Low power 433MHz Telemetry System

MFB

Senior Member
Introduction
Much of the following information has already appeared in my recent post ‘Narrow-Band Radio Telemetry ‘ in this section. Whilst that project covered FSK modem techniques for use with a wide range of communications and storage devices (walkie-talkies etc) this posting is aimed at specific low power radio modules. The RF Solutions low-power 433MHz Transmitter & Receiver Pair available in the UK from Maplin Electronics (order code VY48C) for £9.99.

FSK Modulation
The telemetry data is Frequency Shift Key (FSK) modulated to make it suitable for transmission over a narrow-band radio link, without having to resort to specialised communication routines (e.g. pre/post -amble code and other data padding). FSK modulation is a well-established method of reliably transmitting and storing digital information on general-purpose audio equipment. It was used with early generation home computers for storing programmes on audiocassettes and for digital communication over dial-up telephone lines. Many amateur radio operators still employ FSK modems (sending to a mic input and receiving from a speaker output) for long distance digital communication over voice quality links. Although they increasing use PC sound cards in place of dedicated hardware to process and demodulate the received signals.


PICAXE -14M code to scan three digital and five analog inputs at a rate of 15Hz.


setfreq m8 ‘8MHz operation
let dirsc = %000 'Change C0-3 from outputs to inputs
pause 2000 ‘Pause for 1 second

serout 2,N600, ("!RSET",CR) 'Automatically selects the StampPlot operating mode….
serout 2,N600, ("!RSET",CR)
serout 2,N600, ("!ERRT",CR)
serout 2,N600, ("!TMAX 20",CR) ‘N.B. set-up parameters must be received before dataserout 2,N600, ("!SPAN 0,255",CR) (or entered via the PC keyboard).
serout 2,N600, ("!SHFT ON",CR)
serout 2,N600, ("!FLSH ON",CR)
serout 2,N600, ("!NUMB 6",CR)
serout 2,N600, ("!CSUM ON",CR)
serout 2,N600, ("!PLOT ON",CR)
serout 2,N600, ("!USEB ON",CR)
pause 2000

scan:
b0=pins AND %00001110 * 16 'Shift digital inputs 1-3 into 7-5
readadc 0, b1 'Analog input, leg 7
readadc 1, b2 ' ,, ,, ,, 10
readadc 2, b3 ' ,, ,, ,, 9
readadc 3, b4 ' ,, ,, ,, 8
readadc 4, b5 ' ,, ,, ,, 3
b6=b0+b1+b2+b3+b4+b5 'Generate checksum
serout 2, N600, (b0,b1,b2,b3,b4,b5,b6) 'Send digital and analog channels in binary format @ 1200bps
goto scan 'Next frame

Modulator
The 1200 bps modulator employs a CMOS Schmitt trigger RC oscillator to generate the two standard FSK frequencies of 1200 and 2200Hz. This is achieved by switching in a second capacitor via the collector of a transistor that is driven from the serial output of a PICAXE-14M. The trimmer must be adjusted to obtain the lowest error rate at the output of the FSK demodulator. Adequate long-term stability was achieved by operating the circuit from a regulated supply and using polycarbonate capacitors. By using a 5-pin CMOS devices (and other SM components) rather than part of a 14-pin DIL, the circuit can be made much smaller than would be possible with a dedicated FX614 modem chip.

Demodulator
A second FX614 demodulates the audio FSK signal from the radio receiver output and presents the digital data to a PC serial port. In addition to demodulator circuitry the FX614 also has a band-pass filter, which is able to extract data from signals that sound like noise only. Although the FX614 is quite tolerant of input signal amplitude variations, it may still be necessary to initially adjust the receiver’s output for minimum data dropout. The demodulator is powered from the RTS output (pin7) of the PC serial port using a 5.1V zener diode and series resistor.

PC Display software
An ideal way to display and archive the received serial data is to use the StampPlot Pro application from SelmaWare Solutions. This Windows software accepts serial data and presents multiple channels on a real-time scrolling display. Selected blocks of data may also be exported to a spreadsheet for processing. An evaluation version of StampPlot may be downloaded from www.selmaware.com. This site also provides extensive documentation, including how to use the optional on-line maths functions (using this feature is much easer than trying to perform PICAXE maths before transmission.)

StampPlot expects to receive ASCII values by default but also has the option of accepting data as binary values. To make the most efficient use of the available bandwidth, the latter format is used and selected by the set-up parameters at the start of the PICAXE programme. The binary operating mode offers the option of including a checksum byte with each transmitted frame. StampPlot rejects frames that fail the checksum test and retains the last set of acceptable points on the chart, until updated by the next acceptable frame. This is a particularly useful form of display when sending data over interference or dropout prone links. The above software therefore attaches a checksum to each data packet before transmission.

Results
I developed the above telemetry system some years ago for model rocketry and obtained good results over a range of several hundred metres, using a directional receiving antenna. The output from the receiver was often fed into a camcorder audio input and re-played to the FSK demodulator and PC. In order to video the flight and record data for later analysis.
 

manuka

Senior Member
Bravo- I should really have another look at this, BUT initial trials I'd made ~2004 were very unsatisfactory. How has the sender unit stability held up under changing temps? Stan.
 

MFB

Senior Member
Stan, My not very scientific reply to your question about stability is that it was 'adequate' with no noticeable increase in checksum errors over the range 0-30C. This was achieved by using Polycarbonate capacitors that offer a low temperature coefficient and good long-term stability. The other main consideration is the stability of the supply but this had to be well regulated anyway for the 14-M ADC referance. I found that the best way to set the FSK frequencies was to remove the transmitting antenna; to produce a noisy signal, and then trim the modulator for minimum received errors.

Notice that the demodulator is driven from the receiver’s analog ‘Test’ output, rather than from the bit-slicer’s digital output. This is kinder on the modulator input and allows signals to be decoded that are way below the slicer’s threshold. The receiver was mounted at the rear of the antenna to reduce cable loss and the audio signals sent over a cable to the demodulator and PC.
 

manuka

Senior Member
OK- I'm wondering if BOTH freqs really need trimpot tweaking too. Bell 202 specs. say 1200 ±12 Hz & 2200 ±22 Hz (thus within 1%), & my 2004 trials would often give one reliably but not the other sort of thing. I'd used a common 74HC14 Schmitt hex inverter rather than the unusual FairChild NC7S14MS of yours- do you think this may have been an issue?

Modern caller ID still uses Bell 202 (as you probably know), & with diverse standalone CID units now abounding in junk shops I'd pondered using one for DIY display.

In fact my long standing quest relates to using 2 assigned data channels on the 40 channel Aus/NZ ~470 MHz UHF CB band. El cheapo 433 MHz ASK/FSK wireless data modules are of course already thick on the ground, but they're low power (barely ~10mW) & often have weak receivers- ranges of maybe a km are typical even with decent antenna. In contrast the bargain (~US$20) ½ Watt UHB CB hand held sets here in Aus/NZ are sensational performers, with LOS ranges often to 10-20km. I've used them for all manner of quickee audio coupled work in recent years (SSTV, Hellschreiber) but proper telemetry appeals.

I'm still taken with DTMF based telemetry however, as even a sniff of DTMF can usually be reliably decoded- it's usually better than AFSK in this respect. Sure - DTMF is S L O W, barely allowing ~10 ch. per sec, but this is quite enough for diverse data. Some simple look up table could be PICAXEd for A-Z characters- perhaps using ASCII. Hence DTMF "8" ,then DTMF "0" would show as a "P".

As I don't always read this esteemed Forum, suggest you respond to my Gmail => stan.swan@gmail.com Regards- Stan
 
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MFB

Senior Member
The breadboard used a Schmitt inverter from a hex 74HC14 DIL package and it gave the same results as the SM flight version. I did achieve acceptable results using one multi-turn cermet trimmer for both tones (but then I only ever made two examples). The FX614 seems pretty tolerant of frequency drift because there was no problem storing the FSK data on old cassette recorders and camcorder audio channels.

Btw. Pre-launch range tests involved hanging the rocket from a tree (a few wavelength above ground) and moving the 4-element/receiver assemblies away until there was a sharp rise in checksum errors. The tests gave a range of several hundred metres, which is plenty for model rocketry.

Regarding DTMF, guess it’s a case of horses for courses. Could be just the thing for the long-range balloon projects that I would like to get involved with. Thanks for the email address and I’ll be in contact.
 
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