Replace serin/serout with RF.

jims

Senior Member
I am using the attached project which has 2 Picaxe chips communicating via serin/serout and I want to replace the hardwired link with RF. The distance will be about 60 feet. Since I have no experience with RF I will appreciate constructive recommendations about what RF hardware I should buy in order to get started (there has been so much discussion about RF on the FORUM, that I don't know where to begin).
 

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manuka

Senior Member
Welcome ! I'm unsure if 433 MHz is yet allowed in US - 315 MHz may be your friend. However 60 feet should be doable with almost any cheapo 315/433 MHz data pair. The esteemed DS18B20 is very PICAXE friendly, & it's easy enough to send temperatures wirelessly this way - see here.

Numerous issues often arise to frustrate however! IMNSHO the key to RF success is to start simply - VERY SIMPLY - & make incremental progress. Some cheapo UHF data modules are a pain in the neck & should be avoided. More upmarket modules can be near overwhelming to configure ! I've been a fan of Chinese Dorji for some years, & usually recommend folks start with their cheap & cheerful ASK (Amplitude Shift Keying) modules. Check their web site => www.dorji.com/ or Tindie sales outlet

By chance however I've just been reworking a band sniffer based around a SpiritOn RXN3-B ( B=BIG) module that's persuaded into audio duties. This setup may be a great way to get started. It'll also allow checking of your local UHF environment,propagation path effects & eventual data transmitter performance.

Best you see the Forum's long running "sniffer" saga too !
Stan - ham ZL2APS since 1967
 

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hippy

Technical Support
Staff member
Two ERF modules ought to be able to do the job and you can easily add a URF to test sending to the LCD PICAXE and for verifying what the transmitting PICAXE is sending.

It is what I would choose in the same situation simply because being able to snoop on what is being sent and send data has great advantages over other options though it could also be done using dumb 433MHz-style modules or with added NKM2401 buffers or RFOUT/RFIN where supported.

There are other wireless modules as Manuka suggests and also serial-to-Bluetooth which some have had success with.
 

jims

Senior Member
Thanks...are these available from Rev Ed??
Two ERF modules ought to be able to do the job and you can easily add a URF to test sending to the LCD PICAXE and for verifying what the transmitting PICAXE is sending.

It is what I would choose in the same situation simply because being able to snoop on what is being sent and send data has great advantages over other options though it could also be done using dumb 433MHz-style modules or with added NKM2401 buffers or RFOUT/RFIN where supported.

There are other wireless modules as Manuka suggests and also serial-to-Bluetooth which some have had success with.
 

Goeytex

Senior Member
As I understand it, 433Mhz can be only be used legally in the US with an Amateur License,... EXCEPT possibly if the TX power is <= 0 dBm. This apparently does not stop manufacturers & suppliers from selling 433 Mhz RF modules with power levels up to 27 dBm ( or more) to unlicensed US buyers as may be witnessed by offerings on Ebay,Tindie,Alibaba, et al. My guess is that 433Mhz is being used illegally in the US by tens of thousands of US hobbyists who have purchased RF modules/ devices via the mentioned sources.

915 and 2400 MHz are legal in the US under Part 15 of the FCC code. The FCC code is freely available online but you need be be both an RF engineer and an attorney to make any sense of it.
 

lbenson

Senior Member
There is some discussion of the use of 433mHz in the U.S. in this thread: http://www.picaxeforum.co.uk/showthread.php?11057-Continuous-outside-433mhz-transmissions&highlight=regulation*

Particularly relevant posts are here:
This document here: http://fjallfoss.fcc.gov/edocs_public/attachmatch/FCC-04-98A1.pdf along with much other information about an FCC change relating to RFID in the 433.5-434.5 mHz range, states in paragraph 25: "The 433.5-434.5 MHz RFID band we are adopting falls within the 433-435 MHz band that ARRL has designated for auxiliary and repeater links. Auxiliary stations are required by the Commission&#8217;s rules to operate on a point-to-point basis and are permitted to operate with a maximum power of 50 watts. Because point-to-point operations typically use directional antennas, there is less likelihood of interference from other sources."

This document states that unlicensed 270-460 MHz band must have at least 30 times the dead time as opposed to active, with 10 seconds minimum between transmissions: http://www.radiotronix.com/downloads/RCTcommunicationtomicrocontroller.pdf

This ARRL document states: "The current Section 15.231(e) provisions for periodic radiators, however, permit field strengths of less than 5,000 uV/m at that frequency (measured at 3 meters), with duty cycles of less than one second, assuming a silent period between transmissions of at least 30 times the duration of the transmission." http://isotc.iso.org/livelink/livelink/fetch/2000/2122/327993/327946/4495289/4495082/4495496/31N14883.pdf?nodeid=4499240&vernum=0

I had heard of the 3-second-per-hour rule applying in Australia and, I think, Europe, and had heard that a similar rule applied to unlicensed 433 mHz transmissions in the U.S. Apparently, and loosely, "similar" means a maximum of 1 second on and 30 seconds off--much more liberal than the Australian regulation. An expansion of that time specificaly for RFID in commercial and industrial areas was FCC-approved in 2004. ARRL objected to that but did not prevail. There appears to be some power restrictions to the licensed ham use at this frequency.

Disclaimer: I don't know anything definitive about this, and would be glad to hear from those who do.
And here:
Here is a reference to the governing regulation: http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=cb7ed85f6c396203994cd264f44120fe&rgn=div8&view=text&node=47:1.0.1.1.14.3.240.20&idno=47

This refers to: Title 47: Telecommunication
PART 15&#8212;RADIO FREQUENCY DEVICES
Subpart C&#8212;Intentional Radiators
Radiated Emission Limits, Additional Provisions

§ 15.231 Periodic operation in the band 40.66&#8211;40.70 MHz and above 70 MHz.

Tables refer to permitted field strengths.

Two seconds per hour under certain conditions: "(3) Periodic transmissions at regular predetermined intervals are not permitted. However, polling or supervision transmissions, including data, to determine system integrity of transmitters used in security or safety applications are allowed if the total duration of transmissions does not exceed more than two seconds per hour for each transmitter. There is no limit on the number of individual transmissions, provided the total transmission time does not exceed two seconds per hour."

Exemption for weaker transmissions: "e) Intentional radiators may operate at a periodic rate exceeding that specified in paragraph (a) of this section and may be employed for any type of operation, including operation prohibited in paragraph (a) of this section, provided the intentional radiator complies with the provisions of paragraphs (b) through (d) of this section, except the field strength table in paragraph (b) of this section is replaced by the following:" [1,500 to 5,000 microvolts/meter Field strength of fundamental for 433mHz transmissions]
As noted, exactly what this means for particular 433 mHz modules remains unclear to me, in particular the relationship between "1,500 to 5,000 microvolts/meter Field strength of fundamental" and the more commonly expressed rating of dBm--or watts.

Also as noted above, 315mHz RF may be superior in the U.S. both because of interference because of the widespread use of 433mHz for such things as garage door openers and the high power use available to ham operators for specific purposes.
 
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jims

Senior Member
Thanks, Hippy...One more question. I have a data logger which stores logged data onto an EEPROM. When a logging mission is finished, I take the logger board to my PC and upload the EEPROM data to an Excel spread sheet using the Picaxe "data link" routine and the Picaxe AXE027 programming cable. Can I achieve this same "data link" type of operation with a URF/ERF pair"? If YES; I'll probably get a URF/ERF pair. I think I've got just enough information to become "dangerous".
Yes, ERF and URF modules are available from ourselves ...

www.picaxestore.com/rfa020
www.picaxestore.com/rfa030

Documentation at ...

www.picaxe.com/docs/RFA020.pdf
 

hippy

Technical Support
Staff member
You should be able to do the same; I believe the PICAXE side simply waits for a character to be received and then sends the data in response to that so it should work just as well by wireless as it does over a wire. You might need to add some pauses to the code so it has time to send packets and so you don't lose anything while it is sending.
 
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