RC Lost Model Alarm

Overview

Previously I built a tiny, lightweight lost model alarm for ultra-micro RC planes. I was contacted by a reader to design and produce a lost model alarm suitable for use in big, expensive RC planes.

My recently acquired SIG Kadet with 60" wingspan

Features

The large version features a much louder 100dBA piezo speaker and a circuit board that can accommodate a much larger 350mAH LiPo battery providing about two weeks of continuous run time. The basic code and hardware is the same.

• Standalone, no connections or external power
• LiPo power source
• Loud enough to hear downed plane in a backyard while standing on the curb
• After 15 minutes, beep morse "W" for low battery warning
• After 30 minutes, beep morse "SOS" assuming the plane is lost
• Beep every 10 seconds for a total of two weeks
• When LiPo voltage is nearing depletion, increase beep interval to 60 seconds
• Easily mountable inside the plane flush with exterior skin
• remote on/off switch
• remote LiPo charge port

Design

An ATtiny13 controls the timing and chirping and uses a voltage divider to measure voltage against an internal reference and chirps out the battery voltage upon startup.

After an initial delay period, the device chirps 'W' in morse code to suggest fuel or battery are depleted and that it is time to land.

After another reasonable delay, the unit chirps morse code SOS every 10 seconds to assist in locating the plane if it is lost.

Once battery voltage gets to dangerously low levels, the chirping interval increases to eke out a little extra audio location time.


A socketed 8-PDIP ATtiny13 is used for easy replacement and reprogramming.

Rendering of an older interim design

The initial version of this Big LMA seems to work well, but there are a few revisions planned for the next version that will make it a little easier for the hobbyist to install.

Power Conservation

It delays using a deep sleep mode to conserve power, and uses the watchdog timer interrupt to very briefly wake up every second.

As with the Ultra Micro version, this version also runs at a very low clock rate to conserve power. The PUI Audio piezo speaker (datasheet, pdf) is efficient drawing only 9mA at 100dBA.

Software and Schematics

Firmware (for AVR Studio 4)
Hardware (Eagle files)