Lil Cooter

I don't exactly have a lot of money to spend on my CEN MT 2. I think the next thing to get is a failsafe to prevent the throttle sticking or run away problems or anything else. I was wondering if anybody has a good failsafe that wasn't outragously exspensive. (I was thinking venom.)
Also could someone tell me exactly what a failsafe does.

Thanks

popa71

The VENOM. Ive had the same fail safe on three trucks and its still working like a charm.

Element

I use the venom failsafe works great!

MattLarson29

The OFNA one is nice, too and is smaller.

A failsafe looks at the signals coming from the receiver to the throttle servo. If they become disorganized, as when there is a loss of signal or interference, it moves the servo to a position you set, usually full brake.

It may or may not help if someone turns on a transmitter on your channel, but will help for loss of signal or other interference.

Matt

DCLXVI

Some fail-safes kicks in when your tx pack are running low also...(if your running alkalines that would be a good feature)...

MattLarson29

Good point, I forgot the LVC (Low voltage cutoff) feature. Actually saved me from running with a low Rx pack once...

Matt

nitro maniac

1 of the reasons y i want 2 buy a failsafe is because if someone turns on their transmitter on the same channel it will stop my car from going crazy and does the ofna have a light like the venom to show whn it kicks in

ThermalRD

Nope, wrong, nada, dont believe that!!! I tested that theory right here with my own equipment. Test subject was the original venom, the mini venom, and the micro ofna failsafe. I had a full battery, and set the frequency on my synth radio to the same as my buddies MX3 radio. I turned on the test gear, and all worked great. Turned off the radio, and failsafe worked against signal loss. Turned the radio back on, and then turned on my buddies MX3 radio that was on the same frequency. All of the tested fail safes freaked out. They bounced from full brakes, to full throttle. NOT a good thing. I have a friend that claims his XTM failsafe will go full brakes when another identical frequency comes on at the same time, but i have not tried it. So basically, low batteries, and when your TX gets shut off, or loses a signal. Thats when the fail safe will work. Atleast with my test subjects anyway. And yes, i've had a runaway with a failsafe on my sons 2.5 maxx.

Matt

mecky33

Like matt said the Failsafe wont protect against frequency interference. The failsafe only looks for low voltage or loss of signal. the reason why the servos freak out when another radio is turned on is due to differing servo trim levels between the two radios. One radio is telling the servo one thing and the other radio is telling it something else, this causes a conflict in the receiver and it will try to follow both commands. There is no circuitry in the receiver that tells it to listen to only one radio. They could do it if they coded the individual frequencies, which when you think about it would be a really cool idea. That way it would be like the little FRS radios that you can buy at walmart. You have a few base Freq's and many channels on that freq to choose from.

nitro maniac

so im stuck ofna mini or venom mini (and does ofna have a light like venom 2 show whn it kicks in)

dryvr

go wit the venom.

MattLarson29

Mecky, don't take this personally but you need to learn quite a bit more about RF theory before you post things like this.

A radio signal is comprised of electromagnetic waves. The waves have a height (amplitude) and a length (wavelength which is related to frequency).

Frequency and channel are the same thing. They refer to the carrier frequency of the radio signal. What you are referring to as 'many channels' on a frequency are in fact subaudible tones that prevent the squelch circuit on the radio from opening the speaker unless that tone is present. If you take two sets of FRS radios on the same radio frequency, each with different PL tones and try to talk on both, they will interfere with each other. There are no 'channels' on the same frequency.

The radios we use are either AM, FM or PCM (a form of FM using digital instead of analog encoding of the data).

In an AM radio, the center frequency of the signal is constant. The information is conveyed to the receiver by varying the amplitude (strength) of the signal.

If two AM radios are one the same frequency, they will interfere to some extent with one another. The receiver will not be able to tell the difference between the signals.

In an FM radio, the amplitude of the signal is constant. The center frequency of the signal is varied slightly (known as deviation) to convey the information.

If two FM radios are on the same frequency, the receiver will select one over the other based on signal strength. This is known as the FM 'capture effect'.

In either case, the transmitter sends a series of pulses to the receiver. Those pulses tell the servos how to move. In the absence of interference, the signal is a clean, orderly series of pulses. The receiver takes these pulses in and distributes them to the servos. The servos see a clean, ordered series of pulses that are predictable in their timing.

A failsafe examines the series of pulses coming to the throttle servo. If the pulses stop, the failsafe activates. If the pulses change in timing, the failsafe activates. Some interference will activate the failsafe, but if the interference received is formatted in a manner cose to the desired signal from the transmitter, then the failsafe may not activate.

None of this has anything to do with 'trim levels'.

Matt

mecky33

Matt, I do know quite a bit about RF theory. I am an Electronics Tech in the USCG. As far as FRS radios, they have "frequencies" and they use "coding" to get multiple transmitters on one frequency. Most manufacturers use the term "channel" for the layman, because its easier to understand. So I was using layman speak so I didn't confuse those that dont have a PhD in electronics engineering. You say four radios, all on the same freq but with each pair on a different code will interfere? I have used Motorola Spirit radios, UHF with the channel coding, extensively when I was assigned to a local cutter. I never saw any interference with two sets of radios on the same freq but different code.

As far as your description of how these radios work. The reciever does not take the pulse and send it to the servo. The reciever takes the incoming RF, filters it, turns it into a control voltage and uses that control voltage to control amplifiers that inturn send more or less positive or negative voltage to the servo to control direction.

Also the transmitter does not send out "pulses" it sends out a stream of RF (waves) as long as the transmitter is on. These waves, as you said, vary in Amplitude or Frequency depending on your radio system. The failsafe is a passive device that monitors the signal coming from the reciever to the servo. If the failsafe sees an abnormal condition, I.E. an abnormal signal, loss of signal, or low voltage it will trip itself and you have to reset the failsafe.

Lastly, Trim levels have a lot to do with this. Different servos have different Center positions compared to the radio. This is why trim levels are there in the first place. As you set up your RC you manipulate trim levels to suit the setup of your car I.E. throttle position, steering center. If you let your buddy use your radio, he will have to reset those trim levels for his car and his set up. That said, If Radio A overpowers radio B, Radio A's trim levels as well as possible servo reversing settings will be sent to car B. This can cause many things to be instantly out of adjustment. This in turn can cause a run away, poor steering, etc.

MattLarson29

Absolutely wrong.

The receiver does not send variable voltage to the servo. The leads from the receiver to the servo are positive and negative power at a constant voltage and a signal line. The signal line sends pulses of varying widths that correspond to the desired position of the output. This is why servos are spec'd in terms of pulse width from one end of the throw to the other and not voltage.

Here is a brief explaination of how this works : http://www.brookshiresoftware.com/how_servos_work.htm

Matt

mecky33

Ok, I got one part of the concept wrong. The reciever is still sending a control voltage (signal), that is what the pulse is. And the Servo (I said reciever), using the control signal, determines whether to give more or less voltage be it positive or negative to control where the servo sits in relation to desired position.

ThermalRD

Man, you're both very informative, yet still manage to give me a headache. LOL

Matt

Mxracer45

Well your are not really protected untill you get a throttle return spring and a fail safe. Thats why your sons tmaxx ran away i bet. U didnt have a throttle return spring

ThermalRD

Throttle return spring wont save you in a runaway. If your bad signal that caused the run-away is wide open throttle, then the return spring wont do much good.

Matt

mecky33

Sorry for the headache matt, but now you know how your radio system works.

Mx, the return spring will only help you if your battery dies on the reciever.

Mxracer45

TRS and failsafe is better than just failsafe lol.

BUGGIES_R_US

Question.. Apparently the R330 RX has a Failsafe Built in.. It does work if my JR R1 TX is turned off.. BUT

What will happen if another radio in the same frequency but not PCM (analog FM) is turned on? My radio uses SPCM.. some marketing thing for a high resolution PCM aka "Super PCM"..

I was thinking that when the RX recieves a bunch of info without its own "packet header ID", it will reject it but I'm not sure..Havent had a chance to test it..

Also what if the TX comes in contact with a normal non PCM RX? Will it go nuts trying to make sense of the encoded "data"?

I guess what I'm asking is.. whether the transmission is actually encoded so it wont be affected by other transmissions in the same frequency like in WIFI..
(I know its totally different.. but you know what I mean)

ram

mecky33

They wont interfere with each other. The information coming from the transmitters is different and will be ignored by the receivers.

A good site though, with some more links at the bottom.
http://www.mh.ttu.ee/risto/rc/electr...dio/signal.htm

dryvr

This is getting good.

MattLarson29

Where on earth did you get the idea that two transmitters on the same channel won't interfere with each other? The PCM radio would go into its fail safe mode and move the servos to their pre-set positions.

Try it sometime - get and FM set and a PCM set on the same channel. They will definately interfere with each other.

Matt

mecky33

Read the three articles that are on that link that I posted. They run on completely different principals, there for they can not interfere. The PPM is a quasi digital, and the PCM is pure digital. The PCM will appear to have more interference from outside sources due to the way that the receiver decodes the signal, and the fact that the failsafe will activate more. The PPM will have less appear to have less interference because individual servo control signals become corrupt, instead of an entire series of signals.

In one of those articles it puts it the best way. PPM is like a Record player in that the needle bounces back and forth in the groove on the record to create the signal, thus making it analog. PCM is like a CD in that the CD is a series of numbers that tell the decoder what to make the signal, thus making it digital.

Anways, read the articles and you will see what I am talking about.