to understand the differences in RF circuits.
For a project I use cheap 433mhz modules to send data over small distances up to 50cm but through some thin sheet metal and everything works perfect. The transmitter I bought looks like this
Now I want to design my own pcb with all used parts on it.
On the web I found different circuits for the RF transmitter.
I found a single picture wich, from looking at the PCB, seems to be the circuit used in the module I bought, but my module is missing L2 and C1 (it has the necessary pads to solder them on the PCB).
I found transmitters that look pretty much identical but one has two inductors and one has none at all.
Additionally I found a lot of circuits that look like this
which seems to be kind of standard circuit for transmitters.
After all I'm not sure which circuit I should use.
Are the benefits or disadvantages of the different circuits?
I guess copying the module I bought would be the easiest way, because I could also copy the exact values of resistors, capacitors and inductors.
First off where in the world are you. In many places it is illegal to build a transmitter without a license. Don't confuse building with operating, in the license free bands.
It is hard to comment as non of your pictures showed up. Read the image guide for how to post images here, as well as How to use this forum.
Building circuits to work at 433MHz is not an easy task, you need a printed circuit board and some expensive test equipment.
send data over small distances up to 50cm but through some thin sheet metal and everything works perfect.
You will find that the signal did not go through the sheet metal but the signal might get through by being bounced off other surfaces and getting round the metal.
Are the benefits or disadvantages of the different circuits?
Yes there are. A lot depends on your components, tolerances, layout, and the material the PCB is made of. Some circuits are more tolerant of some sort of deficiencies.
In short buy don't build if you want to stay legal and have not got the skills to work at such high frequencies.
I second Grumpy_Mike's recommendation here Terence. There's nothing dishonorable about removing the header pins and soldering your working transmitter and receiver modules directly onto another circuit board -- and it may save you a lot of agony (and probably some money as well!).
Otherwise though, there's nothing preventing you from building your own transmitter and receiver, at the very low powers we're talking about here -- unless you're planning to build a lot of them to sell. Then, as has been noted, you may need some sort of government certification of your circuit board designs.
The transmitter looks to me like a "Colpitts oscillator" (see here ) tuned by your SAW filter, and I suspect your module costs less than you would pay for the individual components if you bought them separately. The oscillator is just turned on and off by the "TX" signal.
The receiver module is based on a dual op-amp (the LM358 in the photo). You can get some idea of how it works here. The receiver doesn't appear to be "tuned" very well to 433MHz. Try this experiment: bring your mobile phone near it and see how it behaves when you make a call.
Hope some of that is helpful. If you're only making one or two of your designs, why not just build on a piece of "vectorboard" like this:
CurtCarpenter:
Otherwise though, there's nothing preventing you from building your own transmitter and receiver, at the very low powers we're talking about here -- unless you're planning to build a lot of them to sell.
As has already been mentioned, in a lot of places in the World it can be illegal to build a radio transmitter.
There are some exceptions for those that have Amateur radio licences, but the OP did not say if they had one or where in the World they were.
There is no universal, Worldwide exemption for such devices based on power level that I am aware of.
So whilst you can build such a device, it may well be illegal.
In the EU changes in the law last year would require even a home built device to follow the CE approval process.
I know of no law in any country, srnet, that forbids the construction of a Colpitts oscillator -- and that's all that is being discussed here. If you can present a concrete example to the contrary, I'd be interested.
Different story if you want to sell them of course, but even then I doubt if any of those 433MHz modules from eBay or Ailexpress have certifications.
You can, in fact and if you want to, build a working "transmitter" out of a single 74AC04 IC that will be useful over a few meters or so. It can serve as a useful demonstration of how just about anything can be made to radiate (whether you want it to or not).
Rather than spread fear of some imaginary technology police, try the 74AC04 experiment. As I recall, something like a 45cm length of hookup wire will serve as a quarter wave whip -- no other components required. And your 'scope with a similar antenna at the other side of the room can serve as a receiver.
I know of no law in any country, srnet, that forbids the construction of a Colpitts oscillator -- and that's all that is being discussed here.
Once you put any form of antenna on it then it becomes illegal in the EU. In the USA there are provisions for from FCC about building very low power transmitters, although getting them to run in accordance with this limits takes a lot of expensive test equipment.
However in the EU it is illegal to make any form of deliberate electromagnetic transmissions no matter what the power is. You need to get at least a ham radio license to do this.
Grumpy_Mike:
Once you put any form of antenna on it then it becomes illegal in the EU.
And yet those 433MHz modules that Terrance illustrates in his post are being sold in France, Belgium, Italy, Hungary, Greece, Poland, Spain, the U.K and other countries on the continent as you can easily discover with a little effort. And I don't think anyone has been taken to the Place de la République and strapped to the guillotine as a result.
Things radiate. Scotland Yard has better things to do. Get some perspective.
His record at being right when answering questions here is woeful. I would recommend people just ignore him.
And yet those 433MHz modules that Terrance illustrates in his post are being sold in France, Belgium, Italy, Hungary, Greece, Poland, Spain, the U.K and other countries on the continent as you can easily discover with a little effort.
Yes that is correct but you can not construct them yourself, that is what "build" means.
The reasoning that it is OK to do illegal things because your chance of being caught and punished is very small, is morally bankrupt.
Just for fun, here's a picture of a "transmitter" offering continuous coverage from about 1MHz to 120MHz. I used it in Europe for a while and somehow managed to avoid arrest! Maybe there was some uncertainty about the "antenna"
First off, those receivers suck. The superhet ones are much better - and the syn470-based ones are so cheap. Syn115 for transmitter is comparable to the cheap ones (surprisingly).
I actually made a board that mounted a tiny1634, syn115 transmitter, and syn470 receiver. Surprisingly (considering that it was RF and I didn't have a VNA yet) it worked almost as well as it would have if I'd just connected cheap off the shelf assembled modules to the board. The one I assembled is still running now!
In practical terms, it's smarter to just mount cheap modules for receiver and transmitter.
