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Communicate without wires . . . Smart radio modem for microcontrollers This cheap and simple radio modem will enable your PICAXE, Stamp or other micro to communicate without wires! L By NENAD STOJADINOVIC OW COST, simple construction and easy interfacing makes this project ideal for a whole range of low-speed wireless data applications. Remote control and sensing are two obvious uses and there are undoubtedly many more. Even if you’re just learning about microcontrollers, you’ll be able to get two PICAXEs talking in no time! 62  Silicon Chip The seeds of this project were sown when I got a call from Mr Vineyard, whose grapes kept freezing during the depths of winter. He said he needed a system that would turn on a misting water spray over the vines when the temperature dropped below a certain level. Apparently, Jack Frost would then freeze the water rather than the grapes. This seemed a bit dubious but I was assured that this is a well-known method of frost damage control. The only complicating factor was that the temperature sensors needed to be amongst the vines which were up to half a kilometre away from the shed that housed the water control valves. Given half a dozen sensors, the amount of wiring needed was clearly impractical. And Mr Vineyard was very keen to have a temperature readout in his home so he could keep an eye on things. Going wireless Wireless networking was an obvious choice for the job. Microcontroller-based temperature sensors placed www.siliconchip.com.au www.siliconchip.com.au November 2003  63 Fig.2: one end of the radio link can also be connected to a PC (or any computer with an RS232 port). As shown here, the receiver includes an RS232 interface on-board, whereas the transmitter requires an add-on interface. Fig.1: a block diagram of the radio modem, showing how two microcontrollers can be linked together. Fig.3: the UHF receiver module uses a “bit slicer” circuit to convert the linear signal into digital format. strategically amongst the vines could transmit their readings back to a central computer, which would then control the pumps. Since commercial wireless networking gear was too expensive for this application, the alternative was to design the wireless network from the ground up, with the aid of pre-built UHF radio modules. These miniature modules operate in the 433.05 - 434.79MHz LIPD band and do not require a license for operation at up to 25mW of output power. The advertising blurbs suggest that it’s simply a matter of pumping serial data into the transmitter module and recovering it at the receiver (“data in - radio out”). Discovering that this was anything but true is what people ruefully refer to as a “learning experience”. As it turns out, the data must be “massaged” (encoded, decoded, error checked, etc) at either end of the link to achieve reliable transfer across the airwaves. This was achieved with the aid of Microchip’s PIC microcontrollers and many hours of program- Main Features • • • • • • Point-to-point, one-way wireless data link Error-checked data transfer Low cost & easy to build 1200 bps serial interface speed 465 bps end-to-end speed 150-200m range in built-up areas ming. The fruits of these labours are presented here. Project overview The radio modem consists of a transmitter and receiver pair. The designs use pre-built “Laipac” brand 433.92MHz UHF modules, with PIC12C508 microcontrollers handling the “smarts”. Both transmitter and receiver include a TTL-level (0-5V) 1200 bps (bits per second) serial interface for data transfer. This makes it very easy to hook them up to your Stamp, PICAXE, or other micro (see Fig.1). In many cases (such as the vineyard application above), one end of the link will need to be connected to a PC (Fig.2). The receiver board includes an RS232 interface for this purpose. As it’s usually the remote part of the link, the transmitter doesn’t include an on-board RS232 interface. This saves space and reduces power consumption. Where required, it can be mounted on an (optional) RS232 interface board which also supplies power. The receiver and RS232 interface can be powered from either a 9V battery or DC plugpack. When used without the RS232 interface, the transmitter must be provided with a +5V supply. This is usually available from the sensor or associated circuitry. Serial data Most of our readers will already be familiar with the basics of asynchronous serial data transfer. Those new to the subject will find lots of useful information on the Internet. Two informative sources can be found at: (1). http://janaxelson.com/serport.htm (2). www.beyondlogic.org/serial/ serial.htm The word “serial” simply refers to the fact that data is transferred from sender to receiver a single bit at a time. At a minimum, this requires only one complete circuit (two wires) between the sender and receiver. With wires and logic signalling levels, it’s all pretty straightforward. But how does it work over the airwaves? UHF radio modules Fig.4: the complete circuit diagram for the transmitter. Not much to it is there? An 8-pin PIC microcontroller (IC1) receives serial data from the host (PICAXE, Stamp, etc) and transmits it over the airwaves using a UHF transmitter module. 64  Silicon Chip The radio modules used in this project transmit data by simply switching the carrier signal on and off. The terms “On-Off Keying” (OOK) and “amplitude modulation” (AM) are used interchangeably to describe this method of transmission. The transmitter module consists of a SAW-stabilised RF oscillator tuned to 433.92MHz. When a logic ‘0’ (0V) is applied to the data input (DIN) pin, the oscillator is off and when logic ‘1’ (+5V) is applied, the oscillator is on. An antenna coupled to the circuit radiates the carrier signal into the ether. Things get a bit more complicated at the receiver side. Unfortunately, the manufacturer’s data sheets don’t reveal www.siliconchip.com.au much about its operation. Of course, we do know that it amplifies and rectifies the narrow-band 433.92MHz (±1.5kHz) signal picked up by the antenna, with the result appearing on the Linear Output (LOUT) pin. Data slicing A digital version of the signal also appears on the Digital Output (DOUT) pin. Conversion between analog (linear) and digital is performed with a “bit slicer” circuit. As the name suggests, the bit slicer looks at the incoming signal and decides whether it should be a logic ‘0’ or logic ‘1’, “slicing” it up accordingly. This is achieved with a circuit similar to that shown in Fig.3. IC1 is configured as a comparator and once the capacitor is charged up, a signal peak at the input will result in a high at DOUT while a signal minimum will result in a low. The frequency of ‘1’s (transmitter on) and ‘0’s (transmitter off) in the data stream determine the accuracy of the slicer. If the time between ‘1’s is too long, the capacitor voltage sags and ‘1’s will be detected as ‘0’s instead. Conversely, if the data stream contains Fig.5: this add-on interface connects the transmitter to an RS232-compatible serial port. The MAX232 chip (IC1) handles the RS232 (±10V) to TTL (0-5V) level conversion, while 3-terminal regulator REG1 also powers the transmitter board. Fig.6: the receiver circuit is almost a mirror image of the transmitter. PIC micro IC1 receives data from the UHF receiver module and after decoding and error checking, passes it on to the host via the DATA output. Unlike the transmitter, an RS232 interface (IC2) is included on-board. www.siliconchip.com.au November 2003  65 Fig.7: follow this diagram when assembling the transmitter. If you’ve opted for a more elaborate antenna (instead of the single length of wire), the coax shield can be soldered to the ground pad right next to the antenna connection point. too many consecutive ‘1’s, a ‘0’ will go undetected. Transmission speed also affects the average voltage on the capacitor. Circuit time constant is optimised for a particular “baseboard” data rate, which for these modules is specified as 3000 bps. As you can see, the ideal situation exists when the transmitter is fed with an alternate stream of ‘1’s and ‘0’s at the prescribed data rate. In fact, data transmission must begin with a preamble of alternating 1’s and 0’s of sufficient length to “initialise” the data slicer at the receiver end. Of course, during “normal” transmission, data can consist of any combination of ‘1’s and ‘0’s. This is easily accommodated by encoding the data before transmission. Manchester encoding A number of encoding techniques can be employed to ensure that the data stream contains a balance of ‘1’s and ‘0’s. This project uses “50% Manchester” encoding, which simply involves sending every bit along with its complement. Thus ‘0’ becomes ‘01’ and ‘1’ becomes ‘10’. It is simple and robust but does take twice the time to send each byte. However, this is not of particular concern for our “lowspeed” link. Error detection With all the potential for lost or corrupted data over a radio link, some kind of error detection system is mandatory. Along with data encoding, error detection is another of the main functions of the PIC microcontrollers in the transmitter and receiver pair. The PIC micro in the transmitter sends data in blocks or “packets”. Before transmission, all data bytes in a packet are passed through a polynomial generator, with the result being an 8-bit number called a “Cyclic Redundancy Check” (CRC). This byte is appended to the end of a packet before transmission. On the receiver side, incoming data is passed through the same polynomial generation algorithm and the result is compared to the received CRC byte. If they match, the data is deemed good. Otherwise, it is assumed bad and the entire packet discarded. If you’re interested in the algorithm and microcontroller code required to generate CRCs, then check out Microchip’s application note AN730, entitled “CRC Generating and Checking”. It can be downloaded from www.microchip.com Bytes & packets As mentioned above, data received from the “host” (your PICAXE, Stamp, Fig.8: receiver assembly is also quite straightforward. The UHF receiver module must be oriented with its inductors (coils) facing the two ICs. The “SPARE” signal line is not used and should be left unconnected. 66  Silicon Chip www.siliconchip.com.au Fig.9: the overlay diagram for the optional RS232 interface. Install all components before mounting the transmitter board. Note that the electrolytic capacitors go in different ways, so make sure that you have their positive leads oriented as shown PC, etc) is assembled into packets before transmission. Each packet is preceded with a preamble, two “authorisation” bytes (FF 00) and a length byte to indicate the number of data bytes to follow. Data length may be from 1-16 bytes, with a CRC byte appended to the end. Of course, the receiver returns only the data part of the transmission to its host; the other bytes are strictly for housekeeping. This means that apart from a certain amount of latency, the radio modem link looks just like a piece of wire between the sender and receiver! So far, we’ve only described the radio side of the link. Let’s now look at how you connect your PICAXE, Stamp or whatever to the transmitter and receiver. Transmitter hook-up Your microcontroller project interfaces to the transmitter via a 3 or 4-wire interface (see Fig.1). For a basic setup, you need connect only the DATA, SEND and GND lines. Serial data must be sent on the DATA line at 1,200 bps using the standard format of 8 data bits, no parity and 1 stop bit. The SEND line is used for handshaking and in the idle state must be held high (+5V). To transmit data, send 1-16 bytes and then bring the SEND line low (0V). Data transmission begins immediately and after an appropriate delay (see the “Radio-Modem Performance” panel), the SEND line can be brought high www.siliconchip.com.au Listing 1 symbol symbol begin: SEND = 1 TX_DATA = 2 high SEND pause 1 serout TX_DATA,T1200,(“A”) low SEND pause 500 goto begin again and the transmitter is ready to accept more data. The PICAXE microcontroller program to transmit a single character at a time might look something like that shown in Listing 1. For maximum transmission speed, the BUSY line can be connected as well. This line is an output from the transmitter and indicates its status. When BUSY is high, the transmitter is sending data, and when low, it’s 'transmitter SEND line on pin 1 'transmitter DATA line on pin 2 'raise the SEND line '1ms delay 'load the ASCII character “A” 'lower the pin to send the data 'wait 0.5 sec while the data goes 'loop to repeat forever ready to accept the next packet of data. Listing 2 shows a simple example. Note that attempting to load more than the maximum of 16 bytes at a time will result in BUSY going high and the additional bytes going into the bit bucket. Receiver hook-up The receiver interface is even simpler and requires only a 2-wire connection. Again, a fragment of PICAXE Listing 2 symbol symbol symbol begin: waitrdy: SEND = 1 TX_DATA = 2 BUSY = 3 high SEND pause 1 serout TX_DATA,T1200,(“Hello”) low SEND pause 1 'transmitter SEND on pin 1 'transmitter DATA on pin 2 'transmitter BUSY on pin 3 'raise the SEND line ‘1ms delay ‘load the ASCII string “Hello” 'lower the pin to send the data '1ms delay if BUSY = 1 then waitrdy goto begin 'loop until not busy (data sent) 'repeat forever November 2003  67 Radio Modem – Performance Range: maximum output power with a 5V supply is listed as 25mW (14dBm) into a 50Ω antenna. This provides a range of 150 - 200 metres in the suburbs and rather more over open terrain. Maximum range is heavily dependent on antenna efficiency and environmental conditions. Speed: data is transferred between the transmitter/receiver and the connected device (PC, PICAXE, etc) at a rate of 1200 bps. However, due to the overheads involved in the radio transmission, actual throughput is slightly less than half that speed. Calculated on a maximum payload of 16 bytes per transmission, the radio link speed is equivalent to about 465 bps. That’s about 343ms per transmission, plus the time taken to load and unload the data at either end (about 8.33ms per byte). Power consumption: with a 9V supply, the receiver consumes about 16mA. More than 10mA of this is used by the MAX232, so for battery-powered receivers, don’t install this chip if it’s not needed. When idle, the transmitter requires less than 1mA. During transmission, this peaks at about 6mA. When plugged into the RS232 board, total consumption increases to 12mA at idle and about 17mA (peak) when transmitting. code illustrates how to receive a byte – see Listing 3. As you can see from this listing, it’s simply a matter of listening on the DATA line for the incoming serial data. PC Connection One end of the link can also be connected to a PC or other computer system with an RS232-compatible serial port (see Fig.2). The receiver board includes an RS232 interface, so it’s a simple plug-n-play proposition. Alternatively, for remote control applications, the transmitter end can have the RS232 connection. A simple add-on RS232 interface board (is required for the hook-up (see Fig.6 and the photos). A PC connected to the receiver board can display and/or capture incoming data with a simple serial terminal program (see the testing procedure below). If the data is in ASCII format, Windows “HyperTerminal” will suffice. However, if you want to see the “raw” binary data, then you’ll need a program like “RealTerm” instead. RealTerm is available free from realterm.sourceforge.net To send data from a PC connected to the transmitter, you need more than a simple terminal program. Your application must take control of the SEND line (RTS), and optionally read the status of the BUSY line (DSR). Note: the radio modem is not intended for PC to PC data transfers. Attempting to move “PC-sized” amounts of data across a 465 bps link would be pointless. Transmitter assembly With only nine parts on the board, you’ll have the transmitter assembled in no time at all. Fig.7 shows the component placement. The three 1kΩ resistors must be mounted vertically rather than horizontally and note the orientation of the 2.2µF capacitor and microcontroller (IC1). In addition, make sure that you have the transmitter module in the right way around – the SAW resonator (in the round metal can) must face towards IC1. Receiver assembly Install the single wire link first, using 0.7mm tinned copper wire. All components can then be installed in Listing 3 Symbol RX_DATA = 2 serin RX_DATA,T1200,B2 68  Silicon Chip 'receiver DATA on pin 2 'wait for a byte & store it in variable B2 order of height (see Fig.8). Again, take care with the orientation of the polarised components, these being diode D1, the 22µF capacitors and the ICs. The receiver module goes in with its coils facing toward the ICs (see photos). If don’t intend connecting the receiver to a PC, you can leave out the MAX232 receiver/driver (IC2). This will save power in a battery-powered setup. However, you may prefer to socket the chip and remove it later, as the test procedure (below) requires a PC connection. RS232 interface assembly As before, install the two wire links first, then all components in order of height. Take particular care with the orientation of the four 1µF capacitors, as they go in different ways around on the PC board. The transmitter board mounts vertically near one edge of this board via 90° header pins. Install the 2-way and 3-way SIL header pins on the transmitter board first and then fit this assembly to the RS232 interface board. Before soldering into place, check that the edge of the transmitter PC board contacts the RS232 PC board and that the whole arrangement is sitting “square”. Antenna For testing purposes and many real-world applications, the antenna can be as simple as a 165mm length of light-duty hook-up wire. Strip and tin one end of the wire and solder to the transmitter’s antenna connection point. Repeat for the receiver board (see Figs.7 & 8). For best results, the antenna wires should be kept clear of large metal objects and human bodies! Testing Both the receiver and RS232 interface boards can be powered from a 9V battery or 9V DC plugpack. The battery clip leads (or flying leads from a panel-mount DC socket) can be soldered directly to the ‘+V’ and ‘0V’ pads. Note: 12V DC unregulated plugpacks are not suitable for this project due to their excessively high output voltages at light loads. If you’re not using the RS232 board, connect a regulated 5V supply to the transmitter’s ‘+5V’ and ‘GND’ pads. Next, connect the receiver to a free www.siliconchip.com.au Where To Get The Parts Kits of parts for this project are available from the author. Kits include the PC board and all on-board components (battery, plugpack, enclosure & antenna are not supplied). At time of writing, prices are as follows: Transmitter............................................................................................... $25 Receiver................................................................................................... $40 Transmitter & Receiver pair...................................................................... $60 RS232 Interface....................................................................................... $25 Programmed PICs can also be purchased separately: PIC12C508A for Transmitter (including 4MHz resonator)........................ $15 PIC12C508A for Receiver (including 4MHz resonator)............................ $15 If you’re interested in a “rubber duck” or other specialised antenna, write and ask for a current price list. All prices include postage within Australia. To order, write or email the author at: Nenad Stojadinovic, PO Box 320, Woden, ACT 2606. email: vladimir<at>u030.aone.net.au The Laipac UHF transmitter & receiver modules are also available from Commlinx Solutions, online at www.commlinx.com.au serial port on your PC using a standard 9-way “pin-to-pin” cable (not a “null modem” type). To be able to “see” the incoming data, launch your favourite serial terminal application. HyperTerminal (supplied with Windows) will do the job. Set the terminal’s communication parameters to match the chosen COM port, with a data rate of 1200 bps, 8 data bits, 1 stop bit and no parity. Right, we’re all set. Make sure that transmitter power is off and place a jumper shunt across the “TEST” pins (JP1). Now power up the transmitter and you should see the characters “0123456789:;<=>?” appear in the terminal window. A built-in test routine transmits this string of characters continuously when the SEND line is held low (0V) at power-up. This, of course, is the purpose of the “TEST” jumper. Fault-finding No go? First, check the supply This view shows the completed RS232 interface with the transmitter board mounted in position. www.siliconchip.com.au rails. To do this, use your multimeter to measure between pins 1 & 8 of IC1 on both the transmitter and receiver boards. On the receiver, expect close to 5.0V, whereas on the transmitter, your reading should be about 4.7V. Next, use a logic probe or oscilloscope to monitor the signal on pin 7 of the transmitter’s micro (IC1). With the jumper shunt (JP1) installed at power-up, there should a burst of pulses each time the 16-character test string is transmitted. If that checks OK, then it’s over to the receiver side. Examine pin 4 of the receiver’s micro (IC1). Normally, background noise picked up by the UHF receiver module appears on this pin as random “garbage”. However, you should see a distinctive change in the pattern whenever the test string is received. Assuming that you see signs of activity, then measure at the micro’s serial data output (pin 7). Again, brief bursts of pulses should appear here if the test string is received successfully. The last link The last link in the chain is the MAX232 (IC2) on the receiver. As shown on the circuit diagram (Fig.6), serial data from the micro (pin 7) is applied to the MAX232 on pin 11. Therefore, it should appear on pin 14 after conversion to the ±10V (nominal) RS232 signal levels. This pin should sit near -8V when idle and pulse to about +9V when sending the test data. One option is to fit the transmitter into a small metal diecast case complete with a “rubberduck” antenna. November 2003  69 Parts List Receiver 1 PC board coded 06111031, 63mm x 55mm 1 Laipac RLP-434 transmitter module 1 PIC12C508A (programmed) (IC1) 1 MAX232 RS232 receiver/driver (IC2) 1 78L05 +5V regulator (REG1) 1 1N4004 diode (D1) 1 4MHz 3-pin ceramic resonator (CR1) 1 D9 female connector, 90° PCmount (CON1) 1 9V battery & battery clip -OR1 9V DC 150mA plugpack & panel-mount DC socket to suit Capacitors 2 22µF 25V PC electrolytic 7 100nF 50V monolithic Resistors (0.25W, 1%) 2 1kΩ Transmitter 1 PC board coded 06111032, 37mm x 29mm 1 Laipac TLP-434 transmitter module 1 PIC12C508A (programmed) (IC1) 1 1N5819 Schottky diode (D1) 1 4MHz 3-pin ceramic resonator (CR1) Capacitors 1 2.2µF 16V tantalum 1 100nF 50V monolithic Resistors (0.25W, 1%) 3 1kΩ RS232 Interface (optional for transmitter, see text) 1 PC board coded 06111033, 51mm x 46mm 1 MAX232 RS232 receiver/driver (IC1) 1 78L05 +5V regulator (REG1) 1 D9 female connector, 90° PCmount (CON1) 1 3-way 2.54mm 90° SIL header 1 2-way 2.54mm 90° SIL header 1 2-way 2.54mm SIL header (JP1) 1 jumper shunt (JP1) 1 9V battery & battery clip -OR1 9V DC 150mA plugpack & panel-mount DC socket to suit Capacitors 4 1µF 16V PC electrolytic 2 100nF 50V monolithic Resistors (0.25W, 1%) 1 1kΩ If you’ve successfully traced the test data from start to finish, then the problem must be related to your computer! Double-check the terminal program settings and the cable connection between the unit and the PC. A good vintage, indeed What of the “radio thermometer” project? Well, I built it into one of those cheap solar-powered LED garden lights. Using ‘sleep’ mode on the micro, the device now sends temperature and humidity readings to the water control unit every minute or so and is working very nicely after six months of totally unattended operation! More reading Technical data on the RLP-434 & TLP-434 UHF modules used in this project can be downloaded from the Laipac Technology web site at www. laipac.com For details on government regulations regarding LIPD radio communications devices, visit the Australian Communications Authority web site at www.aca.gov.au/aca_home/ legislation/radcomm/class_licences/ lipd.htm A high-performance commercial radio modem was reviewed in SILICON CHIP, February 2003. Details on the WM232-UHF modem featured in the review can be obtained from http:// SC www.radiotelemetry.co.uk/ New From SILICON C HIP THE PROJECTS: High-Energy Universal Ignition System; High-Energy Multispark CDI System; Programmable Ignition Timing Module; Digital Speed Alarm & Speedometer; Digital Tachometer With LED Display; Digital Voltmeter (12V or 24V); Blocked Filter Alarm; Simple Mixture Display For Fuel-Injected Cars; Motorbike Alarm; Headlight Reminder; Engine Immobiliser Mk.2; Engine Rev Limiter; 4-Channel UHF Remote Control; LED Lighting For Cars; The Booze Buster Breath Tester; Little Dynamite Subwoofer; Neon Tube Modulator. ON SALE AT SELECTED NEWSAGENTS Mail order prices: Aust: $14.95 (incl. GST & P&P) NZ/Asia Pacific: $18.00 via airmail Rest of World: $21.50 via airmail Or order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 70  Silicon Chip www.siliconchip.com.au