433 MHz UHF Lost Model Radio Beacon




About: Retired educator/writer

Here's a simple 433.92 MHz short range radio beacon that may be handy for locating downed R/C planes, lost ballons, model rockets - or perhaps hidden transmitter "fox hunts"! 433.92 MHz is internationally a free band slot, and although only low power transmitters (10-25 mW) are legal, UHF signal punch thru' light vegetation etc is superior to 2.4 GHz microwaves.

It's based around a cheap (~US$5-10) low power (a few mW) Keymark/Spirit-On TXC1 data transmitter, fooled into transmitting audio tones generated by a cheap PICAXE-08M microcontroller. Ranges are up to a km line of sight, dropping to 100-200 metres thru' light vegetation and wooden buildings etc. Compared with flashing lights & beepers, the beauty of a wireless locator relates to all hours convenience & simple direction finding even thru' light vegetation etc. A cheap "sniffer" receiver ( based around a companion RXB1 Keymark receiver module) is shown at => http://www.picaxe.orconhosting.net.nz/433rx.htm

NOTE- keep this simple locator beacon approach in perspective! Do not expect it to locate the likes of your $$$$ FPV (First Person View) model plane downed miles from anywhere in deep woods. For serious locator use, more powerful transmitters & GPS encoding should be considered. These of course can be costly, have higher battery drain and may be tedious to configure .

UPDATE - early 2012: Chinese firm Dorji have recently released some cheap (< US$10 a matched TX/RX pair) ASK 433 MHz data modules that significantly improve this beacon! Their transmitter is particularly appealing, as it's power is boosted to a (legal) 25 mW, giving ranges some 4-5 times greater than the KeyMark/SpiritOn equivalent ! A sensitive UHF scanner can still detect it thru' light vegetation etc to ~500 metres, and LOS (line of sight) several km ! The Dorji receiver, which usefully can also be persuaded as a band monitor, is somewhat more sensitive than the Keymark as well.

Since these Dorji ASK modules have shown themselves clearly superior, more versatile, smaller and far cheaper they're now recommended instead of the Keymark! As circuitry & layout will need to be slightly changed however (see => www.picaxe.orconhosting.net.nz/dorjiask.htm & a Dorji based 433 MHz "sniffer" receiver => www.picaxe.orconhosting.net.nz/dorji-tonerx.jpg ), the Instructable is presently still Keymark module based.

Update -Nov. 2012 : A superior Dorji based tape measure antenna Instructable is now available => www.instructables.com/id/433-MHz-tape-measure-antenna-suits-UHF-transmitte/

Update -Sept 2014: An improved PT4302 engined SpiritOn RXN3-B module is now stocked as Jaycar's ZW3102 receiver. Performance has been found very pleasing, & the versatile supply voltage ( 2.4V-5.5V) is especially welcome. Refer RSSI wiring details under comments at this Instructable end.RECOMMENDED!

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Step 1:

Classic Keymark ASK data modules are only modest performers, but they're cheap (~ US$5-10), very popular, widely available,reliable and easy to use. More sophisticated powerful and sensitive 433 MHz transceiver modules are now being marketed by the likes of Appcon & HopeRF,but these new offerings can be a real pain to configure!

Step 2: Circuitry

A popular & cheap (~ US$3) PICAXE-08M can modulate the 433 transmitter with a wide range of encoded beeps, tones, morse ID, tunes or even Sequentional Multi-Tone (SMT) Hellschreiber! Refer the makers web site (UK firm Rev.Ed) at =>http://www.picaxe.com , with skinflint programming details => http://www.picaxe.orconhosting.net.nzThe PICAXE can further allow microAmp level "snoozing" between locator transmissions, potentially thus offering greatly enhanced beacon battery life.

Step 3:

Assembly onto solderless breadboard is initially recommended for evalution - all the UHF circuitry is on the TXC1 module, with just simple signals & voltages elsewhere. Circuit layout is not critical,so even "rats nest" layouts to fit available space may do,but a dedicated PCB is being considered. This may assist those who want a compact and easily assembled device.

The trial shown here explored attaching the module directly to the antenna (a "Slim JIM") - no real benefits arose however.

Step 4:

Although many may cringe with this dated hobbiest technique, assembly can be made onto a 10 hole x 12 hole "stamp of Veroboard. A PICAXE programming socket (made from a cut down DIP8 IC socket) is even provided for easy program tweaking. Note -the 3V lithium cell shown here is cheap,light and convenient, but has only limited service life of course. With low transmission duty cycles (averaging perhaps a current drain of under a mA) a few days of beacon service may be expected.

Step 5:

Here's the matching Veroboard "stamp" layout- note that the mid board conductive strips MUST be broken or short circuits WILL occur! The classic way to do this is with a drill bit spun between your fingers. Ensure all copper scraps are removed. The PICAXE-08M has it's I/O channels referred to as "pins" - pin 4 here is feeding the TXC1 module Vcc supply voltage, while pin 2 delivers the tones and pin 1 has a "spare" LED to perhaps indicate beacon action pre lift off. This LED could be swapped for a pulsed ultrabright version to assist in after dark location.

Pre programmed PICAXE control of these channels allows tremendous versatility. The beacon could even hibernate after lift off, only turning on perhaps after 30 minutes (when the model may be assumed lost).

Step 6:

The controlling PICAXE naturally will need to be programmed, although commands such as HIGH, LOW, SLEEP and GOTO are easy enough to master! The high level PICAXE Editor (download free from http://www.picaxe.com ) runs on even an old W98 clunker PC, and is very user friendly. The program is passed into the PICAXE via a simple 3 wire serial lead- the DIY one used here is made from some ribbon cable & 3 header pins. PCs without a D9 serial port will need a USB-serial adapter of course..

Step 7:

For simple use a ¼ wavelength vertical antenna can be made from a scrap of wire. A length of about 170 mm should do it- this is just under an adult handspan. The antenna works best when elevated and in the clear, but even a flexible wire may do OK for nearby (~50-100 metres) locations.

Do not expect brilliant performance unless the TX antenna and receiver have a good propagation path between each other! Ranges in cluttered terrain may well be quite small- although even then invaluable when trying to find a nearby downed plane/rocket/balloon in tall grass etc.

Step 8:

Superior performance results however when the antenna is elevated. For pre arranged hidden transmitter "fox hunting" a "Slim JIM" type can be made onto a wooden strip or peice of bamboo etc. Slim JIM radiation is near horizontal so this type may not suit vertical hunts so well!

Step 9:

Weather protection can be made by encasing the entire beacon and antenna inside a plastic pipe offcut. Battery life with cheap AA cells may be months if low duty cycles are programmed. Note - 4 x fresh AA cells = 6 Volts of course, which is above the rated supply of both the PICAXE & 433 TX module. Using a dummy cell (a nail inside a piece of bamboo) reduces this to 4½ V, but still allows 4 older cells or NiCd/NiMH types to be inserted.

Step 10:

A slim JIM antenna, along with the attached beacon transmitter & battery pack, can be hauled up a tree with a light cord. It may be even possible to solar power the setup of course, but naturally good sunlight would be needed, which is not often available under a forest canopy!

Step 11: Companion "sniffer" Receiver

A UHF scanner or ham radio ($$-$$$) suits reception of the locator, but for tight budgets & modest needs consider a simple "sniffer" receiver. Aside from use here, these may be very handy for assorted 433 MHz device checking as well- wireless doorbells,back yard weather stations, energy meters etc. Output from the companion ASK Keymark RXB1 receiver can readily drive a piezo sounder when simply NPN boosted, and an undocumented RSSI tap may even be handy. See details =>http://www.picaxe.orconhosting.net.nz/433rx.htm

Step 12:

A simple 170mm whip can again be used on the receiver, but a "cotanga" Yagi antenna made from coat hanger wire will usefully boost ranges and allow better beaming. See Yagi constructional details =>http://www.picaxe.orconhosting.net.nz/yagi433.jpg

Step 13: YouTube

Check the YouTube field testing, c/w research assistant "Jack" =>

http://www.youtube.com/watch?v=rXhKWvZAl1M For receiving clarity a morse "HI" ( = .... / .. ) was PICAXE generated, as the hillside test site experiences considerable 433 MHz "hash" from Wellington city -some 10km away cross harbour! The 170mm whip transmitter was not detectable at 500m LOS (line of sight) with the RX whip, but easily heard with the ~ 6dB gain Yagi.

Update- October 2012: Use of the 25mW Dorji ASK transmitter in the beacon gave far superior performance, with the TX beacon signal then easily heard LOS to several km- even without a directional antenna ! The Dorji TX module is hence recommended instead- it's also cheaper!

UPDATE #2 - Sept. 2014: Aust/NZ electronics outlet now stock a pin for pin compatible PT4302 RF engined SpiritOn module as their ZW3102. RSSI (received signal strength) on this module has turned out as a breeze to organise- refer pictures! Signal clarity and module supply versatility (down to 2.4V) are appealing, and sensitivity is quite amazing. A Darlington pair driven LED allows visual signal confirmation. Reception was found good thru' light vegetation & wooden buildings to 100m, and LOS ( line of sight) signals were still "armchair copy" at 500m !

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    27 Discussions


    4 years ago on Introduction

    Recently released LoRa™ devices are showing amazing range boosts (~ x10 ) that of regular 433 MHz setups. Yes - even in built up areas &/or vegetation, signal coverage may be 100s of metres rather than 10s !

    Although somewhat more costly (US$20 range), for more demanding beacon needs these SX127x based modules are looking a wiser approach. They're easy to drive with a simple PICAXE micro too.

    See => https://www.instructables.com/id/Introducing-LoRa-...


    4 years ago on Introduction

    "Sniffer" details for the new ZW3102 ( SpiritOn RXN3-B) module.


    4 years ago on Introduction

    Jaycar's classic ZW-3102 receiver module has recently been upgraded with a new PT-4302 RF IC. The module is still pin for pin compatible with the orginal Keymark one, & performance may even be superior. It pleasingly accepts a supply between 2.4 -5.5V, but the RSSI tap seems not as straight forward. ( Refer picture)


    Question 1 year ago on Introduction

    Hey, I am having trouble finding the parts to create this device. Could you send me a parts list with the parts you would use if you were going to make this right now?


    5 years ago on Introduction

    I just found out about the concept of transmitter hunting last week and I am attempting to absorb a lot of information. I was hoping to get a little help from this group. I like the idea of the "fox" hunt but would like to scale it down for kids. I am looking to have the hunt limited to a medium size field and have it for teams of 9-13 year olds. Range 100-200 Ft and fairly robust equipment. No licenses for ham radio. I am looking at WiFi finders, Bluetooth, etc. Any suggestions?



    5 years ago on Introduction


    I'm on lookout for a simple RF beacon transmitter and receiver. I want to track a device that will be require 0.5km to 1km tracking on ground. It should be small to fit on 0.5-2inch plate (both sides) and weight should be less than 15gram. Any suggestions or help is much appreciated.


    5 years ago on Introduction

    Phew-this sounds a big ask. What is this robot intended to DO? Do you have pictures & schematics available? Where are you living? Any help available locally? Budget? Resources? Who is the robot for ? Whees or "walking" type?

    Realistically it may take you ages just to get to grips with the nature of microcontrollers (& their programming). Linking them to devices which will control the motors (stepper types are considered better) may be even more of a challenge. The robotic rover at => http://www.picaxe.orconhosting.net.nz/rover.jpg took 6 months & US$1000 to construct.

    Best you start with micros REALLY SIMPLY! There are numerous types (Arduino etc), but I usually find the PICAXE family the best starters - refer to the UK makers site => www.picaxe.com


    5 years ago on Introduction

    I’m working on a project which is about making a rescue
    robot . I have struggling with it for 4 months and now all the mechanical works
    had been done . but I’m facing difficulty in make the robot to work with rf
    module can u plzz help me…

    I want make transmitter and receiver that can control the
    motion…I mean that receiver should have control 8 dc gear motor and each dc
    motor should have a forward and backward botton to control the motion ……can u
    kindly help me with this project ….if u can make the circuit board diagram for
    the PCB ….or any other suggestion plz reply……mail me at “sunny1995gagan@gmail.com”

    2 replies

    Reply 5 years ago on Introduction

    A "rescue robot"! Sounds useful but just how big ? Actions ? Power supply? Range ? Budget? Time frame? End use ? Your wireless data experience ? Best tell me more!

    Reliable wireless control of such devices may be quite a challenge I'm afraid...


    Reply 5 years ago on Introduction

    The rescue robot is about 80 cm in length and 55 cm in breadth. it is powered by 12 Volt dc gear motor. Till now I have invested $120 for this project and all the work has been completed but the wireless part is still incomplete.

    I want all the 8 motor to work separately., and each motor should have a forward as well as backward button.
    Truly speaking I'm new to these microcontroller so can u tell me the easiest way to control these 8 motors. rf module will be best... As it's cheap and simple to use..

    Plz help tell me the easiest way u can
    Thnx for ur active participation. To this problem


    6 years ago on Introduction

    Thanks for the recent update . . . this is an awsome instructable . . .
    I am thinking this would work perfect for a "hide and seek" sort of game for my kids - we are working on some map reading and orientation skills - before a big backpacking trip.
    Would this be a good fit for a game where I used 6 or 9 of the transmitters as hidden "treasures" (say in a waterproof box or pvc pipe) then gave the kids a map with the locations marked and let them have a receiver where a LED light or group of lights would come on as they got closer???
    I have seen some references (including one in the link below) to these being used in "fox hunts" or geocaching - but hadn't seen anything specific so as a relative newbie I thought I maybe should confirm. :-)
    Thanks again for all the great information!!

    1 reply

    Reply 6 years ago on Introduction

    Gizmobot: Happy to help- combining electronics with the outdoors makes for an appealing activity for high energy youngsters!

    Further sleuthing with the Dorji module has revealed a valuable signal strength tap that suits enhanced work- refer attached pix. In conjunction with a 433 MHz tape measure Yagi antenna absolutely remarkable performance has resulted.

    I'm still organizing this as a brand new Instructable, but have uploaded pix to =>https://picasaweb.google.com/113235065280579741262/Dorji_DF



    6 years ago on Introduction

    Oct. 2012 updateYes ! Dorji's DRA886RX  receiver module readily converts to general 433 MHz band monitoring. Check the quick "proof of concept" breadboard layout below -diverse circuitry alterations naturally can be made. Refer Dorji ASK module insights => www.picaxe.orconhosting.net.nz/dorjiask.htm

    Although of course NOT as sensitive as a dedicated $$($) UHF scanner, such a 433 MHz data module hack is dirt cheap,educational,dead easy to use & is particularly handy for diagnostic checks on all manner of 433 MHz devices. Investigate (by HEARING/SEEING band activity) the likes of wireless door chimes, backyard weather stations, car remotes.garage door openers, wireless data devices,local 433 MHz band interference,pickup antenna and device siting benefits etc etc. HIGHLY RECOMMENDED!


    Very cool idea. I saw these modules on Sparkfun the other day and thought they might be used for robot navigation (distance and bearing).


    8 years ago on Step 6

    Happy to help ! Perhaps initially check my crusty old PICAXE resource site http://www.picaxe.orcon.net.nz for ideas. However the best PICAXE resource is at the makers http://www.picaxe.com  This Hellscreiber circuitry is really an engine - it can modulate almost any transmitter. I simply used the 433 MHz one for simplicity. Stan

    1 reply

    Reply 8 years ago on Step 6

    thanks, again -
    great resource & helpful script which allows to create different types of rf sending/receiving devices !


    Reply 8 years ago on Introduction

    please accept my apologies for my late reply-
    and, much more important:

    thanks so much for posting the links!
    will have a look at it asap - (late over here - 4.20 AM)

    (did some research meanwhile and found another really helpful example on instructables
    https://www.instructables.com/id/Wireless-temperature-sensor/ )

    orcon has plenty of sources too, thanks a lot, again!

    Best Regards


    8 years ago on Step 6

    am a total newbee to picaxe (not to mention microcontroller coding)
    hence commands such as HIGH, LOW, SLEEP and GOTO are not easy enough to master for me yet :

    - would you mind to hint me towards how to code this
    probably post a link for similar examples ?

    apologies for being that helpless - i did my googling
    and did not find one example for a newbee-easy-to-understand-
    picaxe rf transmitter-receiver example
    including code and schematics example yet
    (not to mention a "breadboarded" or "circuited" image coming with it)
    and was happy to find your instructable!

    however: thanks for posting all this!