Friday, August 19, 2011

DIY Lost Model Alarm for RC Plane

My Champ with bush tires
Earlier this summer, I bought and quickly lost a Hobbyzone Champ ultra-micro RC plane. I needed a Lost Model Alarm. Correction: I need to make my own LMA!

On my maiden flight, wind blew the light, 1.5-ounce plane away and I lost sight of it over the trees.

I'm not the only one. Many other Champs, and maybe other ultra light UM (ultra micro) planes, like those from Parkzone, have been lost by newbie pilots. (Some neighbors found mine, thankfully).



Ok, What Is a Lost Model Alarm?

A simple Lost Model Alarm (LMA) beeps so you can listen for it while you carry out your search and rescue mission. Most LMAs on the market are too heavy for UM planes and they plug into a standard 3-pin servo connectors that RTFs don't have.

Here's an ultra light, standalone LMA that runs off a coin cell battery.

Requirements
  • Standalone, no connections or external power
  • Less than 5g weight
  • Loud enough to hear downed plane in a backyard while standing on the curb
  • After 7 minutes, beep morse "W" for low battery warning
  • After 15 minutes, beep morse "SOS" assuming the plane is lost
  • Beep every 10 seconds for a total of 24 hours
  • Easily mountable inside or outside the plane with remote on/off switch
Design

 The design maximizes efficiency, runtime, size, and weight, the main tradeoff being loudness.

The schematic. Click to enlarge.
An ATtiny13V MCU handles the timing and drives an 80dB@10cm 14mm piezo beeper requiring only 1mA to make noise. The ATtiny13's single timer/counter is used to generate a 50% duty cycle pulse train to drive the buzzer at its resonant frequency.

To save power and extend runtime, the AVR chip sleeps and is woken up by the watchdog timer only every second where it checks elapsed time since startup and goes back to sleep unless there's some morse code beeping to do.

The whole thing is powered off of a 3V, 12mm, CR1225 coin cell battery with about 50mAH capacity. The entire board (the dual layer version) measures about 7/8" by 5/8". The AVR is in an 8-SOIC form factor to save weight and size.

Board Layout

I stuffed the board as best I could. Rather than mounting the buzzer through-hole, I created a custom package in Eagle to solder the buzzer leads onto surface pads horizontally.

Two-layer PCB, note "SW" in upper right.
One side contains the battery, the other holds the MCU and passives. To save weight I converted to a smaller pitch pin header. The header may be omitted in later designs and MCU programming accomplished with a SOIC-sized clip. That'd save a little more weight, too.

My first pro-fabbed boards! From DorkbotPDX
The two pads labeled SW are for a remote switch; nothing more than a shorting block on the end of two wires. If the unit is mounted inside a plane, the shorting block can be mounted outside.

Here's the current version of the hardware Eagle files as of this writing.

Software

The software is pretty simple. Beeping is achieved by setting up a 50% duty cycle PWM on the Tiny's single counter/timer then turning that on and off on demand.  The chip sets up a watchdog timer and interrupt to wake the chip up every second to check if it's time to beep or not.  For now the warning and SOS times are hardcoded but could easily be externally configured.

Here's the source code for the current version of the software as of this writing.

Tradeoffs

The buzzer is normally rated 80dB at 10cm running at 5V. It's quieter at 3V. The use of a CR1225 limits runtime and increases operating costs, with the advantage of weight savings.

Meeting Requirements

It is entirely standalone, small, light, and the code works well. It's a little quieter than I'd prefer but I'm considering some options to improve that.

The first prototype weighs 5.5 grams, slightly exceeding my somewhat arbitrary weight requirement. The second prototype above with compact 2-layer design, smaller battery holder, and smaller pin headers should weigh a little less.

First prototype is a little heavy at 5.5g
Runtime hasn't been tested but estimated to be on the order of 2 or 3 days.

Here's one of the assembled beta versions.


Cost overall is pretty low, far less than the cost of replacing a plane! So the next time I foolishly lose my Hobbyzone Champ, I should have no trouble finding it again. Just listen for the beeps.

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