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Aug 14, 2015, 08:00 AM
Better SAFE than sorry!
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Mini-HowTo

Make a Lost Plane Alarm


The club field where I fly is in the middle of a large farm, which is wonderful because there are very few vertical obstacles and thereís open sky for thousands of feet. But of course itís inevitable that planes will go down in the fields. Typically the farmer plants corn, which is both good and bad for us pilots. Itís good in the sense that corn is tall enough and planted densely enough to save a lot of planes from making impact with the ground. The corn will ďcatchĒ a foamie and usually the plane will come out with minimal damage. Corn is bad in the sense that thereís zero visibility through the field when itís more than a few feet high, which is true during the entire second half of the flying season. So while the corn softens the crash landing, the challenge becomes finding the plane.

Thatís when a lost plane alarm comes in handy. Sure, thereís an assumption that the electronics on board the plane are still functional. But if that assumption works out, then you could find the plane by following the noise.

Iíve designed and built some alarms for my planes using some commonly-available parts on the Internet. It's the kind of thing you hope you never need, but it's nice to have.

First, you need a piezo buzzer to make noise. I lucked out and made a good choice from eBay on the first try. This is a buzzer thatís rated for input voltage anywhere from 3 to 24 volts, which means itís happy running off of one cell on a LiPo battery as well as other common kinds of batteries. Itís 41.8mm in diameter, or a little more than 1.5 inches. Itís rated for 95db of volume based on 12V input. Iím not sure exactly how loud it is with 3.8V, but itís loud enough for what it needs to do in this application. Installed on the inside of a foamie, I can hear the alarm at a distance of 100 feet. If you wanted to, you could choose to install the buzzer on the outside of the plane or give it a new mounting hole to let the sound out in order to increase the volume. The weight is rated at 11g, which Iíd say is a non-issue.

http://www.ebay.com/itm/New-5pcs-Pie...-/300798524104

The next piece you need is a way to turn it on and off. This Turnigy Receiver-Controlled Switch from HobbyKing works great.

http://hobbyking.com/hobbyking/store...arehouse_.html

The switch has a pair of jumpers on it that allow for a little bit of configuration with respect to when the switch opens and closes. The default setting (both jumpers installed) will leave the switch open normally and close the switch when it sees input of about +20% or higher (on a scale of -100% to +100%, 0% at center) on the radio channel itís connected to.

Finally, we need a power source. First I tried using a 9V battery, but that didnít quite work the way I expected it to. The switch comes with a small instruction sheet that points out that if you use a separate battery for the load on the switch, it must have a common ground with the battery driving the radio receiver or else the switch doesnít work. Thatís correct! I would have needed to connect the negative (-) side of the 9V battery to the negative side of the main LiPo battery. So instead of adding all of this complexity which led back to the LiPo anyway, I decided to just use the balance plug on the LiPo battery to tap the power from one cell. To make that work I used a balance lead extension cable. You can choose the correct size for your LiPo and choose the length you need to reach from the battery to the mounting point for your switch and buzzer. I cut off one end of the extension cable (the end that doesnít connect to the battery!), and I also cut off any wires that I didnít need beyond the primary negative wire (usually the black wire) and one of the positive wires for one of the cells (other colors; I chose the red one for convenience).

Those parts then get wired together using the diagram below. The red wires for the load on the Turnigy switch are interchangeable Ė there is no polarity to worry about. The switch should be inline with the positive voltage side of the load.

With everything all wired up, you can plug the servo lead from the Turnigy switch into an open channel on your receiver. Then, set up your transmitter to control that channel on a switch. The default settings on Spektrum transmitters should be fine. As I mentioned, a value of about 20% or higher will turn on the alarm. For a three-position switch, the transmitter normally sends -100% in position 0, 0% in position 1, and +100% in position 2. That means only position 2 will activate the alarm. For a two-position switch, position 1 will activate the alarm.

The buzzer is LOUD, so cover it with something while you test it on the bench!
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