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Radio Control Fail-Safe

NOTICE: This project has been replaced by RCFS-V2.

RCFS Board
RCFS is a tiny microcontroller based device that adds Fail-Safe features to any PPM (AM/FM) radio control system. It is very simple to build -- only three components!

Expensive PCM based R/C receivers have the ability to detect radio reception problems. On these systems, if the signal integrity is poor, the servos' positions can be programmed to default to emergency conditions.

However, Fail-Safe is not a common feature on the lower cost PPM systems. The Multiplex IPD Receiver family offers it, but none of the other radio makers have bothered to add it to any of their standard R/C systems. However, with the RCFS device discussed here, you can add Fail-Safe to ANY servo that is used on a standard PPM (AM or FM) radio control system. Cost is less than $3 per channel.

What Goes Up Must Come Down

The intent of any Fail-Safe system is to help reduce the danger that might occur when a model aircraft loses radio contact. Regardless of how you may personally feel about PCM radios and their Fail-Safe mode, I believe that most folks agree that programming an out-of-control model to slow down is not a bad idea.

The model will still crash, but hopefully the reduced speed will give it, as well as the folks on the ground, a better chance to survive the mayhem. It probably does not hurt to help stabilize the uncontrolled servos too, something that a Fail-Safe will also do for you.

Unlike the more sophisticated PCM systems, the RCFS can NOT be used to help clean up a "noisy" R/C installation. If your model is plagued with erratic servo glitches then do not expect this device to tame your problems. Yes, a PCM system would help, but blindly fixing the glitches with PCM is often considered a mistake by most experienced R/C'ers. That sort of discussion has already filled megabytes of space on the popular R/C forums, so I will not comment any further. The bottom line is that the RCFS will not help you in this area.

The Fail-Safe behavior on RCFS differs from what you would experience on a typical PCM system. We will talk about that in a moment. But for now, let me say that the RCFS attempts to address some common complaints. Most importantly, I wish to make it clear that using the RCFS device, or any other Fail-Safe system, is at YOUR risk.

Control Freak

RCFS is installed between the R/C receiver and servo. It constantly analyzes the incoming servo pulses and looks for serious trouble. Using a microcontroller, the pulses are checked to see if they fit within a allowable range, in a template sort of fashion. If the pulses are found to be acceptable, they are merely passed along to the servo. You will have full control of the model and RCFS remains a passive passenger at this point.

If the pulses are missing (lost signal) or invalid (noisy signal) then the RCFS attempts to determine the extent of the trouble. If the problem persists for more than a second the Fail-Safe feature is enabled. During Fail-Safe, RCFS takes control of the servo position. It can be set to hold the last valid position or it can move the servo to a preset position.

Complaint Department

My approach attempts to minimize the Fail-Safe's intervention. One of the common complaints with PCM Fail-Safe is that it totally masks reception troubles, especially the brief intermittent type. On a PPM system, these sort of "glitches" can be felt, so detecting trouble before things get out of hand is easier to recognize. I too believe that it is an advantage to be able briefly witness most glitches.

It is for this reason that the RCFS has been designed to NOT mask any brief periods of radio "hits." Random radio hits will be felt during the short windowing period. If the servo signal consists of a collection of good and bad pulses, the RCFS will do its best to remain off in order to allow you to "fly through" the noise. For sure, totally loss of radio contact will cause the Fail-Safe to switch on after the brief delay.

Note: Noisy signals that contain substantial amounts of valid servo pulses will not activate the Fail-Safe mode. This can be a blessing or a burden, depending on the situation. As I mentioned, this is so that your stick commands have a chance of getting through. A PCM system would either (1) mask the corrupted signals or (2) switch to Fail-Safe in extreme situations.

Timing is Everything

Servo PulseThe servo signal is a simple digital pulse. It spends most of its time at a logic low (0V). About every 20mS it goes logic high (4-6VDC) and then quickly goes low again. It is this tiny window of logic high time, called the pulse width, that gets the attention of the servo.

Please refer to the drawing. The period labeled "A" is called the frame rate. In the example it is repeated every 20mS (50 times per second), which is quite typical for many radio systems.

Modern servos define center as a 1.5mS pulse width, as shown by detail "B" in the drawing. Full servo rotation to one side would require that this pulse width be reduced to 1.0mS. Full rotation to the other side would require the pulse width to increase to 2.0mS.

In the eyes of RCFS, good servo pulses will be between 0.75mS and 2.25mS long. Even though a normal servo signal is 1.0mS to 2.0mS, some transmitters offer ATV settings that can extend the timing beyond that. The wider range taken by RCFS allows compatibility with such radio systems.

When servo pulses do not fall within the allowed range, they are flagged as "corrupt." When corrupt pulses are encountered, they are monitored for a one second period to ensure that there is sufficient cause to switch to Fail-Safe. The threshold before switching to Fail-Safe is managed by a complex software algorithm that was developed specifically for this project.

Lost and Found

Besides the Fail-Safe feature, you can also upgrade RCFS to include a Lost Model Finder feature. All it takes is a transistor and a loud Sonalert type piezo transducer. If you do not need the Fail-Safe support then a single IC circuit is all that is needed to make a complete Lost Model Finder. The schematic shows the details.

With the Lost Model Finder option you will be able to locate your downed aircraft simply by turning off the R/C transmitter. Just listen for the screaming beep tone. You may not be able to see it, but you will now be able to hear where the model is hiding.

Less is Best

The parts count in this project is minimal. All it takes is an 8-pin PIC microcontroller, 14-pin CMOS Nand Gate IC, 3-position DIP switch, and a capacitor. Shopping for these parts will be a breeze, since they are all available at Digi-Key. Your wallet will be happy too, since material cost is very low.

The chosen microcontroller is from the vast offerings of Microchip Technology. Actually, your exact PIC choices have some flexibility. You can use a PIC12C508, PIC12C508A, PIC12C509, or PIC12C509A.

The PIC12C50x is not a "Flash" part, so you will need a traditional PIC chip programmer to "burn" the hex file's object code into the microcontroller. Be sure to select the configuration fuses during chip burning as follows (these are optional settings within your chip programmer's menus):
WDT: Disabled
MCLR: Disabled
Oscillator: IntRC
Memory: Code Protected

The PIC's Hex file is designed to automatically instruct the programming hardware to chose these values. However, it is always a good idea to check them for accuracy. By the way, after you program the PIC your programmer will report a failure if you attempt to verify the PIC again. Do not be alarmed -- everything is OK. Just ignore the "failure." Whatever you do, do NOT program the chip twice!

If you have trouble burning the PIC, then please check your programmer. Whatever the fault, it is not a RC-CAM hex file issue. The most common problem is that the user has forgotten to burn the PIC's four configuration fuses, as mentioned above. More programming information can be found starting here.

The CD4011 Nand Gate is used as a 2:1 signal Mux. The PIC's SIG_OK control line determines the Mux's signal source (R/C receiver or PIC Fail-Safe). Please note that Pin 14 connects to +V and Pin 7 is ground.

The DIP switch is used to enabled the Fail-Safe preset positions. It is actually three little SPST slide (or rocker) switches in a single package. You could also use a header with shorting blocks, or simply use jumper wires if you do not need to change the settings. The switch settings are as follows (ON = CLOSED, OFF = OPEN):






ON ON OFF Preset Mode, -60º



ON OFF ON Preset Mode, -40º



ON OFF OFF Preset Mode, -20º



OFF ON ON Preset Mode, 0º (Center)



OFF ON OFF Preset Mode, +20º



OFF OFF ON Preset Mode, +40º



OFF OFF OFF Preset Mode, +60º


RCFS Construction:

Even though it is very simple, this project is best tackled by an experienced electronic technician. If you have successfully built any of the other RC-CAM Electronics Projects then you should have no problem with this one. Entry level technicians should plan on getting some hands-on help.

The RCFS board can be built using nearly any technique you wish. It really deserves a printed circuit board, but mine was built on a tiny piece of phenolic perf board. Be sure that your construction technique is worthy of a model aircraft's environment.

Layout is not critical. Perfboard ConstructionThe circuit was point-to-point wired using 30 gauge insulated Kynar wire. This wire is normally used for wirewrapping, but works fine with a soldering iron. I recommend a temperature controlled iron (700° tip).

The circuit can be hardwired to a servo's cable. However, I used a six inch servo extension that was cut in half. Installation in the model plane is a plug-and-go sort of effort.

Check it Out

Simple mistakes can destroy electronic parts and may generally ruin your day, so check your work carefully. Do not install the receiver battery until you have verified that the power leads are not shorted (use an ohmmeter). If all looks good, plug the RCFS into a channel of your PPM receiver.

Do NOT install the PIC chip until you have verified that U2 pin 8 is ground and the pin 1 has 4.5 to 6.0 VDC on it when a battery is connected. Remove the battery BEFORE you install the chip.

Now it's time to test your work. Just follow these simple test steps:

  1. Turn on your transmitter and verify that the stick controls the servo.
  2. Turn off the transmitter. In approximately one second, the servo should "Fail-Safe."
  3. Turn on the transmitter and verify control is regained within a half-second.
  4. Repeat the test for all eight combinations of the DIP switch settings.

Design Documents:

The technical details are available as file downloads. There is no charge for the information when used in a personal (hobby) project. Commercial users must obtain written approval before use.

Please be aware that the information is copyright protected, so you are not authorized to republish it, distribute it, or sell it, in any form. If you wish to share it, please do so only by providing a link to the RC-CAM site. You are granted permission to post links to the web site's main page ( Please respect this simple request.

Schematic Files Schematic Files: PDF file of the RCFS circuitry. All major components are from
Revision: Rev A, dated 06-17-2002
PIC Object Code Files PIC Object Code: Hex file of the compiled RCFS firmware. You should occasionally check for updates.
Revision: V1.0, dated 06-17-2002.

The Small Print:

If you need a part then please consult the sources shown in the project (see schematics download). I do not work for, nor represent, ANY supplier of the parts used in RCFS. Any reference to a vendor is for your convenience and I do not endorse or profit from any purchase that you make. You are free to use any parts source that you wish.

All information is provided as-is. I do not offer any warranty on its suitability. That means that if you build and use this device, you will do so at your own risk. If you find documentation or software errors then please report them to me.


If you have technical questions or comments about this project then please post it on the rc-cam project forum.

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