"Dollarama Special" Lost Model Alarm
For reasons I can't quite fathom, I've gotten more email about my non-completed LMA, the "dollarama special" that any other single thing on the web site. Go figure. Anyway, I'm sorry to say developemnt is at a stand-still, and never got past the testing device described below, since I've pretty much solved my "lost model" problems (at least in theory) with the Planelocator.
However, in case you're interested, here is the original article from the old site, as-is, with no edits or updates or anything (well, OK, I adjusted the colours of the schematic a bit because it was nearly unreadable with the new colour scheme). So, for you information only, Caveat Emptor and all that, here it is...
AP pilots tend to fly their models higher and "farther out" than your average Saturday model pilot. A number of AP pilots have lost their aircraft. Some have found them eventually, others have not. For example, see the following threads on RCGroups.com:
Most of us, aware of the dangers, give some passing thought to making our planes more "findable." For instance, many throw a cheap Lost Model Alarm (LMA) on our planes.
In September 2004 I had a plane go down in a bean field (fortunately, I recovered it soon thereafter). It was a fair distance away, and (my theory anyway) encountered some turbulence. I had trouble with "orientation" and just couldn't seem to figure out which way the plane was going before it disappeared below a line of trees. It had an LMA on board, a cheap PCB-mount piezo buzzer. It was of no use whatsoever in finding the plane. I eventually found it by sight. The LMA was functioning, but I couldn't hear it above ambient noises until I got to about 20 feet from my plane.
This disappointment, coupled with hearing of the plane lost in the above-mentioned thread All is lost, I decided I really had to do something to make my plane "findable." The "Dollarama Special" Lost Model Alarm is an attempt to cover the "short range" end of the search for a lost model.
The "Dollarama Special" is an entry alarm that I picked up at a Dollarama (shown at the right) and am considering as the basis for an audible Lost Model Alarm (LMA), for the following reasons:
- It is considerably louder than the piezo buzzer and therefore the sound should carry better,
- it has a "warbling" sound, rather than a fixed frequency, which should help the ear to distinguish it from surrounding noises,
- it has it's own batteries, so if the plane's battery or electronics is damaged in a crash, it should still function,
- it's cheap ($1), and
- replacement batteries are readily available (also cheap).
The purpose of this project is to make a test rig that can be carried to the field to verify that a functional LMA can be made from this device, and to check the range over which it is audible under a number of conditions.
It should be noted that the audible LMA will not be useful for longer distances. At most I figure it will be audible over a distance of a few hundred metres. For longer distance tracking, I am planning on making some kind of radio tracking device.
The following are some observations and measurements of the Dollarama alarm.
- Although the packaging says the SPL (Sound Pressure Level) is 90 dB, the alarm is actually much louder than the PCB-mount piezo buzzers I've tried (also rated at 90 dB).
- The active sound element is about 1" diameter. This is quite a bit more area than the piezo buzzers, which perhaps explains how it can produce the louder sound.
- There is a transformer between the electronics board and the sound element. I assume this boosts the voltage available to drive the sound element and achieve a louder sound.
- "On" current (battery drain), i.e. when the alarm is sounding, is 83 mA.
- The "trigger" current (quiescent current through the reed switch) is 2 µA (that's "microamps" or millionths of an amp). This current is drawn all the time when the alarm is armed but not sounding, so the current is kept as low as possible for long battery life. The total current draw from the battery when armed is also 2 µA.
- The type of battery cell used is AG13. A search of the 'net produced the information that this cell is rated at 148 mAH. This cell is also known as LR44. (ref: http://www.durabatt.com, Battery » Button Cell » Alkaline Manganese). So theoretically the alarm should last an hour and three-quarters! By making the signal intermittent, hopefully it can be made to last up to a day.
Modifications to the Alarm
The following shows the modifications made to the entry alarm.
The green wires come with the alarm. Note the solder joints were not very good in my unit, so you might want to check yours. These wires go from the battery holder at the right. The batteries are inserted from the top and are behind the plastic here. I have marked the "+" and "-" using pencil here.
The on/off switch is at the top. At the left, the round black component is, I suspect, a transformer to boost the driving signal voltage to the sound element. The sound element itself is the round component behind the board and transformer. At the bottom (behind the board) is the reed switch. When this switch is closed (by the presence of the magnet), the alarm is muted. If the reed switch is open (magnet is removed), the alarm sounds.
The grey wires are ones I have added. One goes to the battery negative supply. One goes to the battery positive supply, but after the on/off switch. The third wire goes to the sensor connection of the reed switch (the other side of the reed switch is grounded).
The added wires are connected to the "outside world" via a three-pin 0.1" male header. I drilled three small holes in the case 0.1" apart for the header to go through from the inside. The header is epoxied in place. This shows the header pins sticking out of the top of the case:
Alarm field tester
In order to conduct field tests, I coded a PIC12F629 with a program to implement an R/C switch. This code will be developed further to implement a full-function R/C switch similar to the Schieppati Switch (described on the RC Creative Electronics web site). At the moment, however, the code implements a single-function R/C switch.
As mentioned above, the alarm sensor is a magnetically activated reed switch. Within the alarm, one end of the reed switch is grounded (connected to the negative supply). The other is fed by a trickle charge of about 2µA. This current is deliberately kept low in order to prolong battery life. Rather than worrying about trying to switch such a small current (the leakage current of some devices can be more than 2µA), I found it easier to just drive this point high or low with a PIC output pin to activate or mute the alarm.
The following is the schematic for the field test setup.
The receiver is a standard aircraft receiver. Pretty well any channel can be used. The configuration jumper and bi-colour LED are mainly used during configuration to set the trigger point. The LED will show a slow green pulse if all is working and will flash an alternating green/red if the signal from the receiver is lost.
The GP0 pin is low for alarm off. The 10K resistor to ground ensures that the alarm does not sound when the power is first applied, until the PIC has finished initializing and pulls the pin low. The second 10K resistor to the alarm trip point is there simply as protection in case the stupid user (that'd be me) puts the plug in backwards!
As you can see, one of the nice things about this setup is that both the PIC and the receiver are powered by the batteries (three 1.5v cells in series for a total of 4.5v).
This is the test circuit board.
The receiver connection is at the right. The large yellow blob is the 0.1 µF capacitor across the PIC supply lines. The configuration jumper is at the bottom left (shown without the jumper on) and the Bi-colour LED is at the bottom right. The connection to the modified alarm is at the right. Both the off-board wires are made from ribbon cable salvaged from an old computer and soldered to three-conductor female 0.1" headers. (I'm so cheap!) The alarm connector mates with the male header I added to the alarm and the receiver connector fits into the standard receiver connector very nicely (you may have to bevel two of the corners with a file to make it fit).
It should be noted that the configuration and the LED aren't strictly necessary. I could have set up the trip points on the test bench (they are written permanently to EEPROM). However, this way the trip points can be reset at the field if necessary, and it wasn't much extra work to add these components.
I conducted a field test of the above on Oct 27, 2004 at the Central Experimental Farm. Wind was light from the North (I'm guessing about 10 km/h) and tests were conducted east-west (perpendicular to the wind). The test device was placed in short grass about 20' from the roadway. Distance measurements are approximate, based on counting steps.
Here are the field notes:
|Mostly discernable (echoes, direction sometimes not clear)|
|Mostly discernable (can't hear if facing exactly the wrong direction)|
|About 50% heard, direction is a problem|
|Audible at times|
|Faintly audible, occasionally|
The practical limit of the alarm under the above conditions is about 200 m. Different conditions will of course affect this, but I consider this adequate as a near-field locator solution. Once the pilot looking for his/her lost plane is within 200 m (depending on conditions, of course), this should provide an adequate means of completing the search and locating the aircraft. Coupled with an RF tracking device, this should provide the means for locating a lost plane under most conditions. So goes the theory!
Still to check are:
- Removing the covering over the piezo element may increase the SPL.
- Near the end, alarm sounded intermittently. I will have to check if battery capacity is a problem and/or if the PIC should be powered by an external source or perhaps just a capacitor to hold the voltage up.
- Other entry alarms/jogger alarms are rated at 120dB ("Dollarama Special" is only 90dB). I may spring for one of these (C$7) instead.