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“433” Revisited We touched on 433434MHz wireless data back in the July 2003 Picaxe article but here’s an updated workout focusing on the dirt cheap “get you started” modules now around. by Stan Swan* S O YOU WANT to cut the cable clutter and go wireless on your project? WiFi? Bluetooth? ZigBee? Infrared? All offer very fast data speeds but have “fish hooks”, not the least of which is infrared’s need for a totally unobstructed link. Or the other’s need for a computer or two! So how about – gulp – just 2400 bps?! In an age when wireless datacomms push speed boundaries to WiFi’s “g” 54 Mega bps, such a few kilobits per second may seem downright pedestrian and akin to dial-up modems in the XT/AT ’80s! But when crucial data items need sending, for example to unlock your car or raise the garage door, sheer speed is often incidental to module size, convenience and reliability. Naturally, tight budgets and ease of project integration feature too. You don’t want to have to fire up a WiFi PC every time you need to open the garage door ! New cheap modules To cater for experiments with simple-but-reliable shortrange wireless control, Jaycar have recently released a budget pair of 433MHz UHF wireless data modules suiting both experienced users and even perhaps novices needing stimulating “21st century crystal sets”. Classic crystal sets of course were popular in the presemiconductors 1920s-50s and stimulated many a school student (yes, myself included – again) into exploring wireless mysteries when parts were costly and AM radio was king. Given the lament that today’s electronic goodies come so pre-built that users lack investigative curiosity, these 433MHz units may be just the motivational ticket to crack siliconchip.com.au technical inertia. Everyone should build a “crystal set” at least once! Jaycar’s sub-$10, tiny stamp-sized ZW-3100 transmitter and matching ZW-3102 receiver are similar to assorted “key fob” ASK (Amplitude Shift Keying) OOK (On Off Keying) serial data units that have of course been widely marketed for some time. Enhanced FM data transceiver multi-channel versions costing A$30-$100 (such as those sold by Oatley Electronics) are better suited to more demanding or professional data work, therefore have not been considered We mentioned a TX433/RX433 pair from WA firm Computronics back in the July 2003 Picaxe datacomms article. Typically retailing for just A$8 each, they’re often labelled TWS/TLP and RWS/RLP 433 or the like via makers such as Rentron or Laipac. A Google on “433 ISM”, etc, will locate many offerings. Superior Ming or Chipcon versions also exist but being enhanced FM transceiver multi-channel versions, stretch to more like A$40 – rather beyond the scope (and budget!) of this article. All occupy the 433.920MHz licence-free ISM (Industrial, Scientific and Medical) UHF band actually some 1.7MHz wide (433.050-434.790MHz). The alternative LIPD title (Low Interference Potential Device) relates to the very low December 2005  85 SUITABLE ANTENNA: ~170mm WHIP OR YAGI TRANSMITTER (TO PC SERIAL PORT) CON2 DB9 C1 100nF 2 22k 3 10k 5 I/O PINS 1 2 3 IC1 PICAXE-08M 4 7 0 330Ω 6 1 330Ω 5 8 2 330Ω ON λ LED λ LED 3 λ PIEZO TRANSDUCER (OPTIONAL) 10kΩ 433.92MHz ISM TRANSMITTER MODULE (JAYCAR ZW-1300 OR SIMILAR) ANT +V DATA 4.5V GND LED 4 4 1 RECEIVER PIEZO SOUNDER (OR EVEN 32 Ω PERSONAL STEREO HEADPHONE) LED 433.92MHz ISM RECEIVER MODULE (JAYCAR ZW-1302 OR SIMILAR) +5V DATA DATA GND Using the cheap 433-434MHz modules (a selection of which is shown on the previous page, not far off life-size) really is child’s play, especially when teamed up with our new best friend, the Picaxe! Above is the transmitter – the LEDs and their associated resistors (plus of course the piezo) could be considered optional if you want to keep it simple. At right is the receiver – a single transistor amplifier is all that is needed to drive the piezo/headphone). It’s rough – but it works more than satisfactorily! SUITABLE ANTENNA: ~170mm WHIP OR YAGI ANT GND GND +5V 8 C B 10kΩ E DS547, etc (ANY G/P NPN TRANSISTOR) ON 4.5V power transmitter restriction of just 25mW. Most run even offerings and for this alone Jaycar’s units may be worth lower that this, with 5mW being typical! Although just ~1% paying a little bit extra for (you can sometimes find similar of cheap half watt 477MHz UHF CB sets, such flea power items at disposals sources for perhaps $5-$8). will normally still yield a range of a 20-50 metres in builtThey originate, as RXB1 and TXC1, from Taiwanese up areas and even several hundred metres unobstructed. makers Keymark (www.keymark.com.tw) and thankfully A simple “cotanga” Yagi antenna, styled after the turn out to be virtually pin-for-pin compatible with ear477MHz design shown in our February 2005 UHF CB article, can push this to around a kilometre line of sight (LOS), and may be especially useful for crossing valleys or water (perhaps at a marina or lake). Although 433MHz is not so obstructed by buildings and trees as 2.4GHz WiFi, the far lower power means ranges, all up, are much the same. Jaycar’s pair appeal both for their rumoured superior performance and – gasp – quality labelling! In a nanotechnology age when molecules can be stacked like Lego, it’s most frustrating to be faced with devices devoid of details that makers could have readily silk-screened on. Although a close inspection of the units reveals many receiver pins are duplicated and linked on the small PC board, the multiple connections can bewilder even old hands. Knowing here “which pin does Here’s the Picaxe-powered 433MHz transmitter from the circuit above, mounted what” is reassuring after confusion in our new “PICNIK” box. In this case, only one LED is included, driven by the with absent markings on other 433MHz Picaxe data line which also drives the transmitter and piezo. 86  Silicon Chip siliconchip.com.au this has become the default Picaxe supply as well). OK – power needs are now sorted out and the modules fitted to our ever faithful protoboards. For those who’ve just come in and are unaware of my enthusiasm(!) for Picaxe microcontrollers, they really look the data engine of choice for the 433MHz modules. I’ve recently been developing a more compact and cheaper kit for the Picaxe 08M in fact and have managed to squeeze these ISM units into the new Mk.2 PICNIK box as well. See the ~800kB animated gif “slide show” at www.picaxe.orcon.net. nz/picnik2.gif The initial supply detective work lead to further productive tinkering with the modules’ data lines. The receiver protoboard is very simple, with just the module, one transistor, one LED and one resistor – plus, of course, the external connections. lier 433MHz ASK modules I’ve used. Simple swap-over tests with the ZW-3102 receiver (being a quality Himark RX3400-based PLL superhet rather than a super-regen) showed it noticeably more sensitive than cheaper units, which further justify the slightly higher price. Don’t believe the specs! Instead of “boring” old serial data, we can have assorted tones, Morse beacons and even tunes handled by these units! A benefit of this is that any old UHF scanner can receive the info as plain audio. Naturally, this may suit a hidden transmitter “fox hunt”, a simple location/proximity beacon or even audible telemetry and security. Line of sight ranges were some 300m with simple (¼-wave) 170mm whips, but to around 1km when paired with a simple “cotanga” antenna (www.picaxe.orcon. net.nz/yagi433.jpg) at the ZW-3102 receiver. The long established “70cm” (420-450MHz) amateur band brackets the ISM slot, meaning numerous sensitive UHF receivers probably lurk in broom cupboards just awaiting such a fresh task anyway. In spite of other diverse warbling and croaking tone signals at 433MHz, especially originating from keyless car remotes (readily heard near a supermarket car park!), the UHF spectrum has a low background noise level and receivers can be very sensitive indeed. It’s beyond the scope of this article to go into the maths involved but below is a simple table relating RF (radio frequency) signal strengths (in microvolts across a 50W load) to dBm (milliwatt) and communication receiver “S” readings. A 6dB change is equivalent to signal strength (and thus range) doubling or halving. Hence a simple 6dB gain “cotanga” Yagi should double distance, while a 12dB gain antenna (feasible at this frequency) will “double x double” Initial testing of the Jaycar units was most satisfactory, in spite of misleading details provided in both their 2005 catalog (p73) and support web page. The receiver doesn’t just run on the stated 3V but is instead designed for a nominal 5V supply – nicely handled by three fresh alkaline or four NiMH AA cells, delivering around 4.8V. Only very small currents of around 5mA are taken (meaning batteries should last weeks). Much lab and web sleuthing verified this supply and the error must be causing lots of hassles to bright sparks thwarted at 3V power-up stage. Annoyingly the Jaycar support .pdf is mostly in Chinese, limiting even the ready reading of diagrams unless language extras are downloaded. Grrr... Fortunately I have several Chinese-speaking (and reading!) students who were able to help. Happily, the receiver, in common with other such modules, runs well outside the “tight” 5V specification, to as high as 6V or as low as 4.3V before cutout. ISM receivers will normally end up mounted indoors in a garage (or car etc) powered by a mV dBm regulated supply but in contrast, the companion transmitters are destined 50.0 -73 for portable key-fob mounts powered 8.0 -89 by small coin or 12V batteries. 4.0 -95 Because of the associated battery run (2.24 -100) down with use, the transmitter supply 2.0 -101 is usually much more flexible, and they 1.0 -107 were found to work to well between 0.5 -113 2V and 6V, with 12V even a possibil0.25 -119 0.125 -125 ity. A 4.5V (3 x AA) battery supply is (0.1 -127) ideal for them too (and conveniently siliconchip.com.au Abracadabra! Traditional “S” Meter Reading S9 (by definition) S6 S5 S4 S3 S2 S1 S0 (a typical 434 Rx sensitivity) (about the limit of Jaycar’s 434 module) (about 1 strength “bar” on a scanner) (almost lost in scanner background noise) December 2005  87 2x 10kΩ 1kΩ C 1kΩ 100nF 100nF 100nF ON OUTPUT R2 10k C PIEZO TRANSDUCER (OPTIONAL) B B 2x DS547, etc E (ANY G/P NPN TRANSISTOR) E 4.5V OSCILLATION FREQUENCY (f) ~ ~ 700Hz ANT +V ON DATA GND 10kΩ INPUT FROM OSCILLATOR 6 2 8 ON 4 3 555 5 1 4.5V 100nF OUTPUT PIEZO TRANSDUCER (OPTIONAL) 4.5V 1.44 (R1 + 2R2) x C1 WITH VALUES SHOWN: f ~ ~ 400Hz If you want to drive a transmitter module direct (ie, without PICAXE control), here’s how to do it. The two circuits above, with their protoboarded pics above that again, are for simple oscillators – at left is an astable multivibrator which was one of the mainstays of oscillators until the 555 timer came along (above right). As you can see, it is even simpler and doesn’t cost much more – 555s are really cheap! The curly wire disappearing from the photos is the antenna – a piece of wire 170mm long. It’s curled to reduce the overall height. At left is one of the transmitter modules wired to work directly from the oscillator output, via the 100nF capacitor. (or x 4) both this and signal strength. Most of these cheap 433MHz ISM receivers have rated sensitivities around -103dBm to -106dBm (about 1.5mV) and although impressive (for the size and price!), the low UHF spectrum noise means even a 1mV (-107dB) signal is considered quite strong at these frequencies. Modern radio scanners, or more professional ISM receivers, will readily detect signals down to 0.15mV (~-124dBm) with corresponding range extensions to perhaps several km. Audio transmitter circuit Any of the 433MHz transmitters are able to be turned (and held) on by simply pulling their data input line high with a 10kW resistor to the positive supply. Assorted tones can then be fed into this input via a 100nF capacitor (for DC isolation) and served to acceptably modulate the output as FM, rather than the normal On/Off keying. Although 88  Silicon Chip C1 120nF 7 OSCILLATION FREQUENCY (Hz) = SUITABLE ANTENNA: ~170mm WHIP OR YAGI 433.92MHz ISM TRANSMITTER MODULE (JAYCAR ZW-1300 OR SIMILAR) R1 10k the transmitter could be supplied tones by a transistor oscillator or 555 IC (refer to diagrams and pictures), such an approach is now almost redundant given the versatility of a Picaxe-08M instead. Splutter – you’ve not heard of a Picaxe? Where have you been? These darlings are now almost as indispensable as can openers! Easy software readily rustles up pulsating tones, Morse ID or even simple tunes (08M pin 2) and also allows battery saving “sleep” power-downs. Given the 20mA sink/source limit of a Picaxe, it is easily able to deliver the 10mA needed. Even power to the entire transmitter module can be controlled by an output (here channel/pin 4), so as to further save batteries and make hidden transmitter hunts more lively! Check the pictures and schematic for the suggested Morse ID layout and port across the code (433txcw.bas) to the Picaxe-08M. For convenience the code can be copied siliconchip.com.au RECEIVER TRANSMITTER PIEZO OR HEADPHONE ANTENNA (170mm) A LED K 10kΩ A G G +V C E RECEIVER MODULE +V D D G B TRANSISTOR 4.5V Protoboard wiring for the receiver (left) and transmitter (right). The Picaxe needs to be coded with 433txcw.bas – and when you start to play with it, you can change the code to your heart’s content. The Picaxe programming is done via the RS232 port on your PC – pins 5, 3 and 2 of the D9 connector are used. If this is your first time with the remarkable Picaxe chips, refer to the Picaxe series run in SILICON CHIP during 2003-2004. The piezo lets you hear the outgoing signal, if you wish. and pasted from the web sites www.siliconchip.com.au or www.picaxe.orcon.net.nz/433txcw.bas Naturally adjust the beacon code to suit your ID needs – it’s presently sending .... .. (Morse for “HI”) about every 10 seconds. Audio receiver circuit Again any of the 433MHz receiver offerings worked in the simple setup shown. Outputs at the piezo sounder and LED however were weak, so a very simple 1-transistor NPN amplifier was used to boost these to acceptable levels. Of course, this common emitter amplifier should have further biasing resistors etc, but its performance was well suited to the task here. For convenience, even a 32-ohm personal stereo headphone could be used instead of the piezo. Note that the circuit has no squelch, and thus background “hiss” may annoy on weak signals and during transmitter power down. Naturally, a more sophisticated UHF scanner will address this. Applications We’ll extend these basic ISM circuits in a later article but once working here, it’s suggested you use them initially for simple “Easter Egg” hunting of hidden transmitters. Especially in a more open region such as a park, several hidden “fox” transmitters – each sending different IDs – could be activated for the “hounds” to locate. Simple (coiled) 170mm whips can be body shielded (or even removed) when close to the signal, although Yagi beams (see overleaf) offer fair direction finding and triangulation. NB: be careful of poking yourself in the eye with these antenna elements when in scrub and bush – perhaps put blobs of silicon seal or hot melt glue on the wire ends to prevent this happening. siliconchip.com.au Are you into model aircraft/balloons/rockets? How about a 433MHz beacon payload as well, perhaps with a white LED for after-dark locating up a tree. We’re not suggesting you pester your pets but a compacted (35mm film canister?) transmitter could even be attached to a dog’s collar as a DF (direction finding) aid to his whereabouts when chasing rabbits or other dogs! Incidentally, since ACMA regulations specify 25mW effective radiated power, it’s perhaps best to keep the transmitter antenna omni-directional to avoid infringing this limit and put constructive zeal into the receiver antenna. There’s no reason why the circuits have to be as large as shown here – for more compact designs, the receiver especially could be squeezed into a tiny case and powered by a 9V battery suitably 7805-regulated to 5V. You may not even need a PC board; just “dead bug” the transistor and resistor to the receiver module! The more involved (Picaxe-driven) transmitter could even be powered by small PV cells, perhaps rescued from cheap solar garden lamps now flooding hardware stores. Most of these deliver 2V at 30mA in bright sun, so a couple in series will power the entire transmitter, the Picaxe and even charge a couple of NiMH cells for night-time duty. Given the ease of linking in switches, LDRs, thermistors and DS18B20 temperature sensors to a Picaxe-08M, it’d be a breeze to rig up a simple security, proximity or telemetry application sending distinctive audio tones. Mmm – Hellschreiber? Naturally the fundamental DATA handling nature of these 433MHz modules shouldn’t be forgotten either. Stay tuned! References For convenience these are all hosted at web site www. picaxe.orcon.net.nz/433txrx.htm * s.t.swan<at>massey.ac.nz December 2005  89 Want to build a quick-n-easy 433/434MHz Antenna? Here’s the latest antenna that Stan Swan has come up with for fox hunting and general 433/434MHz work. It’s a fourelement yagi built, of all things, from telescopic magnetic pickup tools. They’re cheap and make the antenna very easy to adjust. Gain is approximately 6dB. Being a Yagi, it is directional – it transmits in, and receives from, the direction of the arrow at the top of the picture. It has a driven element, two directors (the shorter elements in front of the driven element) and one reflector. The slight offset of the elements won’t make much practical difference. Since 433.92MHz signals have a conveniently compact wavelength of 692mm (300,000,000/433,920,000), all manner of desktop-sized UHF antenna can be readily rustled up using common materials and household tools. The short coax lead to your 433 RX module can be just video grade that’s probably already at hand as well – however, don’t pinch the family TV antenna cable! For transmitting work, strict attention to impedance matching and the like is needed, but for reception (as here) things can be dead simple. For openers, consider a convenient telescopic element Yagi made from magnetic pickup tools, allowing easy tweaking of element lengths that compact for storage. Element group spacings (starting from the rear reflector) of 123mm, 110mm and 159mm are not critical, but match those of our earlier rigid “cotanga” version. Ensure the driven element pair are NOT touching but that the centres of the other pairs DO connect – perhaps with wire or solder to the (chromed) brass tubes. An even simpler RDF (Radio Direction Finding) style loop can be suprisingly effective too. Simply cut and shape some sturdy wire to suit (it doesn’t have to be an exact circle!) and secure to the coax via “chocolate block” connector strip. Initially, try a wavelength (692mm) but snip the wire shorter – perhaps to eventually 650mm – and retest. Such loops are best suited for finding a “null” when pointing at the transmitter, since they’re bi-directional. Simply step aside some distance for another bearing that should confirm the true direction by triangulation. A “body shielding” technique can be useful too – just hold the antenna close to your chest while you slowly revolve. The weakest signals should be when your back is to the transmitter and you are facing away from it. In built-up areas, many reflections from metallic objects may confuse this a bit! 90  Silicon Chip siliconchip.com.au Here’s what you’ll need: 8 telescopic magnetic pickup tools (we sourced ours from a bargain shop for $2.00 each) 1 length PVC builder’s channel (about half a metre – make sure you don’t use a metal channel!) 1 “chocolate block” terminal block (2-way) 8 solder lugs Hidden transmitter RDF techniques can be very sophisticated but fun – there’s even talk that they may lead to a new Olympic orienteering sport! We suggest that you refer to the references back in the “433” article for more insights. At a serious level, they’ve found extensive use homing in on 121.5/243MHz EPIRB (Emergency Position Indicating Radio Beacon) emergency channels – such as “man overboard” or even boat sinking, lost bushwalker or emergency crashed aircraft beacons. Incidentally, by 2009 the service will become GEOSsatellite based on 406.025MHz, speeding response and reducing the legendary false alarm rate. At present, a common RDF chore involves using a direction finding antenna and receiver to locate which of the safely parked (and usually locked!) aircraft has accidently-activated its 121.5MHz when the pilot made a little-rougher-than-normal landing... siliconchip.com.au 8 screws to suit the end of the magnetic pickup tools (if the tools don’t come with them) Short lengths of hookup or tinned copper wire Suitable length 75W coax cable (thin is easiest to handle but is usually lossiest at UHF) Insulation tape (to secure coax to ducting Construction This is a very easy antenna to construct because the dimensions aren’t critical. And it has a couple of major advantages over other types: (a) the telescopic “whips” can be fully closed for transportation; and (b) the large knobs on the of the antennas make poking eyes out or other injuries less likely (cut coathanger wires can be quite dangerous). The telescopic whips mount right through the edges of the ducting – drill one side to suit the whip diameter; the other to suit the screw diameter. Try to mount them as close as possible while keeping a bit of “meat” around the holes so they don’t break. The halves of each director and reflectors need to connect electrically (that’s the point of the short length of wire under the solder lugs). However, the driven element halves must not touch – if necessary insulate SC them with tape. December 2005  91