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Lost Model Alarm [120139]

Status: Finished
August 1, 2012
There are a number of systems around used to locate lost R/C models. I considered that I could build a system that was a little bit better. The concept specifications were: 
  • Lightweight transmitter for the plane; 
  • Battery backup in case of disconnection of the main batte; 
  • Using commercial, type approved UHF ISM band radio modules; 
  • Range at least 600 ft. (20 m); 

Receiver to be handheld and able to use radio direction finding (RDF) to locate the model.


Mail from author 15-10-2011:

The project is loosely called a Lost Model Alarm (LOMA) , for RC aeroplanes, but it could be used to locate any device. This project is different to to others around, as it is a complete system and it is very inexpensive. The system consists of a transmitter, using a cheap, readily available 433Mhz (or 355 MHz) transmitting module available from a number of suppliers. There are two versions of the transmitter - one uses a
small lithium battery as a backup, in case the plane's battery is disconnected, and a version without any battery backup. Both versions are driven by a simple PICAXE microcontroller, programmed in PICAXE Basic. The transmitted signal can have its signature changed so that many users can use this system in proximity.

The receiver is based on a complementary 433 MHZ (or 355 MHz) receiver module. The difference in my design is that a signal strength voltage is picked off the recever and displayed on a simple bar graph. With the accompanying 4 element Yagi antennae, the modeller can find the
model by direction finding, since the bargarph will indicate the signal strength and hence the direction. There is also an audio output to a piezo speaker so that one knows that one is chasing ones own plane and not some other signal.

The use of standard receiver and transmitter modules means that not only that they are readily available all over the world, but depending on the country, the correct frequency for the ISM band is also available. There is no need to change the design (except the antennae) for the different frequencies.

The project also includes a design for a 4 element Yagi antennae. It is made from coathanger wires, and for stability it is housed in a small section of corrugated plastic.

The project uses readily available parts. The tranmitter uses a small SMD board, but it is only single sided and the receiver is built on a single sided board as well. This makes it suitable for the DIY person constructing this project. As well, there is only the transmitter microcontroller to program, and the software to download the program is available freely on line.

The schematics are attached. The system has been built and tested. The range is an urban area was more than 250 meters. I will make the code freely available. The article also contains some instructions on how to use Radio Direction Finding (RDF) to search for lost planes.

I can write the article myself (I have a lot of technical wrting and publishing experience) , and your editors would just need to adopt it to the magazines style.

I'm looking forward to your response

Robert Budniak

Additional material from the Author
Every radio control plane flyer at some time has had a plane go down outside of the airfield boundaries. Sometimes the location of that plane is easy and at other times they are hard to find. High grass, trees or even uncertainty where the plane went down all make it hard to find that elusive plane.
There are a number of systems around used to locate to lost models. I considered that I could build a system that was a little bit better. The concept specifications were:
Lightweight transmitter for the plane
Battery backup in case of disconnection of the main battery
Using readily available radio modules
Range at least 200 meters
Receiver to be handheld and able to use radio direction finding (RDF) to locate the model.
From this I was able to put to together the following design
Every country has a series of radio frequencies allocated to the Instrument, Scientific and Medical Band (ISM). These frequencies vary between countries, and in Australia it’s on 433MHz. Because there is a demand for simple and cheap transmitters, there are a number of manufacturers supplying RF modules to this market. The type of module used in this project uses Amplitude Shift keying (ASK), and these types of modules seem to have a standardized pin configuration from different manufacturers.
So that’s the RF transmission part taken care of.
The RF module just needs to have a data stream applied its “data’ pin, and for this project I used a simple Picaxe chip (PICAXE 08M). I chose this micro controller as it was readily available, the programming language was easy to learn, the programming language was easy to learn and the chip did not require any special equipment to program it. Really an ideal system for low volume and low complexity projects.
The code is only a few lines, and in the sample code for this project the coded generates three short 500Hz tones and then a pause of about 2.5 seconds. Whilst the code can be copied, would recommend that the line where the tones are generated is changed if you use a number of these transmitters near each other (like in a club environment)
The transmitter is powered of the planes receiver. However sometimes the planes battery gets disconnected in an accident, so a backup battery is provided. The backup battery is a small Li Po battery (around 120mAh capacity) that usually powers small indoor planes. This battery is available for a dollar from online suppliers.
Usually one uses steering diodes to automatically select the power supply. However with the low voltages used in this project (5 volts and 3.7 volts) even the 0.85 volt drop of a Schottky diode was a sizable proportion of the voltage. Looking through some online databases, I found some small signal transistors, which also had base-emitter and base resistors in the chip. They work quite well  in steering the voltages of the batteries, and the voltage drop across the transistor is just 100 millivolts. The only down side is that when the main battery is disconnected the backup battery takes over. So that the backup battery needs to be disconnected when the plane is not in use or it will run down.
So that the Lost Model Finder can be left permanently in situ (it’s cheap enough that one can have one for each plane), there is a combined charging port and on- off switch. This is achieved by using a switch 2.5mm jack socket and a 2.5 mm jack. With the jack inserted, the backup battery is disconnected. If a jack that is connected to a charger is inserted, then one can charge the battery. 
The receiver is based on a complementary module to the receiver. However a little more care is needed in the selection of the receiver.
The first output of the receiver is from the data pin. This passes to a small signal FET and the output goes to a piezoelectric transducer (not a piezoelectric buzzer). What one will hear from this transducer is the tones and pauses generated by the transmitter. In use one listens for one’s personal ‘call sign’ that was embedded in the transmitter.
The second output from the transmitter  is the RSSA output. Basically this is a voltage that is proportional to the signal strength and used as part of the automatic gain control in these receivers.
This signal is applied to an LM2917 LED driver. The RSSA signal on the prototype varied between 0.4volts and 2 volts, so the upper and lower limits of the LED driver have been set to those values.  I won’t go into the design of the circuit around the LM2917, since this is explained well in the manufacturers data sheet (If one wants a more detailed tutorial then point your web browser to HTTP://
The receiver is powered from four AA or AAA cells. Despite the transmitter’s data-sheet claiming a maximum voltage of 5 volts, the module has been tested to work up to at least seven volts.
The only other major item associated with the receiver is a 4 element Yagi antenna. This antenna was chosen for its simplicity and its directional properties. A feature which will becomes obviously necessary in the how to use section.
The transmitter is built on a single sided PCB used SMD components.
Once the PICAXE is soldered in, solder in the two transistors if using the battery backup version. Solder a length of servo cable and connector making sure that the polarity is correct.
At this stage one can test the transmitter by connecting a piezoelectric transmitter between the data and ground pins that go to the RF module. After applying power one should be able to hear ones call sign.
Once testing is complete, continue by soldering in the 2.5mm jack. The length of wire used depends on where in your model one mounts the transmitter and the jack. The jack can be left off completely if desired but then the backup battery will not work.
The RF module lies flat over the micro-controller board. This will require removal of the pins as supplied and joining the RF module with short wire links to the micro-controller board.
The final part is the installation of an antenna. A piece of stiff wire about 173mm in length will do (I use one core of an Ethernet cable)
The micro-controller board, RF module and backup battery (if used) are wrapped in a short length of shrink wrap tube.
The receiver can be built on a piece of perforated board, but the PCB makes it somewhat easier. Insert and solder all the parts into the PCB.
The antenna SHLUD be connected with a length of  coax cable, But in practice a short length of shielded cable would also be suitable with little loss of performance.
The antenna radials are made from coat hanger wire. Any stiff wire will do. Try to cut the radials to the dimensions shown in the drawing. For my prototype I cut some 3mm corrugated plastic sheet and inserted the radials into the cores. The spacing does not work out exactly, but works well in practice. Connect the antenna to the receiver via the previously installed cable.
Fortunately there is no calibration required. Leave the transmitter off at this stage. Turn on the receiver and one should hear white noise with the occasional pop. The LED indicator should be showing the lowest LED on.
Turn on the transmitter. One should now hear your call sign, and the bar graph should be moving up and down in synchronization with the tones of the call sign. The bar graph should go to full scale when the transmitter and receiver are within a few meters of each other.
In use
It’s going to take a little practice in order to successfully use the Lost Model Finder. Its best to initially have a fiend hide the transmitter in a park or garden and then try and find the transmitter.
The antenna is directional and the ‘sharp ‘ end is the end with the greatest sensitivity.
When looking for the transmitter, hold the antenna in front of one’s body and rotate a full circle. Listen for your call sign to make sure you’re chasing your transmitter. The bar graph will indicate where the maximum signal is. Walk in the direction of maximum. Continue doing this, stopping occasionally to check ones bearings. One should see that the bar graph increases.
At some point one will get close to the transmitter and the bar graph will be at maximum. 
Now point the insensitive end (blunt) end of the antenna in the direction where one thinks the transmitter is. Now when one rotates the antenna around one’s body, one is not looking for the maximum signal but the minimum signal. You’re sort of sneaking up from behind.
One can also change whether one holds the antenna radials parallel to the ground or perpendicular or any other orientation.
The last few meters are the hardest in finding a plane, especially one lost in vegetation or scrub. Also don’t forget to look up to the tree top canopy.

Additional material from the LAB:

The transmitter and receiver modules (both from manufacturer Jaycar) used in the original design are difficult to get, at least in this part of the world. They were substituted by types from Quasar. This doesn’t change much for the transmitter circuit.

For the receiver circuit however there’s a bonus. Instead of having to tap a signal from an integrated circuit on the module which is not provided on the standard connections the quasar receiver puts out the RSSI signal and can directly be connected to the display driver. The driver resistor (R1, receiver circuit) for the piezo transducer was increased to 1 kΩ, this reduces the current consumption somewhat. Most current is used by the display driver when a led lights up. Total current consumption is 40 mA (15 mA without a led). The receiver module only uses 6 mA. So with 3 penlights (AA batteries) as a power source the receiver can last more than 3 days continuously (assuming the capacity of an AA alkaline battery is about 3000 mAh with a light load). The piezo transducer (AC) is a small one, only 14 mm in diameter. The resonant frequency is 4.8 kHz and the rated sound level is 85 db. This particular one is not mandatory and any other one will work just as good. It was simply chosen because of its size. We’ve mounted the receiver module vertically on the PCB but if the legs are bent, it can also be mounted parallel underneath the PCB. The antenna, battery and transducer connections on the PCB are all pin headers. Of course wires can also be soldered directly to the PCB. The dimensions of the PCB are 44.22 mm x 30.25 mm.

The PCB for the transmitter is contains only the PICAXE processor (DIP-8), two digital transistors, 2 SMD resistors (bottom side) and 2.54 mm spaced connectors on every side (including the connection for the transmitter module K3). The dimensions of this PCB are only 21.77 mm x 17.78 mm. The transmitter module is even smaller and the 150 mAh single cell Lipo battery slightly bigger. The dimensions of the battery we use are 25,4 mm x 17 mm x 7 mm. But that may vary with other brands and capacity. If necessary a bigger capacity can be used, if the extra space and weight is not a problem. Not all batteries have the standard Molex connector (on bigger batteries with more than one cell this type of connector is used for balancing). That’s why we put a standard 2.54 mm pin header as a connector on the PCB. For a mechanical sturdy connection (especially after a crash of the model plane) we strongly suggest soldering the battery wires directly to the PCB and not use a connector at all. Beware not to short the battery when doing so. By bending the legs of the transmitter module the two PCB’s can be positioned parallel to each other (take care to bend the pins the correct way; otherwise the connections will be mirrored). Place a peace of thick insulating material between the two PCB’s (the sharp endings of the through hole components should not be able to perforate it) to avoid a short circuit as a result of mechanical vibrations or a crash. The serial download pins of the PICAXE processor are made accessible to facilitate ISP programming (K2). This way the processor can be soldered directly to the PCB and avoids having to use a socket. Instead of using the AXE027 PICAXE USB CABLE the well-known FTDI TTL-232R-5V cable can be used, but not without a little extra circuit. The little adapter consists of two inverters from a 74HC04. See schematic 120139-4_adapter_for_ftdi_ttl-232r-5v_schematic_v100.jpg. The two TXD signals need to be inverted to make the FTDI cable work with the PICAXE processor in the PICAXE Programming Editor. A suggestion for adapting the firmware is to tune the modulating frequency closer to the resonant frequency of the piezo transducer used should the sound level not be enough. We used PICAXE Programming Editor 5.5.1 (developed and distributed by Revolution Education Ltd).

The use of a 150 mAh single cell lipo battery compelled us to design a fitting charger. Of course there are single cell chargers commercially available but we designed a charger specifically for this little battery.

The selection of the proper IC was dependent on several criteria. Maybe not everyone knows but the regulation voltage of some lipo batteries is either 4.1 V or 4.2 V. Usually this voltage is not mentioned on the battery itself and you have to look for it in a datasheet or the specifications from the manufacturer. Many IC chargers only have a fixed 4.2 V regulation. It also would be nice if the charger can use a USB port as a power source and is able to charge with only a small current. The need for a minimal number of external components would be nice. Eventually a SO-8 from Maxim was chosen: the MAX1811. It uses an internal FET to deliver the charge current. In principle only two decoupling capacitors and for indicating the connected battery is being charged an additional resistor and led can be connected to a dedicated output. Two inputs control the mode the charger is operating in. One input selects the regulating voltage, 4.1 V or 4.2 V (jumper JP2, SELV). The other input sets the charge current, 100 mA or 500 mA (jumper JP3, SELI). A nice feature of this IC is the ability to precondition a near-dead battery before charging. The enable input (EN) is not used and thus permanently connected to the power supply. The general description in the datasheet states specifically it can be powered from a USB port and can handle input voltages as low as 4.35 V, the minimum of a USB port. With higher input voltages (the MAX1811 can handle 6.5 V maximal) and with the high current set the IC will limit the charge current to keep the die temperature at a maximum of 125 °C. In case only a power source with a higher voltage is available a low-dropout 5V regulator is added (IC2). A jumper selects the input voltage for the MAX1811 (JP1:AUX or USB). Do not connect an input voltage to the regulator if the USB port is selected as input source. The input decoupling of the MAX1811 (C2) is also de decoupling of the output of the regulator. This was done to save space. Cooling of the regulator is done by a copper plain on the back of the PCB. This method has its limitations so when using the regulator (JP1 set to AUX) we advise to use the 100 mA charge current only. D2 is added as a polarity protection. Voltage drop at 100 mA is less than 0.3 V. The reverse voltage is 20 V which should be enough in most conditions. Charge time of our battery (when it’s completely depleted) is probably less than 2 hours. The dimensions of the PCB are 29.84 mm x 20.95 mm. Instead of placing the pin headers for the jumpers wires are also an option, if it’s only for a specific battery. In our case the voltage and current setting don’t need to be selectable. And maybe the power source is a standard mains adaptor so AUX can be selected with a wire. On the model plane a socket will be used to make the battery accessible. So on the charger PCB two wires can be soldered from the output to a plug instead of using a pin header/socket.

We tested the range of the transmitter/receiver combination with two simple 17 cm wires as an antenna. The transmitter was lying on a desk in our office and we went for a walk with the receiver. We are situated in a castle with thick wales and surrounding the caste are also buildings. To be short at a distance of roughly 100 meters we couldn’t get a signal anymore. Later on we build an antenna using a Yagi calculator for VHF/UHF version 2.6.6 (2.6.5?; for Günter Hoch DL6WU Yagis; by John Drew VK5DJ). We entered the data and materials we used. The elements are made of 2.5 mm2 copper wire and the boom is made of a piece of wood of 570 mm x 53 mm x 12 mm (so non-metal). The boom is long and wide enough to hold the receiver PCB and battery holder and there’s even space left at the end to handle the antenna.

The dimensions of our Yagi-antenna (elements in order of placement):
Element           Length             Distance
Reflector          338 mm
Radiator           327 mm           138 mm from reflector (single dipole, length independent of gap)
Director 1        310.8 mm        51.8 mm from radiator
Director 2        307.9 mm        124.4 mm from director 1
Director 2 is placed 30 mm from the top of the boom
Again we went for a walk and the signal was lost at a distance of 233 meters. This means that the Yagi-antenna has more than 7 dB gain compared to the 17 cm wire (theoretical this is 8.6 dB). We build the Yagi with only 2 directors but the program can also calculate the antenna with 8 elements and more. The theoretical gain with 8 elements is 13.9 dB, so maybe this way the distance can be doubled again. The only downside is the length of the boom, which would have be 1.5 m (taken in to account the extra space for the electronics and handle). Of course other Yagi-antennas are possible and everyone is free to experiment. Simulating the antenna before building it could give a better result.

BOM Transmitter (120139-1)

R1 = 10 kΩ, 1 %, 125 mW (SMD 0805)
R2 = 22 kΩ, 1 %, 125 mW (SMD 0805)
T1,T2 = DTB123YK (SMD SC59)
K1,K2,K5 = 3-way pinheader SIL, straight, pitch 2.54 mm
K3 = 4-way pinheader SIL, straight, pitch 2.54 mm
K4,BT1            = 2-way pinheader SIL, straight, pitch 2.54 mm
Socket, 2.5 mm jack, chassis (connected to K5)
Plug, 2.5 mm jack, mono
Socket SIL, straight, pitch 2.54 mm (for K1-K5,BT1)
QAM-TX1 AM transmitter module (connected to K3)
BT1 = LiPo Battery 3.7V/150mAh
PCB 120139-1 v1.0
BOM Receiver (120139-2)
R1 = 100 Ω, 5 %, 0W25
R2 = 620 Ω, 1 %, 0W5
R3 = 360 Ω, 1 %, 0W25
R4 = 2k7, 5 %, 0W25
C1 = 100 nF, 5 %, 250 V, lead spacing 5/7,5 mm
D1-D10 = Led rectangular, 2.5 x 5 mm, 20 mA
T1 = 2N7000
IC1 = LM3914
BZ1 = piezo transducer, flying lead
K1,K2,BZ1 = 2-way pinheader SIL, pitch 2.54 mm
MOD1 = AM SuperHet Receiver 433MHz QAM-RX3-433
PCB 120139-2 v1.0
BOM Charger (120139-3)
R1 = 1k5, 5 %, 0.1 W (SMD 0805)
C1 = 100 nF, 10 %, 50 V (SMD 0805, X7R)
C2,C3 = 10 µF, 10 %, 10 V (SMD  0805, X5R)
C4 = 10 µF, 10 %, 25 V (SMD 1206, Y5V)
D1 = led red (SMD 0805)
D2 = PMEG2010AEH, 1 A, 20 V (SMD SOD-123F)
IC1 = MAX1811ESA+ (Maxim, SMD SO8)                 
IC2 = NCP1117ST50T3G (SMD SOT223)
K1 = mini USB receptacle type B, SMD
K2,BT1 = 2-way pinheader SIL, pitch 2.54 mm
JP1,JP2,JP3 = 3-way pinheader SIL, pitch 2.54 mm
JP1,JP2,JP3 = jumper 2.54 mm
PCB 120139-3 v1.0


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