Demo of a failsafe board for the Basic Stamp autopilot

In an earlier post, I discussed the importance of having a redundant failsafe board so you can regain control in the advent (likelyhood!) of an autopilot failure. Basically, what that board does is sits in-between your autopilot, RC receiver and servos. You switch it between receiver and autopilot with a spare channels on your transmitter. If your autopilot fails, even if it loses all power, the failsafe board, which is independently powered, will allow you to switch control back to the RC system.

Here's a quick tutorial on the board we chose, an RxMux from Reactive Technologies:.

You'll need to do two bits of soldering first. The board doesn't come with connectors, so you've got to order those. It's a little funky, but the way to do that is to "build a part" at Smatec. The connectors you need are Samtec TSW-108-25-G-T-RA, and you just enter in the right numbers or letters in each data field, and it then confirms that this is a valid part and lets you order a sample for free! They sent me three of these right-angle connectors, which was just what I needed. Thanks Samtec! Once you get them you solder them on to the RxMux.

The second bit of soldering is that you've got to make a couple of female-to-female Y-cables. Simply take three of these cables and cut them in half, then resolder the wires, two connectors on one side, one on the other, with heatshrink tubing at each junction.

Then just plug it all together. The channels that don't go through your autopilot can go straight to Input A on the RxMux. The ones that do go through your autopilot will use the Y connectors, with one lead going to the RxMux and the other to the autopilot. And all the servo-out leads from your autopilot go to Input B on the RxMux. It's a lot of wires, but it's actually pretty straightforward. There's no configuration required of the RxMux--whatever RC channel you connect to the Channel 8 input of the RxMux's Input A is the switching channel, and it just transfers control of the servos from Input A to Input B and back again as desired.

Here's a video of it at work:

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Comment by James Hall on February 18, 2008 at 2:10pm
Comment by Scott James on February 20, 2008 at 3:59am
That looks great!

I've been thinking about this....
Out of the three wires from the servo, is the only one we really need to switch (for control) the white one?
Would it be possible to just use a small relay by putting the servo on the COMMON and the autopilot on NO and the receiver on NC?? If so then all that would be needed is a simple circuit to convert a gear or aux channel to power a relay. I have seen similar circuits to control lights on models.
Obviously this would only be useful if you only were controlling one or two servos, I wouldn’t want 6 relays on my model. Do you think that could work??

3D Robotics
Comment by Chris Anderson on February 20, 2008 at 8:10am

Yes, I think that would work. Also, note that Dean Goedde (AttoPilot) is planning on releasing a much cheaper RxMux than the ReactiveTechnologies one. The advantage of doing it with a microprocessor is, as you note, that it scales to multiple channels much better.
Comment by rad man on February 20, 2008 at 5:49pm
this project is beginning to cost me more and more so i think ill just check and check EVERYTHING again

Comment by Sgt Ric on February 20, 2008 at 7:20pm
The point is, the cost of any of the systems highlighted here, is minor compared to a commercial product.

Another interesting aspect of the developement of these DIY auotpilots, is that they are increasingly cutting edge using processors, GPS's and other devices not yet seen in most commercial products.
Comment by Georgios on February 27, 2008 at 7:21am
I am new to this and I wonder where to buy a Basic Stamp autopilot. Is that a good choice?

3D Robotics
Comment by Chris Anderson on February 27, 2008 at 8:21am
Georgios, please go to the home page a diydrones and follow the link to GeoCrawler 3. After you do some more reading you'll understand what we do here. You can't "buy" a Basic Stamp autopilot--you build it.
Comment by neil on May 23, 2008 at 11:15am
i have all the component ready and have following your step by step process ,is there any new updates besides the fail safe board?has anyone airborned with this set up yeat?
Comment by Mike Anderson on May 29, 2008 at 11:03am
Does this RxMux replace the servo driver (either parallax servo driver chip or the FT 639)? I don't see either there in the photo or the vid. I assume you just removed it for simplicity?

If the servo driver is still needed, I assume it would go in between the AP and the RxMux's input B?

Great stuff!
Comment by Mike Anderson on May 29, 2008 at 11:06am
Another stupid question, sorry...I'm guessing the elevator and rudder are the only servos you run through the autopilot because they are all that are needed for navigation?

How is throttle handled? Do you just keep it constant? "set it and forget it"?


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