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by Stan Swan 2nd Generation UHF Telemetry for the PICAXE PURR-FECT! Telemetry (from the Greek tele = remote and metron = measure) refers to the remote measurement and reporting of information, typically using wireless links to carry the data. Such technology is well established in climatic monitoring (especially temperature), water management, motor sport, security, medicine, defence and even space – Martian probe style. W ireless telemetry (using high voltage valves) was utilised even back in grandpa’s era. Perhaps one of the more exotic installations was the World War II German automatic weather station “Kurt”, secretly installed on the Canadian coast by a U-boat in 1943. Its 150W short-wave transmitter, powered by an array of nickel-cadmium and dry-cell batteries, produced 14  Silicon Chip coded signals (derived from weather sensors) receivable thousands of kilometres away in Europe. Distant Atlantic weather conditions could then be monitored but – fortunately for Allied shipping – jamming thwarted the station’s eventual mission! Modern motor racing telemetry allows trackside engineers to view and interpret live race data and use it to rapidly tune their racecar at even- tual pit stops. When every second counts, the ability to promptly work on tele-monitored faults can make for improved race performance. Such “mission critical” applications usually have heavy duty telemetry budgets but the availability of cheap data modules in recent years has allowed UHF wireless data links to proliferate, with many homes even now having several quietly at work – siliconchip.com.au siliconchip.com.au SUITABLE ANTENNA ~170mm WHIP OR YAGI I/O PINS (CHANNELS) CON1 DB9 1 6 7 8 9 2 3 22k 4 5 2 1 7 IC1 6 PICAXE-08M 10k 3 TO PC SERIAL PORT 4 8 5 0 ANT HOPERF HM-TR UHF DATA TRANSCEIVER ON +5V NC GND DATA GND +5V typically at 433.920MHz. As wireless links on the higher (near microwave) 1.8-2.4GHz bands are almost line of sight (LOS), many field telemetry setups in fact prefer low UHF (300-900MHz) or even lower VHF (30-300MHz), as this ensures better signal penetration of vegetation and buildings. The popular 434MHz slot, globally reserved for low power (25 mW) unlicensed Industrial, Scientific and Medical (ISM) wireless data, increasingly abounds with weird signals arising from home weather stations, power meters, car locks, garage door openers, security systems and wireless doorbells. In many suburbs at peak times, a UHF scanner tuned to 434MHz can issue sounds akin to an African dawn chorus! Although now very cheap, these consumer devices typically encode the serial data using ASK (Amplitude Shift Keying) on/off streaming and may be prone to interference from neighbouring services. However, it’s no good complaining – LIPD (Low Interference Potential Device) users on this 1.740MHz-wide (433.050 to 434.790MHz) spectrum slice have no prior channel rights. Failing repositioning, superior FSK (Frequency Shift Keying) encoding approaches may be needed instead. FSK data is largely immune to amplitude-modulated impulse noises – a major FM broadcast radio benefit, of course. The crashes and static you hear on an AM radio station as a thunderstorm approaches (even hundreds of kilometres away) are virtually nonexistent on an FM radio station. Fortunately most 434MHz services are very low power (a few milliwatts) and of very short range (a few tens of metres) and all are usually distinc- ZW-3100 (tx) and ZW-3102 (rx) gave a good account of themselves, with ranges to several hundred metres in open areas. Although this hardly gave the Bathurst Supercar “tele-techo’s” a scare, serial data rates in the 300-2400 bps range readily allowed PICAXE-monitored transducers to be wirelessly read and remotely recorded. However, such links were only one way (simplexbroadcast only), with no easy method to correct corrupted data, although CRC software evolved to at least detect possible errors. For 2-way (½ duplex- 2-way radio style) another tx/rx pair could have been added but the resulting cost 1 2 3 4 5 6 1 4.5V 2 330Ω λ SC 2008 RED LED 8 4 1 Picaxe serial encoding – uhf TRANSMITTER Circuit diagrams for the HopeRF UHF transceiver, powered by a PICAXE 08M. The code for the PICAXE is shown overleaf, while the programming for the transceiver can be downloaded from the HopeRF website. Note that there are slightly different connections for the transmitter (above) and the receiver (below). SUITABLE ANTENNA ~170mm WHIP OR YAGI I/O PINS (CHANNELS) CON1 DB9 1 6 7 8 9 2 3 4 5 TO PC SERIAL PORT 10k 22k 2 1 7 IC1 6 PICAXE-08M 3 4 8 5 0 ANT HOPERF HM-TR UHF DATA TRANSCEIVER ON +5V DATA GND NC GND +5V The HopeRF module that has Stan so excited this month! Shown here approximately life size, it operates in the 434MHz “LIPD” band and mates perfectly with Stan’s other favourite toy, the PICAXE. tively encoded to match their sender. Hence although receiving neighbouring wireless doorbells may occur, yours can easily be re-coded to use different data signals, even though spectrum noise may still decrease sensitivity (and thus range). Regular readers may recall the July 2003, December. 2005 and January 2006 SILICON CHIP articles on serial telemetry using cheap 434MHz transmitters (tx) and receivers (rx), controlled by PICAXE-08Ms. Cheap 434MHz wireless data units and PICAXEs are almost made for each other! Although any of the cheap modules then on sale could have been used, it was found the ~$10 Jaycar 1 2 3 4 5 6 1 4.5V 2 330Ω λ SC 2008 GREEN LED 8 4 1 Picaxe serial decoding – uhf RECEIVER October 2008  15 ANTENNA ANTENNA HOPERF TRANSCEIVER MODULE TRANSCEIVER MODULE 1 2 3 4 5 6 1 2 3 4 5 6 PICAXE08M 330Ω A K 22kΩ 5 3 2 PICAXE08M 4.5V (eg, 3xAA) (RS232) 330Ω A LED 10kΩ K TRANSMITTER LED 22kΩ 5 3 2 4.5V (eg, 3xAA) (RS232) 10kΩ RECEIVER Protoboard wiring for the transmitter and receiver. Differences in the receiver board are subtle – connection to the HopeRF module is to pin 2 and LED drive is different. The LEDs are perhaps overkill as the HopeRF module has red & green SMD LEDS on the modules. There are some differences between these layouts and the photograph shown earlier. doubling, multiple module mounting and “push to talk” control software became daunting. Given the continuing integration and refinement of electronic circuitry, it became apparent that classic 20th century 434MHz units were well overdue for enhancement and I for one have been watching for successors. Features hankered after included better use of the 434MHz spectrum, improved receiver sensitivity and faster data speeds. We’re in an era when electronic finesse often comes with trivial price tags but the only offerings that had arisen were for wellheeled professionals. It was hence with some anticipatory hand rubbing that the wireless data products of Chinese firm Hope Microelectronics (www.hoperf.com) were greeted! The firm, based in the mega factory Pin 6: Enable Pin 5: Configure Pin 4: DRX Pin 3: Ground Pin 2: DTX Pin 1: VCC SMA Antenna Socket Atmel ATMega48 44-pin 20MHz 4kB 8-bit SMD microcontroller 16  Silicon Chip city of Shenzhen (nearby to Hong Kong), produce a broad range of highly integrated UHF wireless data units at budget prices. Some of their offerings blending both transmitter and receiver into one package. Such a data transceiver combination naturally makes for great convenience and reduced circuit layout and is recommended, since prices are only slightly more than equivalent discrete units. Their six-lead HM-TR especially appealed, as it promised interferenceimmune FSK, programmable settings (via an on-board ATMega micro with 32-byte buffer), sensitive reception, rapid send/receive switch-over, data status lights and a quality SMA antenna outlet (gold plated!) – all for not-much-more than a classic 433MHz tx/rx pair. Combined with easy serial links to MAX232 Connection and construction details for the HopeRF HM-TR. Various pins are connected depending on whether it is in transmit, receive or configuration modes. our ever faithful PICAXE workhorses, these “transparent” data units look just what the doctor ordered. The units are programmable (using software downloadable from the HopeRF site) and a simple ~4.5V (3 x AA) breadboard setup with a repositioned PICAXE cable allows configuration tweaking. Frequencies cover four UHF bands (315/433/868/915MHz), complying with US FCC and European ETSI regulations, although some may be outside legal ISM slots. Transmitter power attenuation, receiver bandwidth, frequency fine tuning and the usual plethora of serial baud rates and communications protocols can also be set. As PICAXE serial works normally at 2400 bps, 8 data bits, no parity and 1 stop bit (2400,8,N,1) this was also written to the HopeRF transceiver – out-of-the-box default settings are at 9600 bps and 434MHz. When wired for communication, subsequent breadboarding of a PICAXE-08M-controlled pair – one transmitting (tx) and the other receiving (rx) – proved very straightforward indeed. The unit’s tiny SMD red (transmit) and green (receive) LEDs indicate tx/ rx status, so the extra LEDs (and dropping resistors) added to the controlling PICAXEs may not be strictly needed. However, as their inbuilt 32-byte buffer needs filling before wireless data is sent, the extra LEDs confirm data handling at the PICAXE itself. As ranges were of keen interest, siliconchip.com.au CON1 DB9 1 2 3 4 5 6 1 6 7 8 9 * MODULE ENTERS CONFIGURATION MODE WHEN PIN 5 IS PULLED HIGH (IE, TO +4.5V) 2 4 3 5 TO PC SERIAL PORT ON +5V DATA GND NC GND* ANT HOPERF HM-TR UHF DATA TRANSCEIVER 4.5V CONFIGURATION MODE To get the HopeRF module into configuration mode, all you have to do is take pin 5 from low to high. To program, you can use the same DB9 serial port connector as used for the PICAXE programming. simple code (using ASCII “85” for 10101010 strings) was passed between the pair to allow a distinctive “purr” when heard via a UHF scanner. A small piezo across the winking receiver LED will also give this output, in my case bemusing passers-by who thought I had a contented cat in my jacket! Using the supplied 434MHz “rubber ducky” antenna, line of sight (LOS) links of 0.5-1km proved feasible (the latter across water), although vegetation and wooden buildings cut this to about 200 metres. Since the transmitter is only rated at 5mW and was found pleasingly “clean” on a spectrum analyzer, this testifies to a sensitive receiver. The quality SMA sockets allow an external antenna to be attached, although it’s probably best to use SMA-BNC adapters (such as Jaycar’s) to utilise more standard connectors. As even a “cotanga” Yagi will give 6dB gain (enough for range doubling), simple Yagis at each end should give a 6dB+6dB = 12dB gain, allowing point to point links of perhaps several kilometres – four times that of the basic supplied antenna. As it’s apparent that the HopeRF HM-TR data transceiver offerings look worthy of a “2nd generation 434MHz” title (especially when PICAXE driven), a more demanding half-duplex data workout will be presented in a follow-up article. Stay tuned! Australian suppliers for Hope Electronics are MicroZed Computers on the NSW Central Coast, phone 1300 735 420 (www.microzed.com.au). References, links and software are hosted at www.picaxe. orconhosting.net.nz/hoperf.htm SC Transmitting PICAXE code Receiving PICAXE code purrtx: serout 2,n300,(85,85,85,….,85,b1) pause 500 goto purrtx purrrx: serin 2,n300,b1 pulsout 1,200 goto purrrx Here’s the code you’ll need to get the two modules talking . . . or at least purring . . . to each other! siliconchip.com.au The configuration software (downloadable from www. hoperf.com) is very simple to use – much esier than some of the software we’ve used in recent times. Advantest R41310 Spectrum Analyser display of HopeRF HM-TR serial (300bps) data transmitter on 434MHz. Note the “pure” output! 4dBm transmitter and supplied “rubber duck” antenna was approximately 1m away from a short pickup wire connected to the SA antenna input. Other UHF signals visible in the “grass” are from unknown external sources – a lot of devices use 434MHz! Range testing near Wellington harbour gave ~1km LOS with the rubber duck antenna. By attaching a simple Yagi antenna at both ends, data links over water as far as the island 5km away should be possible. October 2008  17