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by STAN SWAN 433MHz SNIFFER Here’s a simple – and cheap – little 433MHz receiver that has many uses. It is ideal for checking that a suspect 433MHz “wireless” device (and there are scads of them) is actually working. It’s great for finding out where interference is coming from. But most importantly (we believe!) it makes a great tracker for “fox hunts” and the like. I t’s not widely appreciated that the popular UHF telemetry band – more correctly called the ISM (Industrial, Scientific and Medical) band, centred on 433.92MHz, actually covers a generous 1.7MHz between 433.05MHz and 434.79MHz. It’s probably just as well that more than one spot frequency is available, as an army of wireless door chimes, energy monitors, toys, car remotes, garage door openers, backyard weather stations and the like now festoon this licence-free spectrum slot. Although such LIPD (Low Interference Potential Device) signals normally travel only a few hundred metres (as just a tiny 25mW transmitter power is permitted), its increasing popularity means that in urban areas a scanner tuned to the band may reveal a near-bewildering “African dawn chorus” of beeps, buzzes, pops, whirrs and scratches associated with nearby wireless data. The ability to monitor local activity on this ever-more-crowded spectrum slice may ease device fault-finding or interference location, yet the cost and complexity of a UHF scanner may not be justified. Hence it’s with some satisfaction that we present a cheap (~$25), simple siliconchip.com.au and sensitive “433” band monitor. In an electronics age when almost anything seems possible, such receivers have not normally been available. With increasing band “noise”, every “433” user should have one in his toolbox. I’ve used mine extensively for wireless data monitoring and device activity checks and find it a near-indispensable “bang for buck” test item. This recently again showed its worth, with a “no go” neighbour’s 433.92MHz wireless door chime (an Arlec DC149). Although exceedingly efficient (two AA cells last around a year as the receiver spends most of its time on a ~200µA snooze), they use a super-regenerative receiver which, as with all regenerative types, radiates a small RF signal even while receiving. It was the work of moments to bring the 433 monitor close to it and hear a suitable increase in background noise, ceasing when the receiver batteries were removed. A computergenerated image (mostly created by Altium Designer) of the PC board version of our 433MHz Sniffer. This is the one to build if you want to make it a permanent project! anuary 2011  21 2011  21 JJanuary within range. Although the module itself draws only draw a few milliamps, the 10mA or so drain from glowing LEDs and hissing speaker meant a costly 9V battery (of perhaps 200mAh capacity) would have soon been depleted. However, it transpires the module’s HiMARK RX3400 engine is happy with a supply as high as 7V, so even four fresh alkaline AAs (which can be ~1.6V each) would be quite OK, so no voltage regulation was eventually used. AA cells of course are universal, with even the cheapest far more “energetic” than a 9V battery. Circuitry Although considered digital data devices, acceptable audio (simply amplified by an NPN transistor) was indeed found available from the Jaycar module. However, don’t expect orchestral quality! RSSI TAP (SEE TEXT) 1k +5V DATA DATA GND ON 330 22  Silicon Chip C C B B Q2 SQUELCH 10k E Design ULTRABRIGHT LED 1M RECEIVER TO MODULE RSSI TAP A G G +V Q1 E +V D D G 5V* * SUPPLY CAN BE 4.5 - 7V Q1, Q2: DS547, etc (ANY G/P NPN TRANSISTOR) 433MHz SNIFFER WIRE ANTENNA (~170mm) Fig.1 (above): the circuit diagram of the 433MHz Sniffer. As you can see, there’s not much to it (all of the hard work is done inside the module). PIEZO OR HEADPHONE 330 1k Our monitor is based around Jaycar’s widely available 433MHz receiver module (Cat ZW-3102). These reliable modules, sourced from Keymark/ SpiritOn, sell for around $13 and find much use with 2400bps PICAXE wireless data. But at around -105dBm they’re nothing special sensitivity-wise. Although crammed with tiny SMD (Surface Mount Device) components, the modules are, at heart, just specialised ISM band receivers which only need a few connections to work – you don’t have to do any assembly on the module. Initially their biggest weakness appeared to be a need for 5V (±½V) supply. Initial thoughts were perhaps to use a 9V battery/7805 regulator, or maybe four “AA” cells and a series diode or two to drop the voltage to PIEZO SOUNDER (OR 32  PERSONAL STEREO HEADPHONE) 433.92MHz ISM RECEIVER MODULE (JAYCAR ZW-3102 OR SIMILAR) SQUELCH SWITCH 10k At the bell push itself, the outgoing transmitter data was readily heard but the fault turned out to be a weakening transmitter battery which as you would expect, reduced range. And while we were sleuthing, a long misplaced (but still active) “CENTAMETER” mains energy sender was located in a backyard shed electronic junk box! SUITABLE ANTENNA: ~170mm WHIP OR YAGI ANT GND GND +5V The heart of the project is this 433MHz ISM receiver module from Jaycar (cat ZW-3102). Both front and back are shown in the above picture. The wire connection is for the RSSI (strength indication), as explained in the text. Q1 2x NPN TRANSISTORS C B E C B E Q2 1M LED A K 4.5V --7V Fig.2 (left) shows the protoboard layout of the above circuit. It’s quick and easy to build E but it’s not exactly permanent! siliconchip.com.au (Left): the PC board version of the 433MHz Sniffer with the component overlay underneath. Inset is the connection to the RSSI terminal on the receiver PC board. While there is a tiny hole through the board, it’s easiest to solder the tap as shown. Parts List – 433MHz Sniffer RSSI TAP 1M C C 330 Q2 K LED + SQUELCH SWITCH In the interests of prolonging battery life, a rugged, low-profile highimpedance piezo transducer was used. Although a high frequency responder, it gave very efficient sound generation at a good level. Note this is NOT a piezo buzzer – they won’t work at all! A small low-impedance speaker (perhaps even one recycled from cheap 32Ω headphones) may also be consid- B E E A Q1 1k 10k B – 6V BATTERY PACK ered but the output circuitry may need modification to suit and the current drain would no doubt be higher. Squelch For prolonged monitoring, receiver noise in the absence of signals may become annoying. Although not essential, fitting a 3.9nF capacitor between the NPN base and ground was found to give hiss-free 1 PC board, 70 x 28mm, code 06101111 or 1 small protoboard 1 433MHz ISM receiver (Jaycar ZW-3102) 2 NPN G/P transistors (eg, DS547) 1 ultrabright red LED 1 piezo sounder (NB: NOT a piezo buzzer) 1 4-way “AA” cell holder 1 SPST power switch (if required) 6 PC stakes 1 175mm length stiff copper wire (for antenna) Hookup wire as required Resistors (0.25W, 5%) 1 1MΩ (brown black green gold) 1 10kΩ (brown black orange gold) 1 1kΩ (brown black red gold) 1 330Ω (orange orange brown gold) Australia’s Best Value Scopes? You decide! Priced from just $69.95. Over 20 different models available to suit your needs. Colour display. USB host for USB memory stick FFT and Math functions Up to 1000 Waveforms record and playback USB device - PC software and cable included 60 MHz and 500 MS/s *** 3 year warranty *** CHRISTMAS SPECIAL * SUPER SPECIAL* 60 MHz Colour UQ2062C only $495 ex GST NZ orders welcome. Postage at cost. * While Stocks Last! Visit our website for more Christmas bargains! Contact TRIO Smartcal now! 1300-853-407 or visit www.triosmartcal.com.au to learn more. Email info<at>triosmartcal.com.au ADELAIDE BRISBANE MELBOURNE SYDNEY SALES: PH 1300 853 407 FAX 1300 853 409 sales<at>triosmartcal.com.au siliconchip.com.au www.triosmartcal.com.au January 2011  23 Here’s the receiver on the alternative presentation, a protoboard. The diagram overleaf has a few minor differences (use the diagram when placing components to avoid any mistakes). The black object at right is a 4 x AA battery pack. No actual power switch is used – simply remove the batteries when not in use! squelch, albeit at the cost of a slight decrease in sensitivity. If the second stage is attempted squelch can also be achieved with a 1MΩ resistor between the RSSI transistor base and ground. RSSI activation This one’s for those with “macro” vision and a steady hand but it makes a very worthwhile “extra”! An innovative circuitry feature, detected after data sheet scrutiny and very fine probing, relates to an undocumented RSSI (Received Signal Strength Indicator) tap on the Jaycar module. RSSI is a measurement of the power present in a received radio signal, which the module (thanks to its HiMARK RX3400 “engine”) offers as a small voltage swing at low current – even an ultra-bright red LED was only dimly lit. A thin flying-wire feed at the tap point, taken to another NPN transistor and amplified gives an extremely useful LED brightness variation with signal strength. The ZW-3102 RSSI tap point – although in the clear on the module (refer picture) – is very tiny and may be even covered by flux residues. Clearing it with a very fine needle or craft knife may first be needed. Even when amplified, the brightness is still only modest, so select a modern, high efficiency red type – in my case, a discarded LED from a cheap 2009 Christmas decoration was found to be ideal! Feeding the RSSI voltage into a PICAXE for READADC attention is tempting but the resulting increase in circuitry complexity and cost was not considered warranted at this stage. But being ever the optimist, some time in the future I may reconsider! 24  Silicon Chip Presentation We’re showing this project in two forms. First is the way the circuit was originally developed, on a standard breadboard. All hobbyists should have one or more of these handy devices in their armoury simply because they can be used over and over again. Circuit layout on the breadboard is not critical but the layout shown is easy and logical. Although normally we’d be pretty wary of breadboarding a project at UHF, all the receiving work is being handled on the compact module. Only low frequency audio and LED feeds need be taken off this. The second, more elegant method is on a specially-designed PC board, measuring 70 x 28mm and coded 06101111. This is obviously a more permanent way to build the project and we would almost certainly mount it (and its piezo sounder) in a small box, complete with battery pack, on/off switch and squelch switch. Assembly It’s recommend that assembly is done in two stages – in fact, the first (audio) part may be all many users will require. So first build the project with Q1 and its associated components (ignoring Q2, the LED, squelch switch etc) and confirm that it works as intended – that is, when you turn it on in the presence of any 433MHz signal you should hear an output from the piezo. The second part, connecting the RSSI tap for LED brightness related to signal strength as detailed above, is extremely handy for RDF (Radio Direction Finding) but requires a fine wire connection to the module. Which ever method you choose, simply follow the component overlay diagrams and you can’t go wrong – that is, unless you put something in the wrong way around or in the wrong spot! The receiver module, transistors, LED and of course power supply connections must all be correct or you could let the smoke out. The entire monitor (PC board or breadboard version) can be powered by a 4 x AA battery pack and the setup could be housed in a cheap plastic box. If you use a clear plastic type, the RSSI LED will be visible through this and a few simple holes will accommodate a simple (RCA?) antenna socket or allow the piezo to be better heard. A small, (cheap!) on/off switch and (if required) a similar switch for squelch can be mounted on the lid of the case. Performance The circuit readily receives 433MHz transmissions (at unobstructed ranges) of several hundred metres using just a quarter-wavelength wire antenna (around 170mm). Even through vegetation and wooden buildings, reception ranges of 50-100m are typical. A wireless doorbell sender makes a handy transmitter but first ensure it’s not disturbing your neighbours! A PICAXE-08M driving of a matching Jaycar ZW-3100 433.92MHz transmitter can however easily be organised to send distinctive tones or a simple Morse beacon. Attaching a directional antenna to the receiver will not only boost the range but also allow possible interference location and simple direction finding (DF). Ah yes – direction finding. Wireless location, although perhaps at its peak locating the three “esses” (submarines, ships and spies) during WW2/Cold War, is still a VERY serious and fun pursuit. There’s even whispers of it as a future Olympic sport (but don’t hold your breath!). Aside from locating emergency rescue beacons or tracking animals, an important RDF need relates to finding sources of radio and TV interference from bizarre electrical problems. These can sometimes be miles away and arise due to some really offbeat causes, such as a rubbing wire on a power pole, faulty power supply or suspect electric fence and so on. siliconchip.com.au DIRECTOR 1 (D1) = 328mm DIRECTOR 2 (D2) = 328mm 123mm 159mm ALL ELEMENT LENGTHS ARE END TO END DRIVEN ELEMENT (D) = 346mm MOUNT ALL ELEMENTS AS CLOSE AS POSSIBLE TO EACH OTHER (DIRECTORS AND RELECTOR SHOULD BE SHORTED; DRIVEN ELEMENTS MUST NOT BE SHORTED) ENSURE DRIVEN ELEMENTS ARE INSULATED FROM EACH OTHER REFLECTOR (R) = 383mm 433MHz 4-ELEMENT YAGI -- (~6dB GAIN) 110mm 2-WAY MAINS TERMINAL BLOCK (MOUNTED ALONG CONDUIT) COAX CABLE (TO TRANSCEIVER) ~450mm LENGTH 2-PART (SNAP FIT) PVC ELECTRICAL DUCTING KEEP AS SHORT AS POSSIBLE SCREW SHORTING WIRE SOLDER LUG SHORT LENGTH OF WIRE SOLDERED BETWEEN LUGS UNDER ENDS DUCTING As wavelengths at UHF are modest (being ~70cm at 433 MHz), antennas can be quite compact and mildly directional. A major UHF RDF (Radio Direction Finding) issue however relates to the terrain and nearby reflective surfaces (especially metallic), which may cause signals to apparently come from unexpected directions. Serious searchers prefer sophisticated Doppler RDF gear but a lot of fun is possible in open spaces with simple receivers and plain body shielding or a simple directional antenna The antenna The modules are sensitive enough “as is” to detect even weak nearby signals but normally a quarter-wavelength vertical whip will be needed. At 433MHz wavelengths (~690mm), just a 170mm whip made from a piece of stiff wire does well. Radio waves in fact slow down slightly in conductors, meaning the normal wavelength (and therefore antenna length) will be slightly shorter than 690mm. While the length won’t be too critical, the RSSI LED may even help you cut the antenna to the right length – start somewhat longer and trim the wire to suit for maximum brightness when the receiver is receiving! siliconchip.com.au INSULATION TAPE ELEMENTS EITHER TELESCOPIC WHIPS OR “TELESCOPIC MAGNETIC PICKUPS” ADJUSTED TO LENGTH Note: for eye safety ensure the top of this whip is capped, folded over and/ or marked with a simple tape “flag”– it can be hard to see such slender wires when working close to a circuit board! It’s even harder to spot at night or when tracking something through the bush. Although simple whips have omnidirectional coverage, a technique of “body shielding” can allow the transmitter direction to be broadly estimated. This exploits the RF shielding of your own body – just hold the receiver close to your chest while slowly rotating yourself. At some point, (ie, when the transmitter is behind you), the received signal will significantly decrease. Repeating the technique nearby should then allow triangulation clues on the transmitter location. A better antenna For serious work however, a directional antenna will be needed. There are numerous designs available with the classic Yagi arguably being the most popular (Google 433MHz Yagi and you’ll find quite a few!). Making one’s own antenna further also demonstrates resonance and wavelength/frequency relationships. An easy-to-build 4-element Yagi for 433MHz. The boom is made from a length of plastic electrical ducting with tele-scopic whip antennas for the elements. These can be adjusted for length once mounted to tbe boom and laer telescoped back in for easy storage. Alternatively, stiff wire (eg coathanger wire) could be used but mounting is more difficult. At 433MHz a half wavelength is only a few hand spans, so quite a compact classic Yagi beam can readily be rustled up using stiff wire mounted on a broom handle or plastic rod. Shielded TV grade coaxial wire can be run to the module, or even the entire receiver mounted on the antenna itself. Compared with an omnidirectional whip, even a 4-element version will give some 6dB gain – equivalent to doubling the range. Perhaps more useful is that the enhanced front-to-back pickup ratio improves direction finding. A large part of the RDF fun relates to disguising the transmitter as a plant, or everyday item such as sunglasses, candy bars or clothing etc! See http://members.aol. com/homingin/ or Google it. Well, there you have it. Not only a useful RF test item but also a handy RDF “engine” suiting outdoor use (once encased). Youngsters, such as scout groups, can run off excess energy “fox hunting” hidden 433MHz transmitters while triangulating signals or mastering map reading. A parent’s dream! Resources and references For convenience these are hosted at www.picaxe.orcon.net.nz/433RX.htm SC January 2011  25