Black Box (alarm system)

Introduction:

Looking around the web it looks like most alarms for planes are pure crap. I don’t know about you, but I’ve crashed plenty of times that resulted in ejecting the battery. (I externally mount my batteries.) All alarms that are designed for RC planes require the use of the battery that you use for flight. I thought that was pretty useless.

Real planes have a black box. A device that is designed to survive a crash. Real black boxes are used to record all of the data needed to figure out why the plane crashed, but the RC version would be used as an alarm so the wreckage could be located.

I need a separate power supply for this black box incase I crash hard enough to damage a connection or to throw a battery. I thought about using a LiPo, but that seems like a very bad idea. If my plane gets stuck in a tree, it could take a long time before I retrieved it. If a LiPo battery becomes too drained, it permanently damages it.  An alkaline 9volt battery seems like the ideal choice to me. Actually, a lithium ion 9volt would be better because of how much lighter it is. But I went with alkaline.

I went to my local Radio Shack to look at alarms. Radio Shack calls them “buzzers” for some reason. They had some 80db, some 90db, but I settled on the 102db. Is it overkill? You betcha! But I know that the black box could land in the mud and be muffled. Besides, you want the alarm loud enough to be able to easily hear it from a hundred feet away, no matter where it lands, and with the alarm mounted inside of the plane. Foam tends to act like a muffler. The down side is that 102db at a high frequency tends to hurt the ears. And I’ll have to get near it to turn it off.

Originally I was going to make a home made switch to activate the alarm. It would basically be two pieces of foam board wrapped with tin foil. A zip tie would be sandwiched between them, and the whole thing would have a rubber band around it. It would work by having the zip tie either mounted to the plane or to a servo. When the zip tie got pulled out, the rubber band would force the two tin foil ‘plates’ together, closing the circuit and activating the alarm. There would be two ways that the zip tie would get pulled out. Either the crash would cause the black box to break away, because I would only use a very small piece of velcro to hold it to the plane. Or I would have to use a switch to turn the servo so it pulls out the zip tie. The key would be to have the battery mounted better than the black box. That way, if the battery goes, the black box is sure to go too. If the battery remains intact, I can use the servo. I decided that this system had too many ways that it could fail and I never bothered to try it. I only shared this idea incase it’s the way you’d prefer to do it.

After thinking more about it, I remembered about an electronics piece called the “relay”. A relay is basically a switch that is turned on and off by a separate electrical circuit. The way it works is that the circuit that is activating the switch powers an electromagnet. When there is current, the electromagnet pulls on a spring loaded piece of metal which either closes or opens a separate circuit.

After doing some research I found that a SPDT relay would be the one needed for this project. I think a diagram will explain why the best, so here is a picture I stole off of the internet. (The top two wires are for the electromagnet and would be powered from the Rx. The bottom left wire is the "COMM" wire. The bottom bottom right is the "Normally Closed" and the bottom top right is the "Normally Open". You want to use the COMM and the Normally Closed since you want the alarm to be On when there is no power from the Rx.)

When looking for a relay, it is important to know that the volt and amp ratings refer to the electromagnet of the relay. Relays are mainly used to allow a low voltage circuit to control a high voltage circuit. Since you will be using the Rx to power the electromagnet, you need a 5volt relay. The amps that run through your Rx depend on what ESC you are using.

Most relays are a plastic box holding everything together. I don’t think it can survive a crash as well as it should. They also make computer chip relays, but it sounds like those are less reliable. I think the durability of the black box has many factors. Like where you have the black box mounted, how well you put it together, and if you added any padding. I think a plastic box relay should be fine, as long as you take those factors into consideration. Remember, the black box will do no good if it also breaks in a bad wreck.

Most of the parts you need for this project can be found at your local Radio Shack. If you don’t live near one, you can go to www.radioshack.com and order the parts you need. Really any store that sells electronic parts should have what you need. These are rather common parts.

Material list:

 Here is a photo of all the electronics you'll need:

note: In the ( ) is the Radio Shack Product Number for the part I used. You can go to www.radioshack.com , search that Product Number and order the parts that way.

- A Servo. All you need is the chip. If you have a stripped out servo, use that.

 - 9volt Battery. I used Alkaline. Lithium Ion would be lighter.

- 9volt Battery Clip. (2700324)

- 5volt Relay. Apparently the one I used is rated at 1amp. My ESC has a 3amp BEC. Since I am also powering four servos, I'm not that worried. Plus, if I burn out the electromagnet, then the alarm would be activated. Not the worse thing. (2750240)

- 12volt Alarm/Buzzer/Siren. They didn't have any 9volt alarms, but the 12volt is actually 6-14volt. I went with the loudest they had. (2730079)

You will also need:

- solder.

- electrical tape.

- scraps of foamboard.

- hot glue.

- and velcro for mounting.

Electronic schematic:

note: As far as I know, it doesn't matter if you reverse any of the positive and negative wires.

Build:

The bag that the Relay comes in has a wiring diagram to show what each connection is for. Note that this is the same diagram I drew in the schematic. There is a lot of other useful information on the bag. Like the minimum and maximum voltage for the electromagnet.

Just so you know what to look for at your local Radio Shack:

 This is the "Piezo Siren" I used.

A photo of it's stats. Since I'm only using 9 volts, instead of the recommended 12volts, it's not actually 102db loud. But I tested the completed black box last night and I can hear it really well from a hund away, when it's in a wheat field.

I want to mention a few things about the servo that you use for this project. I decided to just cut off the potentiometer and it works fine that way. You use the wires that go to the electric motor of the servo. I do not have a working voltmeter so I didn't test how many amps are actually running to the relay. I have no idea how much resistance the servo chip has. Since I have a 3amp BEC and the relay is rated at only 1amp, it is a concern that the relay may fail prematurely. If you are really worried, you could leave the potentiometer attached to the servo chip and set that, and the channel on your Rx, to a place where it will have just the right amount of amps. That would be easier and cheaper than adding resistors. But who knows, maybe it's actually perfect the way it is. Again, since those 3amps are also powering the servos, it probably isn't actually 3amps that is going to this one. Nor would it be 3amps going to the relay because of the servo chip.

Incase you would like an explanation as to why you have to use a servo as the switch, and why you can't just use a servo extension, I will explain it here, feel free to skip this paragraph. There are three wires that come from the Rx to the servo. As you know, a Positive, a Negative, and a Signal. The Signal wire acts like a ground, even when your Tx is turned off. Since it is always a ground, and the Positive is always hot, the relay would always have power. Unless the battery got disconnected. Since it is always powered, you cannot use your Tx, in any way, to shut the power off, which would then activate the alarm. When your Tx is on, the Signal wire has, well, a signal running through it. It pulsates. A quick flicker of on/off/on/off/on/off. When you turn off your Tx, this signal is dead and the Signal wire is simply on. However, the servo can read this signal. And when there is no signal, when it is simply another ground wire, then the servo cuts power to the motor. So by using the servo, it makes it so when you crash your plane, but it's still very much intact, you can simply turn off your Tx, and the alarm will go off. Without the servo chip, the alarm would ONLY be activated when the battery gets cut. Which is pretty useless.

Okay, on the the actual build. The fist step is to alter your servo. Just remove the bottom four screws and pull out the chip. Cut off the potentiometer unless you want to use it to ensure that only 1amp is going to the relay. Cut off the motor, but make sure you have as much wire as possible, for these are the wires that are going to the relay.

Actually, this is probably all pretty straight forward and self explanatory. Just solder everything together. If you are uncomfortable with doing that, and you really want to make this, hopefully you can find someone that can solder it for you because there really isn't any other way to make this except to solder. This is the first thing that I soldered, so, you could always just learn. I guess you could use my original idea and make a switch with a zip tie in it, but using a relay is far superior.

 Please do a better job at putting it together than I did. I used hot glue on the relay to help ensure that the wires won't get pulled off of it. And to dull the pointy wires sticking out of it. I incased the relay in foamboard to protect it. I should have done the same with the battery, if not only to make it easier to change batteries. I put foam board between the battery and the alarm to cushion it. Then I wrapped it in electrical tape.

Three very important things to remember:

1) If you cover the air hole on the alarm, it muffles it. The louder you have this the better. Yet, you don't want mud getting into it.

2) You want to be able to unplug the battery easily. You don't necessarly need to be able to easily change batteries. (I have no idea how long the battery will last. I wish I did.) But you need to be able to disconnect the battery in order to turn off the alarm when not using it, or when you locate your horribly wrecked plane.

3) You do not want the servo lead to get pulled out. I taped it, folded it over itself, taped it, then folded it back and taped it again. This way it is the tape that is getting pulled on, not the connection of the lead to the chip.

 It's not the best looking thing. But it works great! So, I guess it doesn't matter.

Because some one is going to ask, and I'd want to know too, this weighs 99grams. (Remember, I used an Alkaline battery, a Lithium Ion would be lighter.) Just under what my 3 cell 1300mah battery weighs. Which, depending on the plane, can be rather heavy. However, this is great for FPV or any expensive planes. I think it's definitely worth the extra weight for the peace of mind and the protection of the investment of your plane. Losing $300 worth of electronics in a field is the worst. I assume because I don't fly FPV. But I have spent 20 minutes looking for a plane in a corn field before. NOT FUN!

So, that is basically it. I know this article got a little long but I wanted to have as much information as possible. Since I know a lot of people will want a loud alarm that can be activated by turning off your Tx, or if your plane gets it's battery disconnected. And this project only costs around $25. I don't want to meet the guy that doesn't want one of these.