[Thunder Chicken]

So after loosing a plane in the bush over the weekend and searching for a beacon of some sort to help find it, I'd like to think about building something. A simple ELT for models, that has its own power source, triggered by a crash, not an extra channel or low voltage, with just a tone.

Full scale aircraft have an ELT. (emergency locator transmitter). It senses a crash, and transmits a tone on a certian frequency so other radio rx;s can hear it and home to it. (also satellites). They also have a speaker and emit the sirin noise for all within ear shot to hear.

With small electronics in cell phones, can one buy the accelerometer portion, have it set to trigger a small speaker at a certian g force, all powered by something simple like a 9v battery, or a couple of AA's. (with an on/off switch) This may not be the smallest device, but a .40 sized plane or larger could handle it with ease.

I've seen the beepers that HK sells. None would have worked for me......my aircraft battery pack pooched during flight. Even nicer would be some sort of GPS tracker to use with a phone app.....but that seems to complicated/expensive. Plus, I dont have a fancy cell phone or good cell service at our flying site.

I found a few diagrams of guys building g force meters to use in thier race cars etc. Something like that, set to make the tone go off, with its own power source would be ideal.
It may activate if you have a hard landing or do crazy aerobatics but it could be easily reset after landing.
Any electronics gurus want to comment?

[Thunder Chicken]

here's a link to an accelerometer:
http://www.zagrosrobotics.com/shop/item.aspx?itemid=728

[davidmc36]

I would say you would have to design a circuit that "turns on'" when a certain threshhold on the accelerometer is reached. Probably not too difficult with something like ATTiny and Arduino.

You can program an ATTiny chip to turn on a digital output when a certain analog input level or digital on signal is reached.

Have a look at what sort of signal comes out of the accelerometer.

Ya got my thinking cap heating up.

Edit: I just peeked at the Accelerometer again. Says analog voltage levels. Should be a snap. Have the ATTiny programmed to turn on a certain output if either the X or Y axis reaches a certain level. No problem to have two separate inputs on the Tiny. That way you could set it up for nose in to activate it or also if it hit "flat".

[Dennis Pedersen]

I remember one from years back that turned on once you turned the transmitter off.It would sound a loud buzzer once the signal was lost.

[davidmc36]

Originally Posted by Dennis Pedersen

I remember one from years back that turned on once you turned the transmitter off.It would sound a loud buzzer once the signal was lost.

That would be easy to do also with the ATTiny. It will activate a digital output based on a pulse signal input. So tell it to leave the buzzer off as long as a 1500ms signal is present. If it sees anything else, turn on the buzzer. I expect many devices like that are based around such microcontroller chips. I'm sure I have seen such devices for sale.

If you wanted to do DIY just for something to do, another idea would be if you know anybody in the avionics world they are tossing out loads of the old ELT's that only transmitted on 121, 243, 403. The G-switch inside them is pretty small, for sure doable for 40 size planes, even smaller.

I sure like the idea of the two axis sensor though.

[Dave Holmes]

Just do a search on "Lost Model Alarm" and you will find bunches of these things. I built some years ago from a circuit I found on the internet. Three cheap transistors, a few resistors and a capacitor or two with a Radio Shack piezo buzzer as the output. Similar ones are available at many hobby shops and even Hobby King has one.

The big "gotcha" (there are two) is that the piezo buzzers, while loud, are easily muffled. A sheet of tissue paper over the output side drops the sound level significantly. They must be "vented" to the outside of the plane. I cut a neat round hole in the bottom of my LT-40 to hold it so the sound could be heard from a distance.

The second "gotcha" is they need to be attached to the power source to work. In a severe crash (the kind I am usually subject to) everything gets disconnected. When your plane gets smeared across the landscape and the battery keeps on going, the alarm doesn't work to0 well.

Basically it is looking for the presence of a signal at a servo output, and when the signal goes away (turn off your transmitter) the buzzer sounds. You can usually hear it from 50 feet or more, and in tall tall grass, this is great.

[kip51035]

I still have a couple left that do the job. They work good on Futaba FM but not on Hitec, they jitter too much when transmitter is turned off. They plug into a servo channel and when transmitter is turned off they buzz.

[Max]

I like the idea of a lost model alarm triggered by an accelerometer.

I have a couple of lost model alarms that are useless in my modern receivers because when I switch off the TX, the RX goes into failsafe. This will not trip the alarm because the alarm sees a clean "failsafe" signal, and thinks the model is still flying and thus remains silent

[Thunder Chicken]

Thanks for the ideas guys.
Most of the hobby king and other 'cheap' buzzer alarms do not work they way i'd like. I want one with its own power source, and a seperate means to activate other than tx signal loss.
Another option I thought of....a little different direction though. Get a telemetry unit, with a GPS output. My thought is a seperate telemetry rx, connected with the GPS sensor, and a seperate ground station reading the GPS signal. (and whatever else I'd decide to attach) I run Airtronics, so adding one of the HK modules will not work, exactly. However, in my case, I'd run a seperate rx, small battery and the sensors seperately from the aircraft Rx. This way, it will run on its own and hopefully be more reliable during a crash or rx lock-out/failsafe event. With the GPS co-ords, the aircraft should be easy to find using a handheld GPS unit or cell phone with a gps built in.

I like the DIY of making an ELT though, just for the fun of it if nothing else! Having $100 invested into something that will help find a lost plane in the bush is well worth it considering the value of a larger airframe and equipment attached! Where we fly, it is surrounded by dense forest. Not fun trying to find a plane in. They always end up hundreds of yards farther away than you think!!

[davidmc36]

Originally Posted by Thunder Chicken

....I want one with its own power source, and a seperate means to activate other than tx signal loss....

That should be pretty easy to pull off especially if you are just going for a 'buzzer' type alarm. Package it all in a little box with battery, accelerometer, chosen alarm device, and even if it gets thrown clear the G-load will set if off and get you pretty close to "The Scene".

I didn't get too deep into the thread but there may be some ideas here"

http://www.rcgroups.com/forums/showthread.php?t=1521561.

If I were to make a robust little "Black Box" with a GPS I'm thinking I would want an audible device on it anyway. Probably won't be exactly cheap to make and would be nice to recover it.

You really got my gears spun up on this accelerometer idea. Probably too many projects this summer to spend any time on it but winter is looking good.

ATTiny, accelerometer, Arduino programming, stand-alone power source, two or three buzzers to cover different direction it may end up landing.....just need to hunt down a reasonably priced source of accelerometer, maybe even a three axis.