Mramos, that one transistor transmitter is pretty cute. You have to admire circuits that are so frugal with parts. But I can see that it will have poor frequency stability, depending on what is moving around the antenna. This is because the antenna is tightly coupled to the oscillator and its impedance will partly determine, with the coil, what the frequency of oscillation is. Since this thing will likely oscillate at low VHF frequencies, I guess, the antenna impedance will vary if something moves nearby it (like within about 3 feet or so).

This may not be a problem for some applications but it means that the receiver will have to be fairly wide bandwidth to insure reception under all conditions. Wide bandwidth in a receiver means less sensitivity, and maybe less range, but then it can also be simple. Lots of tradeoffs to make!

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RadioRon

 

what about Microchip's rfPic's?

 

Thanks for suggesting the microchip parts. I have not looked at those before so this was a good excuse to review some new parts.

The rfPIC series appears to be a very easy way to get a UHF transmitter. There are very few parts outside the PIC and with a bit of care, you should be able to make it work. The frequencies and powers available are suitable for a short range wireless one-way link, the same kind of range that you would get from a good garage-door opener, maybe a bit more depending on how good your antennas are.

For the matching receiver, I looked at rfRXD0420/0920. This part is not so simple to implement, but within the capabilities of someone who is determined and follows the advice in the datasheet very closely. For chips like this, it is essential (I mean really critical!) that you use a good pcb layout. The chances of undesired oscillation or poor noise performance are quite high if the layout isn't right. For this reason, I recommend that you ask microchip for an example layout since I didn't see one on the data sheet. They usually have such a thing, so it is worth asking or looking through their site. Copy their example layout precisely and the chances of making it work are very good.

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RadioRon

 

I looked the rfPIC's a few years ago, I thought they looked rather complicated, and required too many external components. Like you say, the receivers are even more complicated though.

In the end I decided to go with licence free radio modules, probably cheaper than an rfPIC solution?.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I agree that using modules is the way to go if you want it to work first time and reliably. Implementing a transmitter or receiver starting with a chip can be pretty frustrating for an RF novice without any instruments.

One other source of simple RF link hardware is one that I've used before, which is to recover the transmitter and receiver hardware from a scrap garage door opener. The receivers are usually pretty good, while the transmitter, which is the thing in the little pushbutton portable plastic case, is really simple. Of course, the data that this is designed to convey is usually one a few bits, which may not be much use.

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RadioRon

 

I've got a number of little transmitters in a box at work, not sure where they came from, but I don't have any receivers to go with them :cry:

The transmitters use the common little round metal transmitter module plus an encoder chip.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk