This project has been replaced by
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
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.
is not a common feature on the lower cost PPM systems. The
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
(AM or FM) radio control system. Cost is less than $3 per channel.
What Goes Up Must Come
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
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.
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
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 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
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
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
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):
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
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
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):
||Preset Mode, -60º
||Preset Mode, -40º
||Preset Mode, -20º
||Preset Mode, 0º (Center)
||Preset Mode, +20º
||Preset Mode, +40º
||Preset Mode, +60º
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
Layout is not critical.
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
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:
Turn on your transmitter
and verify that the stick controls the servo.
Turn off the transmitter.
In approximately one second, the servo should "Fail-Safe."
Turn on the transmitter
and verify control is regained within a half-second.
Repeat the test for
all eight combinations of the DIP switch settings.
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.
Files: PDF file of the RCFS circuitry. All major components are from
Revision: Rev A, dated 06-17-2002
Code: Hex file of the compiled RCFS firmware. You should occasionally
check for updates.
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.