$5 transmitter/encoder module

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moxhamj

New Member
Matches with a -103dbm receiver which is kind of middle of the road receiver sensitivity. A good price. I bet they would work very well with picaxe. There is a comment on the website about just using a bit of wire 30-35cm for the antenna. I think it is better to actually calculate the length based on the frequency. V=F x lambda. V is the speed of light 299 792 458 m/s. F is the frequency in megahertz. Lambda is the wavelength. Round the speed of light to 300 million metres per second and the frequency to 300 million cycles per second and the wavelength of a 300Mhz wave is 1 metre. 434Mhz is a higher frequency so it is a bit shorter. ie 300/434 shorter. So 434Mhz is 0.6928 metres. Divide by 4 gives 17.3cm.
 
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hippy

hippy

Ex-Staff (retired)
Dr_Acula said:
There is a comment on the website about just using a bit of wire 30-35cm for the antenna. I think it is better to actually calculate the length based on the frequency ... So 434Mhz is 0.6928 metres. Divide by 4 gives 17.3cm.
Click to expand...
2 x 17.3 = 34.6 so presumably that 30-35 is for half-wave rather than quarter-wave.

When I was playing with my 433MHz modules I couldn't find any easy to understand explanations on whether it was better to use full, half or quarter-wave aerials and I expect a lot of RF newbies are in the same boat.

I ended up using BNC connected "rubber duck" aerials for 70cm band, about 9cm long so I guess a coiled 17cm aerial inside, quarter wave. Seemed to work for me.

I also tried some 1 inch long "stubby" aerials ( came off old 434MHz pagers ) which I presume are the same with tighter coiling ? They seemed to work okay as well.
 
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MFB

Senior Member
These transmitter and receiver modules should indeed be easy to interface to PICAXE serial input and output ports. The onboard ASK modems enable wireless coms without software trickery (to maintain the average dc level at the input of receiver's bit-slicer).

To quote from SparkFun "We have used these modules extensively and have been very impressed with their ease of use and direct interface to an MCU. The theory of operation is very simple. What the transmitter 'sees' on its data pin is what the receiver outputs on its data pin. If you can configure the UART module on a PIC, you have an instant wireless data connection"
 
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moxhamj

New Member
Re "What the transmitter 'sees' on its data pin is what the receiver outputs on its data pin. If you can configure the UART module on a PIC, you have an instant wireless data connection"

There sometimes is some confusion about what that actually means. Certainly, in the middle of a data packet, if the line on the input goes from low to high, the line at the other end will go from low to high.

What isn't entirely clear is how the device handles DC. If you have a low on the input that lasts for 10 seconds, do you get a low at the output?

I'm not sure ASK works like that, and the test would be whether you could use the module in place of the dowload cable and handle the Break command properly. Even FSK can be a bit vauge - are you talking about shifting the frequency up in the megahertz to send 1s or 0s, or is the shift down in the audio range like the way old dial up modems used to work?

The other issue with modules is what you get on the receiver when there is no signal - do you get a low signal, a high signal or white noise?

I suspect these modules are just fine for 99% of picaxe applications, as long as you do all the tricks that manuka et al suggest - eg putting Us before the packet, keeping the packets short, happy to live with a picaxe hanging in serin etc.

I have yet to find any module that can replace the download cable, though there must be one out there.

Last night I managed to send an entire copy of Microsoft's original Basic, which is a 23k file, from one board to another at 4800 baud, and then get it back again intact. But there were a few tricks along the way - using the Hope RF modules, working within the parameter that they send data in packets of 32 bytes, putting in delays when needed, and using the Xmodem protocol which breaks files up into 128 byte blocks with checksums etc. So even smart modules like the Hope ones are not true transparent replacements for a cable.

For picaxe work though, where the packets are less than 15 bytes and you can put UUUUU at the beginning, these modules would be perfect. Anyone care do do some range tests?
 
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manuka

Senior Member
Bravo Dr_A! My much used Wellington Harbour eagles nest antenna test range awaits! "Deaf" receivers are usually the limiting factor with 433 datacomms however.
 
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moxhamj

New Member
Agree. I'm starting to get the hang of what "deaf" means. -93db on the Rx is deaf. -103db is middle of the road. -110db is very good. And those Friendcom modules claim -119db which would be quite amazing.

Given most data Tx units try to stay under the legal 10mW, the receiver sensitivity is critical.

Knowing this, you can then work out if a particular module is a bargain or not. How many $ per -db of sensitivity are you paying? This is all part of the black art and mystery of radio communications.
 
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Dippy

Moderator
Well, at that price they're cheap enough to buy a few and try.
So much more productive than theorising.

But don't panic, I'm sure the Hopes are far superior... your baby won't be usurped yet :)
 
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moxhamj

New Member
Actually, I'm not sure the Hope ones necessarily are superior. They are just the first solution I've got working that can send big slabs of data.

For instance http://www.radiometrix.co.uk/products/smx2.htm are quoting -120db. That is the best sensitivity I have seen. Just need to get a price (preferably in Oz Dollars, and preferably on stock that was bought 3 months ago when the Oz dollar was worth twice what it is now. Darn those currency speculators.)
 
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manuka

Senior Member
I recall those Radiometrix modules are priced at USAF $1000 hammer levels-allow $$$ rather than $($).
 
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