Black Box (alarm system)

by rcspaceflight | July 29, 2013 | (4) Posted in Tips

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.

COMMENTS

jd7792 on July 31, 2013
Great job! A lot of thought has gone into a relatively simple but very valuable idea. Could I suggest a little heat shrink over the solders would tidy them up and stop them getting damaged?
Also well done on the narrative, it made me even feel confident to tackle the project!

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Gopherdave on August 4, 2013
Nice Article:

I've been looking for something like this for some time, self powered, signal or crash activated, it's all there. I fear loosing my tri in long grass or a crop, I'd never find it. I'm half deaf, can't here my batt chargers go off when they are done, but 120db could wake the dead. I think I will use a diode on the relay, seems to be a common practice for servo to relay switch designs. Theirs lot's of other things you could do to this for your application. Great info well explained concept. Thank-you for taking the time to share with us. I've spent countless hours looking for lost models, mine and others. The field I fly at is surrounded by a farmers crops and he doesn't, like us to trample it all down to find lost models. Someone recently lost a model and drove all through his crop looking for it, that's a great way to loose a flying site! An old servo, relay and cheap battery is within almost anyone's grasp, great idea.

Dave
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#3 on September 24, 2013
Thanks again for your help I use the Turnigy TG9e servos the $2.00 ones from hobby king I will try the Y - connector on the throttle and let you know how that works. I came up with another idea last night I do not use the rudder to fly yet i plug it into the alirons so i could use your setup and add a manual switch to be turned off by the rudder servo if i crash/land and the receiver is still working i could set off the alarm with the rudder and if the receiver is not working then your alarm should sound the horn untill i get a raido with more switches then i can use the rudder for the rudder again thanks again i will let you know how the Y - connector works.
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rcspaceflight on September 26, 2013
I hope you get it working soon. If you have any more questions, feel free to ask.
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Antheman54 on July 31, 2013
Great idea good job

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c.sitas on September 29, 2013
I might have stumbled on to the relay not working. I never paid close enough attention to what I bought. I had the same problem ,the relay would not unlatch. Upon checking a little closer I discovered that I bought a 12VOLT RELAY. DUMMBIE ,I needed a 5 volt. Put that in the system and it works like a charm. Just thought there can't be anyone else that dumb, but posted this anyway.Thanks for the great idea, I also used the warble sound--AWESOM.
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rcspaceflight on August 5, 2013
Thanks for the compliments. I was worried that I let this article get too long, but I felt like it was all very important information. I wanted my logic behind it so people wouldn't make any mistakes or errors. Like when I cut a servo extension to see if that would work, turns out it doesn't.
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#3 on September 23, 2013
I tryed this and it all works except when i turn off the tx it will not drop the power to the relay/servo it is a great idea and i realy want it to work because i spend a lot of time in the corn feild any ideas i can try i am useing a spektrum dx5e tx and a ar6115e receiver and my servo chip/board looks just like yours
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rcspaceflight on September 23, 2013
I don't know. I used the cheap Hobby King 6ch Tx. I guess I assumed that when you loose the signal, from turning off the Tx, the servos loose power. Maybe not all Tx/Rx do that.

I know that you can use a servo as a switch. I think you have to keep the potentiometer intact to do that. It should still work then except for the signal loss part. If the battery gets disconnected, then the servo would loose power and the alarm should go off. If the battery stays connected, then you could use a switch on your Tx to activate the alarm. If you loose signal, then you should be able to get close enough to your plane to regain the signal and use the switch to turn on the alarm then.

Actually, I don't think you need the potentiometer to use the servo chip as a switch. See if you can set that channel up to a switch. Even if you don't get the signal loss feature to work, it should still be useable and help you locate your plane in a corn field.
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#3 on September 23, 2013
I am realy glad you answered because untill i get better at the controls i will be spending a lot more time in the corn feild i even have a long pole with a mirror on it so i can hold it up high in the corn to try to find it my tx does not have and spare switches but the receiver has a spot for gear and aux and they are where i plug in the alarm like i said it works great when i turn it all on but the servo board will not loose power one of the comments above from Gopher dave mentions a diode on the relay but i would not know how to install one or if that would even help is your tx one of the orange ones mabey i will have to get one i have all ready lost a few lipo batteries because they drained to low by the time i found my plane and they will not recharge
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rcspaceflight on September 24, 2013
I have the really cheap $30 Hobby King Tx. Not the Orange one.

I'm not sure on the diode either. I tried looking it up and it sounds like the diode goes between the servo chip and the relay. When the servo gets turned off it can cause reversed voltages. Or something. I know that diodes only allow electricity to flow in one direction. The diode would stop any reversed voltage. And if the voltage reverses then the relay would still be powered and stop the alarm from being activated. One diode from one of the wires to the relay should do. Just make sure you get the correct sized diode and it is installed the correct way. Actually, an LED, or Light Emitting Diode, is also a diode and works the same way. Not that you need an indicator light or anything, but that might be easier to find or you might have an LED somewhere that'll work.

Although I wonder what kind of servo you used. I used a 9gram Hextronik. I wonder if that is your problem. Hobby King sells a signal loss alarm and some one commented that it should work with any Tx when you turn off the Tx. Worst comes to worst you could just buy the Hobby King version but then use what you already made to boost it from 85db to whatever alarm you are using. Just cut the wires from the alarm/speaker and solder that into the relay switch.

Also I see that the Hobby King signal loss alarm acts as a Y-connector. If you have a Y-connector, maybe splice the alarm in with a different channel. If you use a different servo but leave the potentiometer intact, then you could splice the alarm in with throttle and set the pot to one of the lowest settings, then when you turn off your throttle the alarm would sound. Might get annoying at times. But see if just using a Y-connector solves the issue at all.

I say, Y-connector first, if that doesn't help, then try adding a diode or LED. If that doesn't work, then either try a different brand servo or buy the Hobby King signal loss alarm.
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teamhairball on January 5, 2014
This is really cool. I have been thinking about this issue lately. I can't believe this is not commercially available.

What I want is a lost model alarm that is self powered? It would be nice if it took power from the main lipo through a servo conncetion. This way if the main power source remains intact after a crash then the buzzer could be powered for much longer. If the main lipo is ejected on impact the device would use its own battery to activate the buzzer.


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c.sitas on July 23, 2014
I use these all the time now. In my opinion they're a must have.All you have to do is see that your gonna CRASH. Then just turn the transmitter off. The loss of signal starts this thing just a screamin. You should turn your tranny off anyway, you might save your motor from burn out cause it can't spin.
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Black Box (alarm system)