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Lost Model Alarm

Don't go MIA!

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!

LoMA is small but effectiveCan 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 the features:

  • Loud >85dB alarm gets your attention.
  • Built-in R/C signal glitch detector/counter.
  • 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 (~5-grams).
  • Only three electronic components.
  • 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 projects.

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 Digi-Key.

The chosen microcontroller is from the vast offerings of Microchip 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):
WDT: Disabled
MCLR: Disabled
Oscillator: IntRC
Memory: Code Protected

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 here.

LoMA Construction:

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 shown hereThe parts you need are shown on the right. Below is the complete materials list:
PIC12C508 U1 PIC12C508A-04/P
.1uF Cap C1 1203PHCT
Alarm Buzzer, 5VDC/35mA BZ1 102-1124-ND
Servo Cable J1 N/A

Trim the PIC's leads

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 orientation. Install the cap on the PIC

Bend the Piezo Buzzer lead

Bend the "+" lead of the buzzer as shown. Be careful or it may break off.

Solder the PIC onto the Piezo Buzzer

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 yet.

Elevate the PIC 1/6" above the buzzer (the space will be used to route the servo cable in a future step).

Solder in place. PIC pins 1 and 2 will be bridged, as shown in the photo on the right.
Solder the PIC onto the Piezo Buzzer

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.
The final assembly looks like this

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 crash.

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:

  1. Turn on your transmitter, then your receiver. You will hear three "hello" chirps from LoMA.
  2. Verify that LoMA is silent.
  3. 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.
  4. 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!).
  5. Go back to step 2. Each time you repeat the test, the glitch count will increase by one at step 3.

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!

Aircraft Installation

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.

Design Documents:

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.

Schematic Files Schematic Files: PDF file of the LoMA circuitry. All major components are from
Revision: Rev A, dated 09-23-2004
PIC Object Code Files PIC Object Code: Hex file of the compiled LoMA firmware. You should occasionally check for updates.
Revision: V1.4, dated 05-31-2010.

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.


If you have technical questions or comments about this project then please post it on the rc-cam project forum.

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