Losing a radio control
model airplane is an unnerving feeling. You know it must be nearby, but trees,
bushes, and random acts of nature, all seem to work against the search mission.
You wiggle the transmitter sticks hoping to hear the servos buzz, but you
get nothing. If only the model could talk back to you!
That is where lost
model alerting devices earn their keep. You can buy them from the hobby shop,
but how about making your own? Do you have a PIC programmer and a free hour?
So, let me introduce
you to LoMA, a tiny microcontroller based lost model alarm. It is very simple
to build -- only three electronic components!
You Hear Me Now?
LoMA plugs into your
R/C receiver and is silent until you need it to wail out to you. To do that,
just turn off the R/C transmitter. The missing radio signal tells LoMA to
reply back with an attention getting alarm tone. Now you can hear where the
model is, even when you cannot see it.
But wait, that's not
all. It also has an interference glitch counter. After you land, you can
listen to a series of beeps that indicate how many times you experienced
severe radio signal glitches.
Here is a summary of
Loud >85dB alarm
gets your attention.
Built-in R/C signal
Can be used as a reminder
to turn off the R/C receiver.
Works with AM/FM PPM-FM
or PCM R/C systems that utilize standard 50Hz framerates.
Plugs into any standard
4.8 - 5.0 VDC powered servo channel (spare or Y-connected).
Average current draw
is under 1mA when idle, 7mA during alarm.
Small size and weight
Only three electronic
Low cost. All parts
can be obtained from
Digi-Key for just
a few dollars.
Snitch on the Glitch
LoMA installs on any
servo channel you choose. It constantly analyzes the incoming servo pulses
and looks to see if they are valid. If the pulses are missing (lost signal)
or invalid (glitchy signal) then LoMA kicks into action.
In the case of a missing
R/C signal, LoMA recognizes that you want it to scream from its loud alarm.
However, if the R/C signal is present, but corrupt, LoMA will identify the
problem as an interference event. The total glitch count can be determined
by listening to the number of beeps you hear at the end of the flight.
The lost model alarm
is a distinct sound. It is a Beep-Warble-Beep series of tones. This beep
string helps you hear the alarm above the usually background noise. Here
is what it sounds like: Click Me -- LoMA Alarm
Tone (three beep strings).
Despite the simplicity
of this project, it is not recommended to beginners. It requires good soldering
skills and familiarity with building microcontroller based
Less is Best
The parts count in
this project is minimal. All it takes is an 8-pin PIC microcontroller, capacitor,
and alarm "buzzer." Shopping for these parts will be a breeze, since they
are all available at
The chosen microcontroller
is from the vast offerings of
Technology. Actually, your exact PIC choices have some flexibility. You
can use a PIC12C508, PIC12C508A, PIC12C509, or PIC12C509A. Project Update:
You can also use the easier to find PIC12F508 or PIC12F509 instead.
The PIC12C50x is not
a "Flash" part, so you will need a traditional PIC chip programmer to "burn"
the hex file's object code into the microcontroller. Be sure to select the
configuration fuses during chip burning as follows (these are optional settings
within your chip programmer's menus):
The PIC's Hex file
is designed to automatically set the programming hardware to these values.
However, it is always a good idea to check them for accuracy. By the way,
after you program the PIC your programmer will report a failure if you
attempt to verify the PIC again. Do not be alarmed -- everything is OK. Just
ignore the "failure." Whatever you do, do NOT program the chip twice!
If you have trouble
burning the PIC, then please check your programmer. Whatever the fault, it
is not a RC-CAM hex file issue. The most common problem is that the
user has forgotten to burn the PIC's four configuration fuses, as mentioned
above. More programming information can be found starting
LoMA's circuitry does
not need a fancy circuit board. Instead, we are going to build it on
the back of the buzzer. Not a single jumper wires is needed either. Could
it get any easier?
parts you need are shown on the right. Below is the complete materials
|Alarm Buzzer, 5VDC/35mA
Program the PIC using
the firmware file. The first construction step is to trim the PIC's legs
so that just a stub remains. See the photo on the left.
|Bend and trim the
leads of the .1uF cap and solder across pins 1 and 8 of the PIC, as shown
on the right. The cap is not polarized, so you do not need to worry about
Bend the "+" lead of
the buzzer as shown. Be careful or it may break off.
Place the PIC across
the buzzer pins as follows: Install the buzzer "+" pin between PIC pins 1
and 2. Place the buzzer "-" pin between PIC pins 5 and 6. Do not solder
Elevate the PIC 1/6"
above the buzzer (the space will be used to route the servo cable in a future
Solder in place. PIC
pins 1 and 2 will be bridged, as shown in the photo on the right.
On the opposite side
you need to bridge PIC pins 5, 6 and 7, per the photo on the left. It might
be easier to add a piece of wire to help bridge it all together, but solder
alone will work too.
Believe it or not,
the hard part is done.
Now slide the servo
cable under the PIC and pass the two power wires to the "top" of the chip.
Solder the negative servo lead to Pin 8 and the positive to Pin 1. The servo
signal goes to pin 4. It should look like the photo on the right.
Use epoxy or thick
CA adhesive to strain relief the cable. A piece of heatshrink should be used
to cover it all. Build it tough because you want it to survive a
Check it Out
Simple mistakes can
destroy electronic parts and may generally ruin your day, so check your work
carefully. Do not install the receiver battery until you have verified that
the power leads are not shorted (use an ohmmeter). If all looks good, plug
LoMA into a spare channel of your receiver. If you don't have spare port
then use a "Y" cable and share the throttle or gear channel.
Note: LoMA works
with nearly all PPM and PCM receivers. It is NOT compatible with R/C systems
that utilize slow servo framerates (like some Multiplex R/C systems use).
If you are using a PPM receiver, then choose a servo channel that does not
have exaggerated EPA/ATV settings. (use no more than 100% mix). If you are
using a PCM or 2.4GHz R/C receiver, then it must be programmed to send a servo signal that
is greater than 2.05mS during failsafe mode. This requires setting the
EPA/ATV to no more than 100% on the low side (LoMA fail safe off) and 150%
on the high side (LoMA fail safe on).
Now it's time to test
your work. Just follow these simple test steps:
Turn on your transmitter,
then your receiver. You will hear three "hello" chirps from LoMA.
Verify that LoMA is
Turn off the transmitter.
In a couple seconds you will hear a very distinct beep pattern (beep-warble-beep)
that repeats as long as the transmitter is turned off.
Turn on the transmitter.
The Lost Model Alarm will change to a single beep. This is the glitch counter.
The single beep means you had one signal glitch (yes you did -- you turned
off the transmitter!).
Go back to step 2.
Each time you repeat the test, the glitch count will increase by one at step
Power Miser Mode
There is a chance that
your model will be lost for more than a day. Sometimes nightfall gets in
the way or you need to return later with tree climbing equipment. Well, LoMA
comes to the rescue, again, with a power saving feature.
During the first hour
of alarm beeping LoMA will sound off with sixteen beep strings per minute.
This will consume 7mAH (milliamps per hour) from your battery. After one
hour of R/C signal loss, the rate will be reduced to 6 beep strings per minute.
This will consume 2.6mAH. This works out to about 80mAH per day, which is
not bad at all.
Once the alarm beeps
are in the low power mode (after one hour) you can remotely wake up
LoMA and restore the normal beep rate. Just return to the area where the
model was lost, turn on your R/C transmitter for a few seconds, then turn
it off. Pretty neat, don't you think?
Silence is Golden
There is a chance that
you have a model that experiences signal glitches, but still flies fine.
This is typical of some low cost park eflyer receivers. In this situation,
the glitch counter might end up being a nuisance rather than a blessing.
So, LoMA has another
trick up its sleeve. To disable the glitch feature's beeping, all you have
to do is disconnect the PIC's pin 2 lead and jumper it to pin 8 (short pin
2 to gnd). This will silence the glitch counter. Be sure to disconnect it
from pin 1 first or you will have smoke!
The alarm tone from
the buzzer is somewhat directional. So be sure to mount LoMA in a location
on the model that helps ensure it can be heard. If you want better audio
coverage you can modify the design by using up to two (2) of the buzzers.
All you have to do is solder the buzzers in parallel (pos to pos, neg to
neg). Mount them facing different directions for the most effective coverage.
The technical details
are available as file downloads. There is no charge for the information when
used in a personal (hobby) project. Commercial users must obtain written
approval before use.
Please be aware that
the information is copyright protected, so you are not authorized to republish
it, distribute it, or sell it, in any form. If you wish to share it, please
do so only by providing a link to the RC-CAM site. You are granted permission
to post links to the web site's main page
Please respect this simple request.
Files: PDF file of the LoMA circuitry. All major components are from
Revision: Rev A, dated 09-23-2004
Code: Hex file of the compiled LoMA firmware. You should occasionally
check for updates.
The Small Print:
|If you need a part
then please consult the sources shown in the project (see schematics download).
I do not work for, nor represent, ANY supplier of the parts used in LoMA.
Any reference to a vendor is for your convenience and I do not endorse or
profit from any purchase that you make. You are free to use any parts source
that you wish.
All information is
provided as-is. I do not offer any warranty on its suitability. That means
that if you build and use this device, you will do so at your own risk. If
you find documentation or software errors then please report them to me.