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Nov 06, 2004, 07:53 PM the skies...
Cats Eyes's Avatar

Finding a lost model


As a result of a number of lost planes that people have reported on the Aerial Photography forum, the latest being eBird's All Is Lost thread, a number of us Aerial Photography (AP) pilots got to thinking about ways to locate a lost plane.

Mr. RC-CAM kindly designed and shared a Lost Model Alarm (LMA) device in the thread LoMA: Worlds Simplest Lost Model Alarm. This thread is a spin-off from that thread, which I hope can be a discussion of "plane finders" of all sorts.

Although AP pilots tend to fly higher and "further out" (and in more, um, "interesting" locations) than your average Sunday pilot, the problem of a lost plane is by no means unique to AP. In addition to radio controlled planes, free-flight models and model rockets can use locating devices. In addition, some of the equipment used for tracking wildlife can be adapted as a lost model finder.

Background Info

I'd like to start by sharing some links to information on this topic.

Audible LMA's
RF Trackers


I am hoping eventually to end up with a system that I can use to be 99% sure of retrieving a lost model. To that end, I am working on having both an audible and an RF component. The RF tracking portion should allow me to locate the model's general area (within, say 100 yards), and then audible alarm will then allow me to locate the plane exactly.

If you have lots of money, the above links should provide you with the info you need to purchase the audible LMA and RF tracking system (transmitter and receiver) for a few hundred dollars. I am hoping to do it for much less!

The first three links under RF Trackers, above, show DIY projects in the FM broadcast band. Note that they all use the same one-transistor transmitter output stage. The schematic is identical in all cases, although the particular component values vary somewhat. Since I'm pretty handy with PIC programming, I can use a PIC to drive the output stage with whatever signal is needed (both on/off and perhaps to provide an audible modulation signal). It is my hope to build something like this for the transmitter, use a cheap FM receiver, and build some kind of directional antenna to use to search for the model.

Unfortunately, my knowledge of RF is rather limited, and even with poring through a couple of ARRL books, I am still pretty clueless about how to go about building a directional antenna.

If you have any ideas about this, or have any comments on any of the above, please feel free to post away!

-- Kevin
Last edited by Cats Eyes; Mar 05, 2005 at 11:29 AM. Reason: Hyperlink to a "Dollarama Special" pointed to new web space
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Nov 06, 2004, 09:39 PM the skies...
Cats Eyes's Avatar

From the old thread

From the old thread...

Originally Posted by windtrader
What happens if the plane has the speaker pointed upward in the top of the plane and the plane crashes upside down with the speaker pointing toward the ground?? How much sound is lost, how much still radiates outward?

It seems multi-speakers, maybe with less volume, generally would be more effective in the "real world". I've not seen any multi speaker designs that would minimize plane orientation and allow greater coverage. Anyone want to prototype this out?
A very good point. The construction of most piezo buzzers is just a flat disk diaphram, so the sound should be radiated out the back of the disk as much as the front. Most cases are designed to limit the back wave while letting the front wave out. However, I think by hacking the case you could design it in such a way that the back wave could be let out as well, making it bi-directional. That way, no matter how the plane lands, at least one of the "beams" should be directed upwards.

Originally Posted by richard tunstal
it wouldn't the simplest loma any more but free-flighters use
173 mhz as a uk legal model -borne rf tx.
a 3v lithium coin cell will last 2 weeks.
( 150'ish mhz is used as a non-legal uk freq!)
car key fob lock transmitters on 433mhz uk (350'ish mhz in us?)
can also be used legally
a f/f tx type approved in uk costs ~50 ready to go
'surplus' tiny 173 xtals are available in uk @ 1
a 1 transistor tx circuit will give usable range
uhf key-fob tx's usedto be ~10 don't know current prices
add a cheap scanner or a de-luxe free-flighters rx @ 350 (they really are very good
if you can afford) and away you go
Thanks for the info. I would be curious if you could find construction information on these tx's.

350 is way out of my budget. What did you have in mind as a "cheap scanner"?

Originally Posted by zl3vml
I have one of these security alarms that is supposedly 130db that I had a play with, but most of the sound comes from clever construction of the casing around the piezo.
Once you remove it it is not much louder than the std ones.....

I have found that when mine is pointed up in the air it is a lot less softer than when directed at something about 2-3 inches away that reflects the sound again. When I stuck mine to the fuselage side directly under the wing it was suddenly heaps louder.
This concerns me a little. I noticed that my entry alarm seemed louder with the case off. I wonder how much the case and nearby objects affect the radiated sound (direction as well as absolute loudness). I guess this is something to experiment with further.

Originally Posted by richard tunstal
the tracker tx in the photo is intended for f/f
give me a week or so and i'll try it
Yes, I'm very curious about this! I do hope you'll let us know how your testing goes.

Originally Posted by richard tunstal
the circuit is from wildlife radio tagging by r e kenward
its an idiot guide for biologists so it gets pretty basic but is a super book if you can get it.
there's another one by d w macdonald and c j amlaner jr
i think the title is 'a practical guide to radio tracking'
this is a collection of sym posium papers i think
the basic circuits have been around since the '60's!
ther's also an article in the us national free flight society 1995 symposium report
by ken bauer which details his commercially produced design.
i will scan a few to get you started but they're all pretty much the same.
I had a look around for those books. Our library doesn't have them. I checked Barnes & Noble and Chapters (the Canadian equivalent of Borders):
A Manual for Wildlife Radio Tagging
by Robert Kenward
Chapters: C$47.50
Barnes & Noble: US$72.95
Strange B&N is so much more expensive!
A Handbook on Biotelemetry & Radio Tracking: International Conference: Biotelemetry & Radio Tracking in Biology & Medicine, Oxford, 20 - 22 March 1979
Editor: Charles J. Amlaner, D. MacDonald
Chapters: C$560.50 (out of stock anyway!)
Barnes & Noble: price not given, out of stock
I may spring for the Wildlife book if I don't find the info elsewhere. Any chance you could scan some of the relevant pages and post them? I was hoping to do the whole thing for under $100 and if I have to spend $50 on a book that blows half of it right there!

The transmitter is one thing, but I'm still clueless about how to go about building a good direction-finding antenna. I think a good antenna design can not only provide a way of figuring out the direction to your model, but can also increase the range by several times. I just don't quite know where to start.

Originally Posted by zl3vml
Hey Kevin - I found some of those units at our local $2 shop. Took two home and wow - ear splittin sound from it. I can see how you could hear it 200m away!
You are correct, it does use a voltage booster out to the piezo. I measure around 30volts AC!
I am busy modifying it to see how light I can get it. Removing the reed switch and trimming the pcboard all around. Will post some pictures once done.
I am going back tomorrow to get the rest of their stock :-)
Great. Keep us posted.

I'll have a look at the output of the voltage booster transformer with the 'scope next time I get a chance. I'm curious how much the voltage is boosted. It appears the transformer primary is just switched on/off (at the audio rate) across the battery supply, so perhaps by changing that to a push-pull arrangement (H-drive) you could double that.

Another thing I want to try is an driver element I pulled out of a defunct smoke alarm. It's about 1.5 times the diameter (twice the area), so theoretically should produce more sound still.

Keep the great ideas coming!

-- Kevin
Nov 06, 2004, 10:16 PM
Registered User
Mr.RC-CAM's Avatar
I've been thinking about an RF solution for the last few weeks. I have a some strong opinions about what would be effective and practical. Over the winter I will try to bring it to life.

Here are some of my thoughts:

(1) Range: I'm aiming for 300 meters (or more) of reliable range.

(2) Low Power: It will need to work for at least one week on a small dedicated battery supply. The Tx will be activated upon loss of the R/C signal.

(3) Frequency: The solutions that use the FM radio broadcast frequencies are not the ideal choice, at least not for me. Although the Rx would be very low cost and off-the-shelf, the poor sensitivity and tuning stability of the average FM portable radio would be an issue. Also, effective and compact directional antennas would be a challenge at 88Mhz to 108Mhz. Lastly, the longer range I am looking for would violate the FCC's requirements (Tx power on FM broadcast freqs would need to be VERY low).

So, my target is 433Mhz. There are advantages to this spectrum, including antennas that are smaller and the higher frequency offers increased directivity (something we need).

(4) Legal Operation: I'm in the USA, so I'm going to do what I can to create something that does not violate the FCC regs. There are provisions for home built devices, but they will be tough for the average (RF naive) hobbyist to ensure compliance. If I cannot create a "legal" license free project, then second best is something that can be used by a licensed amateur radio hobbyist.

(5) Ease of Assembly: This is the part that is slowing me down. I have learned that most folks will tolerate a small amount of soldering, but will skip projects that get too involved. So, I'm looking at existing consumer devices to hack, mostly key fob based remote control accessories. If I get lucky, this will also go a long ways to satisfy the FCC issue.

(6) Cost: This is a tough one. My experience suggests that if the project goes over $35 USD then the target audience will be quite limited. But, that bridge will be crossed when the cost data is known. The reality is that Tx will probably be under $30, but the Rx cost is a wild card at this point.

(7) Small size: I wish I could say it will be weigh 1 gram. But, to meet all the things listed above, it will probably be 1 ounce or more. It would be less if it is designed for ham radio users. But I will first aim for the license free solution.

Last edited by Mr.RC-CAM; Nov 06, 2004 at 10:33 PM.
Nov 06, 2004, 10:45 PM
Sir Jasper
zl3vml's Avatar
Thanks for the new thread Kevin.
If you have a scope (or access to one) - measure the frequency of the AC into the step-up transformer. My guess is it would be maybe around 25-35 KHz. That would explain the small size of the transformer. I used two 3v lithiums, 6v to get the measured voltage of 30v. Current draw was around 78mA.
I tested the alarm part today and could hear it right accross the field, did not measure the distance as it was more than I need, a couple of hundred meters I would guess.

Now to work on the PicAxe drive circuit. I think I will glue the chip onto a piece of 1/32" ply with the legs splayed and solder the components directly to it.

Nov 06, 2004, 11:05 PM
Sir Jasper
zl3vml's Avatar
Here is a photo of the alarm (bottom) and after some prudent trimming of the pcboard (top) with the weights.

This is only the alarm circuit. The PicAxe timer circuit is yet to be modified and "slimmed" down.

Nov 06, 2004, 11:09 PM
Sir Jasper
zl3vml's Avatar
Err - can someone tell me how to get a smaller size photo? It is compressed down to only 67kB but still comes out so huge on this list - or is it just my browser?
Guess I should set my camera to a lower resolution....
Nov 07, 2004, 01:37 AM
Registered User
I have been playing at 433.92 MHz for quite some time now. There appears to be very few loopholes in the FCC regulations for unlicensed operation there and that will restrict the range considerably if operated legally there. Radiated power and on time restrictions under part 15.206 are the most restrictive. There are many devices on this and similar frequencys that are being sold that violate both the regulation and spirit of the regulation, but that does not make it any more legal for us to do it.

I chose 433.92 MHz as there is a large amount of equipment available on this frequency, and it is within the 70 cm Ham band. With the Technician class ham license so easy to obtain as a result of the rule changes of a few years ago there is no reason for anyone in the US to have to operate illegally. Also many overseas courtries specifiy 433.92 MHz as a specific frequency with rules that are much less restrictive than the US regulations. Of course that varies country by country.

For ham operation this is good news. There a a number of transmitter RF modules available that are intended to be built into end used devices. The small ones have about 10 to 15 mw outputs. If this is coupled to a somewhat resonant antenna, ie 6.7 inch chunk of wire, the field strength greatly exceedes the field strength perimtted for unlicensed operation, however it is no problem for ham operation. The recommended mode of operation for using these module in unlicensed mode for key chain type contollers is to put what ever type antenna they can fit in and then put an attenuator between the module that just brings the radiation within the FCC field strength limits.

I ran one of the 15 mW modules on the bench using a function generator for modulation. Ran it on 5 volts, but the operating voltage range of the module is from 3 to 12 volts. Used a 6-1/2 inch wire antenna. I walked down the street using a using a handi talkie with an AM detector in it and got 600 feet out before the signal started getting into the noise.

The module I was using was a Radiotronix RCT-433-AS. It weighs in at 1.25 grams and gets a little lighter when the connection pins are cut off short. Size is approximately 10 x 13 x 6 mm not including the connection pins. The modulation input directly keys the transmitter in what is normally refered to OOK (On-Off-Keying) and keys the entire transmitter so no power is consumed by the transmitter unless a positive signal is applied to the modulation pin.

Companion Receiver modules are also available. The least expensive is a super-regen with a RF amplifier to provide isolation isolation between the regen detector and the antenna to keep the radition from the detector low. It has excelent sensitivity, sub 1 microvolt. The one I have played with the most is the Radiotronix RCR-433-RP.

Since we are always concerned with cost, the good news is that these are low cost modules. The lowest prices I found were $4 for the transmitter module and $5 for the receiver module is quanity 1 type pricing. They are available from many sources, but I got my last ones from Mouser Electronics, Also similar modules are available from other manufacturers at similar or slightly higher prices.

I think that when used for a Lost model locator, that operation classification would be as telemetry under the Ham regulations and this requires that the transmitter send ID every 10 minutes. But, this ID could be sent contnuously as an A2 Morris code as used for your DF locating tone.

As an aside, I also have a RC Transmitter built up on 433.92 MHz running 500 mW power and have used the above modules for telemetry on a Hummingbird electric micro helicopter. But these are topics for other threads if any one is interested as I have already used up more than my share of this thread.

Gail -- W5MLY
Last edited by GailInNM; Nov 07, 2004 at 01:38 AM. Reason: Typo corrections
Nov 07, 2004, 02:34 AM
Registered User
Mr.RC-CAM's Avatar
GailInNM, All of your points summarize the issues. The FCC compliance is a tough nut to crack. I have some Lynx modules that are part of my backup plan, but like your Radiotronix solution, they would require a ham license to operate.

My psuedo-FCC compliant idea is based on using a stock key FOB. These low power devices could achieve 300 meter range with the proper antenna on a Rx that has good sensitivity. For example, my Ford's remote gives me 75 meter range in a crowded parking lot. With a 12dB Rx antenna I could stretch that to nearly 300 meters.

Hacking into the FOB would be a technical violation for sure. And the signal's expected intermittant use would exceed that which is allowed (it would be activated every 30 seconds or so). But, in the spirit of the FCC rules it would be something that might not be so offensive that it draws a lot of unwanted attention to it.

By the way, I search for FCC ID's to the RF devices described in the first post. I could not find any. Does anyone have the Reg ID for the Walston device?

Nov 07, 2004, 11:27 AM
Registered User
Has anybody checked out any other of the model rocketry (and HPR) location devices, besides the Walston? These are also typically very long range, since the models are unguided, have battery life that occasionally lasts for > a month, and of necessity are quite rugged too. Then there are the GPS-based units that transmit their coordinates...

The Walston has been used for many years, legal for sure in the U.S./Canada, was once the Cadillac of location devices, don't know if it still is. IIRC Walston has virtually zero formal web presence, they used to only accept orders by phone/mail, mainly just advertise in rocketry mags. All my good info/reviews on it are in print form, and of course I can't lay my hands on it when I want could call them.

Re the wildlife tags: I can sometimes pick some up from home with my handheld scanner, course I don't know where the animals are exactly, but they're *supposed* to be around 5 miles away (where their habitat is) range isn't bad considering their small antenna.

Many radio location methods are described in the ARRL (annual) handbook. Everybody who has any interest in any electronics should get this book at least once, it's the best bargain out there for reference, even if you're not interested in ham radio. When the new year issue comes out, the previous year's is dirt cheap, you won't be disappointed.
Nov 07, 2004, 05:56 PM the skies...
Cats Eyes's Avatar
Well, it looks like we've got quite a good discussion going here! Thanks everyone for all your input and ideas.

Mr.RC-CAM: I'm very glad you've chosen to tackle the RF tracker idea. Your list of requirements will certainly make it a challenge. You certainly seem much more aware of both the technical and regulatory issues than I am.

I "chose" the FM broadcast band rather by default -- the only schematics I'd found on the web were for the FM band. Plus the idea of using an old FM radio (I have two just "floating around") appealed to me. However, I'm certainly not married to the idea if something better or more practical can be found.

As far as I can tell, spectrum allocation and regulation is harmonized between Canada and the US. ("Harmonized" being a euphemism for "the US does what it wants and Canada follows suit"!) However, I'll try to find the Cdn regs and check to make sure.

For my personal requirements, you could certainly relax the "ease of assembly" and to some extent the weight and cost requirements. I was born with a soldering iron in my hand, and my brushless Slow Stick won't mind an extra oz. of weight. And since this is supposed to protect me from losing a plane worth, well, more than I care to think about, if I have to spend $100 or so on the solution, I'm willing to do so.

What I mean is, in the course of investigation if you come up with a good solution that is a bit more difficult to build, more expensive, or weighs a bit more than you had hoped, please share it with us anyway!

* * * * *

zl3vml: I hooked up the 'scope to look at the output from the step-transformer. With the piezo element connected, my AC voltmeter measured 26 volts (rms). I removed the piezo element for the rest of my tests to avoid driving my wife (and me) crazy and instead connected a 1.8K resistor. The attached pic is with resistor attached and is at 50uS/division (horiz) and 20 V/div (vert). Sorry the pic is so crappy, but hopefully you get the idea.

The frequency slides quickly up and down for a kind of "warbling" sound. The multiple tracings on the right show where the frequency is sliding through the range. I measured the period as going from 0.245mS to 0.455mS, or about 2.2 to 4.1 KHz. My guess is they do that for two reasons:
  1. To make sure it hits the driver's resonance at least once per "cycle" (avoids having to deal with manufacturing variations), and
  2. it sounds distinctive.
I think my batteries are getting old, so I was supplying the circuit from 4.5V regulated. At that voltage, it was taking 83mA. I haven't tried increasing the supply voltage; another thing to try.

Originally Posted by zl3vml
Err - can someone tell me how to get a smaller size photo? It is compressed down to only 67kB but still comes out so huge on this list - or is it just my browser?
Compression has nothing to do with image size (number of pixels), just the size/quality of the resulting file. Yes, it's huge for me too. If you can't resize in your camera's software, go get Irfanview (it's free), then use the resize/resample (resample is better quality) and type in whatever size you want. When you save, you'll have to check the jpeg compression ratio (under options) to make sure you get a small file too. For instance, I've resized to 640x520 below. Hope that helps.

Incidentally, how did you hack your case? I mean what tool did you use to cut the plastic? I thought that was clever to keep the battery compartment -- think I'll steal your idea!

* * * * *

Originally Posted by zl3vml
Has anybody checked out any other of the model rocketry (and HPR) location devices, besides the Walston?
I would dearly love to, if someone can give me a URL. The Walston (which as you say has zip web presence) and the Rocket Hunter (at tad pricey for my tastes) are the only ones I know about. If you know of others, please post the info.

Originally Posted by zl3vml
Many radio location methods are described in the ARRL (annual) handbook. Everybody who has any interest in any electronics should get this book at least once, it's the best bargain out there for reference, even if you're not interested in ham radio. When the new year issue comes out, the previous year's is dirt cheap, you won't be disappointed.
I had one of those out of the library and read through the section on Radio Direction Finding (RDF). It was a bit over my head, but when I read it I didn't have any particular frequency or anything in mind. Perhaps if I decide (or someone else recommends) a particular frequency, I can read up on antennas for that frequency specifically.

If I remember correctly, RDF transmitters are frequently on 144 MHz. Could we use that?

-- Kevin
Nov 07, 2004, 07:35 PM
Registered User
Heres a higher power FM beeper cct, should give 200M to 1 km depending on instalation and possition of downed model .
Nov 07, 2004, 08:13 PM
Sir Jasper
zl3vml's Avatar
Kevin - thanks, can resize the image size on the camera. Just lazy :-(
I used a Junior Hacksaw to chop the case. Need to take care as the plastic is slightly brittle and tends to splinter.
Just a note - some of the comments above were not posted by me - dues to go to someone else.
Ok on the frequency - I would have though it much higher.
As an aside - dont touch the that output circuit while it is going - has quite a punch in it!
Nov 07, 2004, 09:08 PM
Registered User
Kevin: I also was "born with a soldering iron in my hand", so some of the things I do for fun or challenge might be a real PITA for most...probably not useful for this discussion. I also don't mind doing some firmware etc.

I have not kept up with rocketry stuff much, things got tough for me after 9/11 because I have to get most stuff from the U.S. ... so I don't know what's current, but transmitting GPS coordinates was coming on, big rockets are expensive and travel a long way. Old GPS receivers are fine, and cheap used, you want the NEMA output models that put out ASCII text, that's what's transmitted, not that large when the case/keyboard/display is removed. For larger planes, but don't have to be huge by any means.

Cost effective stuff is modifying existing gadgets. I just used a Radio Shack very loud beeper ("personal alarm") in my rockets, modified to only beep every 30s to conserve battery, lasts a very long time. These don't exist any more I think, used to always be on sale for cheap and quite strong, not to mention annoying and loud, their purpose. As others have mentioned, these are NOT loud when the electronics are removed from their plastic case.

Modifying RF stuff is much more restricted in the U.S. than Canada. The basic mod for most Tx modules is a bigger antenna or an RF amp on the output. Not legal. You can only do so much with a weak signal without getting fancy (like for GPS reception).

For many reasons, AM may actually be better for this than FM. Due to FM's line-of-sight reception...we need range here, and to be able to get the signal out when there's obstructions. AM responds quite well to a rotatable loop antenna, for RDF. BTW, when researching on the web, RDF and "fox hunting" are ham terms that may be useful. Many of the gadget Tx are FM specifically because of their natural LOS range. We don't want LOS, if we did we'd walk straight to the model, or at least head in the right direction.
Nov 07, 2004, 10:49 PM
Registered User
AM or FM makes no difference as to if its line of sight , its the frequency used . higher frequencies are line of sight - lower not so .
Your FM radio works in your house ? in cities ? the advantage of FM over AM is it generally needs less power to get the distance , its a more efficient means of transmitting . the cct posted above should get 200m with only a 10" antenna fitted .
Nov 07, 2004, 11:07 PM
Registered User
Not true!

[Don't get the wrong idea, I only use FM myself. But AM sometimes has some advantages. Especially for distance.]

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