Easy, Cheesy 433 MHz Radio Link


Senior Member
I've had some of these cheap (99 cents, free ship) modules forever but only tried them today: http://www.ebay.com/itm/433Mhz-RF-t...link-kit-for-Arduino-ARM-MCU-WL-/281533665689

Had very low expectations, having read Goeytex' comments that the cheesy super-regen receiver was junk. So I spent as little time as possible breadboarding and coding. Used rfout and rfin (Manchester coding) and son of a gun, I got good data reception up to ~25 feet indoors and 40 feet outdoors.

Hey Technical: I had to use a pair of 20M2's. Why can't the 08M2 support rfout & rfin?

I used this antenna design: http://www.instructables.com/id/433-MHz-Coil-loaded-antenna/ I just coiled wire wrap wire around a piece of a bamboo food skewer (whatever was laying around). Simple code for transmitter and receiver attached. Transmitter sends numbers 1-255 and repeats. Receiver blinks LED with each number received and displays numbers received in serial terminal.

My transmitter and receiver are both powered by 3x AA batteries (4.5V). Some sites (including Ebay seller) report the transmitter power/range can be improved by increasing the transmitter voltage up to 12V. I have not tried that yet.

#picaxe 20m2
pause 200

low b.7 ' LED off
rfin c.7,b0,b1,b2,b3,b4,b5,b6,b7
high b.7 ' LED on
sertxd (#b0,13,10)
pause 300 ' keep LED on a bit

for b0=1 to 255
rfout b.7,(b0,b1,b2,b3,b4,b5,b6,b7) ' transmit b0 plus 7 dummy values for rfout format
sertxd (#b0,13,10)
pause 500



Senior Member
Hey Technical: I had to use a pair of 20M2's. Why can't the 08M2 support rfout & rfin?
Or you could have used a 14M2, 18M2, 28X2 or 40X2.

The 08M2 has only 7Kbyte of Flash memory. The 14M2 and upwards have at least 14Kbyte of Flash so can fit in more functions.


Technical Support
Staff member
As srnet notes the 08M2's have a smaller amount of internal memory for firmware use, not enough to support all that some other PICAXE's do.


Senior Member
Great that they worked, but my past experiences with such cheapies is that range is very low,TX power is very weak (even at 4.5V), RX supply demand is 5V picky, and that the RX is very prone to interference or drifting significantly off matching frequency. We used the likes of these extensively ~12-15 years back in student projects, & FRUSTRATION usually abounded.

But now, & for only a few $, far superior,far more reliable, powerful & sensitive modules are available - here in NZ/Australia Dorji ASK offerings (with a NZ/Australian legal 25mW TX) have become standard for simple 433 MHz wireless data needs. These are good for ranges of ~100 meters thru' light vegetation & wooden buildings. Closer in performance can be so good that NO external antenna need be used either!

N.B. AFAIK the 433 MHz ISM band is not legal in the USA, except at tiny microWatt level powers.
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Senior Member
I'll share my final project in spite of any interest. :)

I made a smallish 12-button remote control using a 20M2 and one of these tiny transmitters. The case length was determined by the antenna length, and its cross section by the 9 volt battery. This was used to trigger 12 different digital outputs in a hidden receiver project for a client. It was desirable to hide the transmitter in hand to trigger the outputs stealthily. 4x3 buttons are easily read (plenty of input pins on a 20M2) plus I have a memory button on the end. Easily and stealthily pressed by thumb, it repeats the last command, also written to EEPROM to survive power down.

This was a rush job without much time to optimize, but long story short, the transmitter (wrapped in electrical tape), Picaxe PCB, battery and antenna were all jammed against each other for a near net fit under light compression :) . The transmitter got 9V, 5V regulated to Picaxe. It works GREAT, indoor range is 20 feet or more, even with the remote inside a tightly wrapped-hand (which is certainly blocking some signal output). Photos attached.

So my story ends happily, I'll use these again despite bad results reported from many other sources. Besides the module's performance, I'm gobsmacked (!) how well and easily the RFIN & RFOUT commands work to send data. There are no serial/baudrate/parity parameters to specify, it's all done for you.



Senior Member
Last photo showing internals. Is there a 2-photo limit per post? Couldn't add this to my previous post.

Transmitter w/electrical tape insulation is flat against the Picaxe PCB. Antenna (shown in first post) butted right up against the metal case of the 9V battery. Far from optimum, but it fit and it worked great!

I can hear the purists and scoffers chuckling already... :)

Edit: Current out of the battery was ~3 mA at idle, 10 mA when transmitting.


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Hey Technical: I had to use a pair of 20M2's. Why can't the 08M2 support rfout & rfin?
For just 60p from the PICAXE Store, you can get the tiny NKM2401 (8-pin PIC) which gives you the same Manchester encoding/decoding functions as in the larger PicAxe devices. The 14M2 does take up less overall real estate, but in some cases, it might be easier to get two 8-pin devices into a small space?


Senior Member
Those chips are dirt cheap! In this instance I needed 20M2s for their many I/O pins in addition to their RFIN/RFOUT commands. But those NKM chips might be handy for use with an 08M2.
I bought some little antenna's for cheap off ebay for these modules, $2 for 10 antennas. I get about 60' and threw the outside of the house with them now, used in a simple weather station. The micro is not picaxe forgive me.I wanted to send temp/humid/ and soil moisture from that raised bed into my house DSCF1316.JPG DSCF1313.JPG
its an Arduino - arrrrgh, but I think you got away with it :)

I construct my helical antennas with a pencil and use solid core wire. It makes (effectively) a bottom loaded antenna and electrically lengthens the antenna as a result of the inductance, not efficient but it helps to get a small foot-print. The best antenna will always have the maximum surface area - generally a quarter wave piece of wire, where at 433MHz a full wave = 70cm so 1/4 is 17.5cm, that then presents a better match to the Tx output stage.


Senior Member
If you want to wind your own helical antennas with stiff wire - skinny copperweld [iron inside copper outside] wire can be found in TV cable coax dropwire core at your local trash bin :) [test with magnet to see if iron core there...]


Senior Member
Legal: Although 433 MHz is indeed NOT a legal ISM band in the USA,consumer wireless devices using it however abound. Perhaps someone on the ground there may care to comment just why this occurs! (Here in NZ/Australia we are legally allowed a paint blistering 25 mW TX power)

Antenna: 433 MHz regs. say these are NOT allowed on transmitters, but can be added to receivers. This ruling is naturally "hazy" if the module comes with a swish external antenna socket etc - most users (myself included) have few qualms about adding a ~160mm 1/4 wave radiator.

A word of caution on DIY antenna -a stiff wire on a bench top module may be near impossible to see when working end on from above. To avoid a poke in the eye while you're tweaking circuitry perhaps add a small flag or bend over the wire top so accidental impact is less traumatic. (Many decades back I'd a ham radio mate who experienced major eye damage from a DIY UHF Yagi).

For backyard/crossroom applications I again mention that using more modern ASK modules may relieve you of the need for an external antenna anyway. Many recent ASK receivers are not only far more sensitive but are 3V friendly too,needing just a couple of AAs or a Li coin cell etc. Typical is a SpiritOn RXN3 using Princeton Tech. Corp's (PTC) PT-4302 RF engine, sold here in Aus/NZ via Jaycar. Older deaf modules demanding a picky 5V supply are a PAIN to work with in comparison...

You want links? Check the numerous offerings from Deal Extreme. The YXR15 receiver module below sells for ~US$3.99, & (to NZ at least) with free shipping.

Extra: for those new to 433 MHz it's worth mentioning that modules offered for this esteemed ISM band increasing may include sophisticated transceivers - Chinese firms like HopeRF & Dorji offer GFSK & LoRa™ modules of astounding performance. Although still quite cheap ($- low $$) these however can be a challenge to master. As mentioned endlessly before hence it's recommended to START SIMPLY using reliable ASK modules.

Stan. ( ZL2APS since 1967)


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Well-known member
Legal: This section of FCC Part 15 could apply:

§15.23 Home-built devices.

(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.

(b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of §15.5 apply to this equipment.
§15.212 Modular transmitters. (vi) The modular transmitter must be equipped with either a permanently affixed label or must be capable of electronically displaying its FCC identification number.


Senior Member
erco, how do you justify operating at 433MHz in the USA?
Hmmm... I guess that's why there's a paucity of applications for these modules! I see some other users, such as at http://www.instructables.com/id/RF-315433-MHz-Transmitter-receiver-Module-and-Ardu/ but of course they may not be in the USA.

My very low power transmitter will be used very rarely for very brief demos. I doubt if there will be any problem with interference. If I see an FCC truck heading my way, I'll know to not push the button!

But I thank you for bringing the matter to my attention, TCH!

You Brits are good with using 433 MHz, though. Correct?


Well-known member
... they may not be in the USA.... I doubt if there will be any problem with interference.
Here's the crux of the matter. That 433 MHz module is transmitting in the upper part of an Amateur Radio band. That is if you actually know what frequency it is transmitting on.

§97.201 Auxiliary station.

(a) Any amateur station licensed to a holder of a Technician, General, Advanced or Amateur Extra Class operator license may be an auxiliary station. A holder of a Technician, General, Advanced or Amateur Extra Class operator license may be the control operator of an auxiliary station, subject to the privileges of the class of operator license held.

(b) An auxiliary station may transmit only on the 2 m and shorter wavelength bands, except the 144.0-144.5 MHz, 145.8-146.0 MHz, 219-220 MHz, 222.00-222.15 MHz, 431-433 MHz, and 435-438 MHz segments.
You could be causing interference, but in true contemporary American fashion you can quote "It ain't illegal unless I get caught!"


Senior Member
Well, the law is the law. I'm still betting that my one-off prototype, rarely used, intermittent remote control signal is small potatoes to the FCC, given that Amazon.com lists 1,420 results for "433 mhz module" : http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=433+mhz+module

Including these commercial telemetry kits: http://www.amazon.com/Neewer®-3DRobotics-Telemetry-433MHZ-Module/dp/B00FFTLLIQ

and http://www.amazon.com/FLOUREON-Standard-Telemetry-Multicopter-Quadcopter/dp/B00SD2RUG4

How can those possibly be legal, given all these previous restrictions?
I think it's worth noting the FCC regulation is for a spot frequency of 433.5Mhz rather than a band and like the EU 433 is a shared band with e.g. radio amateurs. The modules generally available have a nominal 433 Mhz frequency and so I presume be sold legally, even though the exact frequency is likely to be an unknown but about 433Mhz.


Senior Member
Colleagues in the US tell me that parts of the 70cm Amateur bands (circa 433Mhz) are unuseable due to interferance and a high noise floor.

Do you think this could be caused by large numbers of 'very low power' and 'occasionally' used illegal transmitters ?
I think so, like the EU allocation, it is quite wide at ~430-440Mhz and when my cheap hand-held (Woxun) is monitoring GB3UB I can hear these devices going off all the time, it's a cacophony of sound and transmissions, that said with my Yaesu VX8R I hear no interference because it's selectively is so much greater. I've used my radio in the US (Washington, DC) and their bands are ultra busy compared with ours, I might hear one station a month in this area by comparison (70cms).


Well-known member
433 MHz Frequency Allocation in the USA

A page of the "Frequency Allocation" table referring to 420 to 450 MHz

There is this (emphasis mine):

§18.107   Definitions. ... (g) Consumer ISM equipment. A category of ISM equipment used or intended to be used by the general public in a residential environment, notwithstanding use in other areas. Examples are domestic microwave ovens, jewelry cleaners for home use, ultrasonic humidifiers.
§18.203   Equipment authorization.

(a) Consumer ISM equipment, unless otherwise specified, must be authorized under either the Declaration of Conformity or certification procedure prior to use or marketing. An application for certification shall be filed with the Commission on an FCC Form 731, pursuant to the relevant sections in part 2, subpart J of this chapter and shall also be accompanied by:
§18.301   Operating frequencies.

ISM equipment may be operated on any frequency above 9 kHz except as indicated in §18.303. The following frequency bands, in accordance with §2.106 of the rules, are allocated for use by ISM equipment:
ISM frequency	Tolerance
6.78 MHz	±15.0 kHz
13.56 MHz	±7.0 kHz
27.12 MHz	±163.0 kHz
40.68 MHz	±20.0 kHz

[COLOR="#FF0000"]915 MHz	±13.0 MHz[/COLOR]

2,450 MHz	±50.0 MHz
5,800 MHz	±75.0 MHz
24,125 MHz	±125.0 MHz
61.25 GHz	±250.0 MHz
122.50 GHz	±500.0 MHz
245.00 GHz	±1.0 GHz
A page of the "Frequency Allocation" table referring to 902 to 928 MHz


Senior Member
Even "down under" here in Wellington NZ wide band monitoring the 433 MHz ISM slot (433.050 - 434.790 MHz so 1.74 MHz wide) can yield an African dawn chorus of clicks,buzzes,beeps & tones - & even voice (presumed LPD433 ). Weak signals can even be detected here from 10km away clear across the harbour, with predictable surges in band noise presumably arising from garage doors & doorbells etc at commuter time.

My own slant as a ham (of almost 50 years experience) is that the assigned 10MHz available in the 430-440 MHz 70cm ham band still leaves (of course arguably!) "plenty of room".


Well-known member
erco, I've admired your projects and engineering talents here on the forum. I don't mean to be offensive to you personally, and of course you aren't alone.

Just consider the legal documents from the FCC (and other radio frequency authorities) as another DATASHEET in your project.

A great man recently said, "Well, the law is the law."

PS. Just because you can buy something, doesn't mean it is OK to operate with it. Engineers know better. Think of bolts, nuts, hardware stores and bridge engineers.

PPS. I'm done.


Senior Member
But regarding 433mHz rf regulations in the U.S., there is also this from a prior thread:
This refers to: Title 47: Telecommunication
Subpart C—Intentional Radiators
Radiated Emission Limits, Additional Provisions

§ 15.231 Periodic operation in the band 40.66–40.70 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]
I don't have any idea what "1,500 to 5,000 microvolts/meter Field strength of fundamental for 433mHz transmissions" means with regard to the devices under discussion, but if they are under that field strength, they apparently "may be employed for any type of operation".


Senior Member
Googling shows a 1990 era Code of Regulations that indicates these field strengths are measured at 3 metres (assumed from the transmitting antenna). 1 microVolt is -107dBm of course & represents the typical weak signal limit of most 433 MHz ASK receivers. Hence that "1,500 to 5,000 microvolts/meter Field strength of fundamental" would be a good signal at cross room distances.

Diverse converters (refer =>http://www.moseleysb.com/mb/mv2dbm.html ) relate this Field Strength to a mere ~100nW (n = nano) or ~1/10th of a microWatt transmitter power however. This is approx the same peanut power as many MP3 linked in car FM transmitters & drive by "talking signs". In most countries regs. limit these to an output lower than 100 µV/m at 30 meters (for ~1 microWatt output).

Significant forum discussion arose some years ago interpreting this! The consensus was that only very weak sub microWatt 433 MHz transmitters are legal in the USA.

FWIW however -(sigh...): A legal 433 MHz workaround of course is to use directional RX antenna or superior receivers &/or modulation schemes - most GFSK (Gaussian Freq. Shift Keying) types are good to -120dBm sensitivity, while LoRa™(Semtech's spread spectrum Long Range) easily stretch to 130+ dBm. Slower data rates help too. As every 6dB system gain doubles range then a ~24dBm link budget boost (from say -107dBm to -131dBm) would give 2x2x2x2 = 16 times the range. Instead of say 10 metres cross room you could reach 160 metres across the field.

Additionally transmitter battery life would be enormously enhanced at such low TX powers - witness 10/10 claims (10 years battery life & 10 miles range) by Microchip for their recently released RN2483 LoRa™ module. Although immensely versatile these darlings are easily serial fed- Mouser sell them at ~US$15 each .


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Well-known member
manuka, due to following all of your publishing over the last few years, I ended up with this module for my future expansion into wireless for my project. I don't know how I'm going to do it, just yet, but I'm confident that the design is PICAXE capable. Actually, this module is way overkill for what I'm doing, but its a start for me that I can expand the hardware into.

I just wanted to say "Thanks!" to you for your efforts to get the word out on RF modules.

PS. These are actually legal. That is important to me in my designing.

PPS. http://www.mouser.com/ds/2/268/70005219A-607960.pdf


Senior Member
Modest blush ! Yes - Lora™performance is astounding ! But does your "actually legal" mean at 434 MHz on very low TX powers or on US legal high power 868MHz ? (Microchip's RN2483 can apparently cover both). As my popular LoRa™ Instructable indicates, evaluation at least of serial capable LoRa™ modules is possible with merely a 08M.

All manner of AT tweaks are inbuilt in many cheap Chinese SPI interfaced LoRa™ modules, with H.A.B. SRNet (Stuart) in the UK the expert on such using HopeRF's. I assume these RN2483 offer similar- yet to be tried. Stan.


Well-known member
As Amateur Radio operators, we used to practice tracking down ignorant folks that didn’t understand that other people were following the rules so they didn’t interfere with each other’s gadgets. Not a single one of them had “bad intentions.”
I was about to say that where I live, about a mile away from a 70cm repeater that is very high up, I had awful problems with 433 modules being swamped, even by car key fob transmitters. Another thing the operating near the amateur satellite frequencies is dubious. After all though, in the uk the mod are primary users of this band, and I would think they would get a little agitated if they had real problems with interference. This, with a lot of amateurs having access to hundreds of watts into high gain antennas makes you wonder why these modules were ever designed for 433MHz. I found 868MHz to be more reliable even though it is the second harmonic of 433.