RC Plane Piezo Locator Circuit Powered by 1.5V AAA, Triggered by RX Power Loss

G'day all,

I have some very basic electronics experience and my soldering skills are acceptable. My main past time is rc aircraft and I am wanting to build a piezo sounding locating device for a downed rc aircraft.

There are available such units but rely on power to the main rc rc which is not always part of the scenario.

What I propose is the following and I would appreciate any assistance with this project.....

An extremely loud piezo alarm sounding on off on off etc, powered from a single 1.5v AAA battery or similar power source.
The trigger would be a relay of some sort that relies on an aprox 5v source being outputted from a channel off the rc rx. This could be triggered by turning the output of the rc rx off thus removing the 5v source or if power to the rc rx was lost then again the 5v source to the relay would be removed, thus triggering the piezo.

I'd like to keep the weight as low as possible as it's, as stated, going to be mounted into a rc model aircraft. Somewhere as close to or below 100g would be nice but I'd accept the battery would probably be the heaviest component..
The piezo needs to be really loud. I mean really loud. The aircraft could be in thick grass, up in a tree, even behind thick shrub foliage. I'd also like to see a dual output so a piezo could be mounted on the top and underneath of the craft. There's a good chance it could land upside down thus muffling the piezo if it crashes upside down, and vice versa.
I'd like to incorporate a simple switch so as to deactivate the alarm when the rc plane is not powered and sitting on the shed shelf.

I'm not sure how difficult this will be and I will add I'm in Australia so not sure what discreet components I will need are available to me but I am willing to do the work and get the results.

Again, any assistance is highly appreciated.

-Scott

As Colin says, you can buy small piezo sounders from electronics suppliers, which may be adequate for your purposes. But I get the impression you want to build a customised device that will be louder and lighter.

A basic flat piezo sounder element is good, because if its lightness. A simple circuit to drive it would be a 555 timer astable circuit to provide a square wave, perhaps powered by two CR2032 coin cells (or any lightweight battery pack 5 to 15V).
If your soldering skills (with a 1mm tip) get very good, you could use an SMT 555 for extreme lightweight and tinyness.

For longer battery life, you could use two 555s in a circuit that gives a beep every ten seconds.
(Just conveniently, 555s are available in a dual version, the 556.)

Experiment with different frequencies to find the best audible distange range.

Excellent site with calculator for 555 astable circuit component values: http://houseofjeff.com/555-timer-oscillator-frequency-calculator/

You might like to add a flashing light, perhaps a 0.5W LED giving a brief flash every five seconds, to assist location at night.
If used in a flashing circuit, LEDs can be intermittently driven with a higher than normal current for extra brightness, just keep the average power low.

Example circuit attached. Let us know if your need any more help or ideas.