Universal Failsafe

If you've been building electronic bits or modifying stuff you've bought then tell us all about it here. A good place to discuss the installation of 2.4GHz DIY modules too.

Universal Failsafe

Postby MajorTom » Thu Jul 08, 2010 5:53 pm

Hello, I had an idea I would like to share with anyone who may be interested. I have for years now been making small simple projects using PIC microcontrollers, and I think I have thought of a simple, feasible means of building a failsafe, with as many separate programmable outputs you may need. The PIC would measure the pulse width of each servo during programming, save them, and output those timings as soon as the Rx's signals become absent. Two buttons, one to cycle through the channels, and one to save the current position for that channel. The servos position is set via the Tx, and held while the "save" button on the failsafe is pressed. An LED will indicate the current channel being programed. I have never seen or used a commercial failsafe before, but I intend to use one for my FPV ambitions. The actual simplicity of this project makes it too tempting to resist. If there is enough interest in this, I would love to put it together for everyone else to enjoy. Let me know what you guys think!
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Re: Universal Failsafe

Postby soopy » Thu Jul 08, 2010 10:55 pm

MajorTom wrote:and output those timings as soon as the Rx's signals become absent.


I think this is going to be your biggest hurdle. How are you going to detect loss of signal? On the old PPM FM receivers, going out of range simply results in the receiver interpreting noise as valid signals and will output random signals to the servos. Any microcontroller listening would still be seeing valid servo pulses on the channels, only difference is that they are not commanded by the pilot. I'm not sure what these 2.4GHz systems do upon loss of signal. If they simply stop sending servo pulses then your idea should be feasible. If they act in the same way as the old FM units and output random servo pulses then it's going to be pretty hard to detect this.
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Re: Universal Failsafe

Postby MajorTom » Thu Jul 08, 2010 11:49 pm

Thank you for your input, I had not considered this before. I however do not believe that PPM FM systems will output a steady servo pulse train when out of range. They will spurt random positions due to noise from other radios, but I do not believe that the signal timing is regular as it is when you have a link. All one would have to do is wait for say 4 or 5 pulses within the proper time frame to determine its validity. I unfortunately do not have an older PPM FM radio to confirm this. I have a cheapo 3 channel FM radio from a Harbor Freight Wild Hawk plane, but this is not representative of what most people have. I have a Turnigy 9x on backorder, but it is of course a 2.4 GHz system. I have the proper test equipment to develop this type of thing, so I am. But I really don't see myself buying an older system to test it on. Perhaps if someone else can scope the output of a typical PPM FM Rx and share the results when the Tx gets turned off, I can accommodate for this in the code. Thank you for your reply!
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Re: Universal Failsafe

Postby jay-pheno » Fri Jul 16, 2010 12:05 am

sounds interesting , diy el cheapo i love , do you have a schematic an source code ? :)
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Re: Universal Failsafe

Postby MajorTom » Fri Jul 16, 2010 7:01 pm

I have a preliminary schematic. This is still in the "dreaming up stage". There are some issues I still have to sort out. I will try to get around to posting what I have already so anyone out there can point out any obvious mistakes that I miss. I welcome any criticism, anyone has. It helps me avoid making stupid mistakes, I make plenty of them! It's worth mentioning that the more people that seem to be interested in building this thing, the more likely I am to actually finishing it. To be honest I was kind of disappointed by the lack of interest. I know however that this could be very useful to people, and if I can save one aircraft, it will all be worth it! lol!
Thank you though, I will try to get the schematic up for criticism later tonight. Take care everyone!
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Re: Universal Failsafe

Postby ergocentric » Sun Jul 25, 2010 10:53 pm

a lost model alarm works on the same principle, no servo signal -> loud alarm
you simply have to turn of failsafe (if any) for this to work
fm receivers without failsafe will stop sending servo pulses if out of range or transmitter off
or assume no servo movement for one minute = failsafe condition
you can do all manner of things with servo signals (or missing servo signals)
http://www.rc-cam.com/lma.htm
http://www.vermontficks.org/servod.htm
unconcerned but not indifferent - MAN RAY
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Re: Universal Failsafe

Postby RCModelReviews » Sun Jul 25, 2010 11:27 pm

It's worth noting that Hitec's digital servos have a failsafe capability. With the matching servo-programmer, you can pre-set the position they'll go to if the data pulses disappear.
RCModelReviews.com, just the facts.
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Re: Universal Failsafe

Postby MajorTom » Mon Jul 26, 2010 3:30 am

I'm kind of thinking now that this may not really be worth the time. I just stumbled upon a site selling a single channel version of what I envisioned for $4.99 delivered! (www.rctimer.com) I think it even set up in a very similar manner to what I described! lol! I may still do it however. If I do, I will definitely share it with everyone.

How about some other Ideas for DIY projects for this site. I would LOVE to find something that lots of people would build and find useful. I'm just a hobbyist designer, but I've got the tools and know how to use 'em.

So anyone out there, bounce a few Ideas off of me. Get my imagination going.....
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Re: Universal Failsafe

Postby ergocentric » Mon Jul 26, 2010 2:37 pm

the concept behind the weatronics 72/35MHz system was channel diversity, but it is the same concept as failsafe.

the weatronics transmitter module was two transmitter models on two frequencies fed by a common PPM signal.

the weatronics 'receiver' was two receivers with a computer to decide which signal was better and pass that through to the servos
at the same time you can add battery redundancy, pre-programmed failsafe positions, and any other thing you can think of
the expensive part is the wires and connectors
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Re: Universal Failsafe

Postby ergocentric » Mon Jul 26, 2010 2:40 pm

RCModelReviews wrote:It's worth noting that Hitec's digital servos have a failsafe capability. With the matching servo-programmer, you can pre-set the position they'll go to if the data pulses disappear.


and Hitec used to give away windows software with a build it yourself interface, they have since deleted it from all their sites to boost profits on their new servo programmers.
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