RFM42 Lost Model Locator

srnet

Senior Member
This is a PICAXE 20X2 RFM42 Lost Model Locator Radio Beacon intended for locating lost Radio Control models using only the device and a suitable receiver.

When the radio beacon has determined it's lost, either by a failure of a pulse from the remote control (RC) receiver or by elapsed time, it starts transmitting a series of tones,. These descend in power and rise in frequency. These tones are heard on a suitable receiver. The receiver can be a scanner type receiver with narrow band FM capability or a PMR type transceiver. In between tones the Lost Model Locator goes to sleep with a current consumption of 10uA. The beacon is built on a PCB that is 25mm x 32mm.

The RFM42 module can transmit on any frequency between 240Mhz and 930Mhz at up to 100mW. The frequency of transmission is defined by 3 of the program variables f1, f2, f3 which are then written by the program into the RFM42 frequency setting registers $75, $76, $77 respectively. The frequency of operation must be chosen and programmed by the user. The RFM42 has various registers that can be set to compensate for frequency inaccuracies and temperature drift.

As you get closer to the radio beacons location you will hear through the chosen receiver more of the higher frequency tones. Fox Hunting is a sport practiced by amateur radio enthusiasts in which they locate hidden transmitters. Do a search on Google for 'Fox Hunting Body Fade' for some ideas on how to locate a hidden or lost radio beacon (the fox) using just a receiver. Body fade location is a lot easier if the receiver has an adjustable squelch control.

The detection range will vary a great deal depending on frequency, terrain, receiver sensitivity and transmitter power output. In a built up area of houses and the radio beacon on the ground (as a crashed model might be) at 430Mhz the short high power tone (100mW) may be heard from 1kM away. In open country with few trees or other obstructions, the usable range might be 5Km or so. The use of high gain antennas on the receiver such as a yagi or ZL special can increase detection range by a factor of 2 or more.

The RFM42 has a matching reciever, the RFM31, the pinout is the same. The direct modulation pin used is (RFM GPIO1) is available on a pad at the edge of the PCB, so a pair of these PCBs could be used as a transmit and receive pair.
 

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srnet

Senior Member
I would post the documentation but the uploader reports the .doc file as invalid, the .bas program file is too long, and the .zip file containing the Eagle PCB files is also reported as invalid.
 

lbenson

Senior Member
Nice project. I think I recognize a 20X2 soic there. Depending on size, if you rename the files as .pdf, you may be able to load them, e.g., something.doc.pdf, etc.
Users can then rename them.
 

srnet

Senior Member
I will try, but why such craziness ?

I mean not being able to post PICAXE programs on a PICAXE forum is a bit wierd ...........

Yep, 20X2 SMT.

Basic file attached and .doc documentation attached (remove the .pdf extension)
 

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srnet

Senior Member
For completness, I was using the PICAXE as a development platform, its just so easy to work with. I have replicated the code for detecting pulses, driving the RFM42 etc, into Mikrobasic for a 18F26K22, see picture.

This Lost Model Locator, acccepts a GPS input, establishes the home position at switch on and then transmits the lost location as two morse code numbers, distance and direction from home (compass bearing), so if you can hear the morse, you can find the lost model with just a compass.
 

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lbenson

Senior Member
A very complex project in a very small space. Thanks for taking the time to post it, along with your extensive documentation and extensively documented code. All the files downloaded ok and were viewable to me after I deleted the ".pdf" file extensions.

I was glad to look at your eagle board. On mine, I routed connections to avoid the pads for the SMT 20X2 on the bottom side of the board--which don't exist--doh!
 

srnet

Senior Member
Register $09 of the RFM42 can be used to fine tune the frequency of the RFM42 transmitter, useful if you have a frequency counter that is up to the job. This register controls the amount of load capcitance for the 30Mhz crystal, which alters the frequency slightly.

The RFM42 powers up with $09 set to $7F, a load capacitance of around 11pf.

If you set bit 7 to 1 (adds 3.7pF) and <6:0> at mid point ($40) then you can adjust the load capacitance from 3.7pF to 14.7pF.

On one sample if the programmed frequency was 434.10Mhz, adjusting the load capacitance from min to max with the register changed the frequency output from about 434.05Mhz to 434.19Mhz, with a resolution of around 1Khz.

The RFM42 has an on board temperature sensor, so it should be possible to adjust the output frequency to compensate for temperature drift.
 

techElder

Well-known member
Stuart, your documentation is next to impeccable, but could I trouble you for a real PDF printout of the schematic? I don't have Eagle handy, and I do like to peruse a good schematic.
 

srnet

Senior Member
RFM42 Frequency Adjust\Calibration with Morse transmission of adjustment register value

Here is a program that allows you to remotely trim the frequency of the RFM42 transmitter by adjusting the load capacitance of the main crystal.

Since the LML is designed to be connected to a Remote Control receiver, is uses that connection and the transmitter joysticks to carry out the adjustment. In this way you can carry out the adjustment from some distance away so that you have a nice weak signal to use.

There are generic morse routines in the program, the value of the capacitance adjust register is transmitted as slow speed morse. The adjustment info is printed to the sertxd terminal also.
 

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Paix

Senior Member
@Srnet, If you had named your file with a .txt extension instead of the .pdf, then most people would be able to view it directly without having to download it first to change the extension. :)

Now I feel obliged to make the effort . . . :)

Lots of work there Stuart. I'm more apreciative than I sound . . . honest!
 
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srnet

Senior Member
I changed it to a .BAS file, whatever was preventing them being posted appears to have been fixed, althouh that .BAS file is very close to the current limit on posting a .BAS file 19.5Kb.
 
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