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Author Topic: help...... Wireless Emergency stop  (Read 2647 times)

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Offline karmaxTopic starter

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help...... Wireless Emergency stop
« on: April 12, 2008, 09:05:15 AM »
I am making a wireless emergency stop which can stop my robot in drastic condition .i am using two microcontroler and connect them with rf module.and connect receiver end to relay which discoonect the connection to battery when i want to stop it.But it is not working well.....
 sometimes it transmit the signal,when i don want .i cant understand whats the problem .how i remove it .please help me.
 
« Last Edit: April 16, 2008, 08:52:56 AM by karmax »

Offline AndrewM

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Re: Wireless Emergency stop
« Reply #1 on: April 12, 2008, 09:15:15 AM »
Are you sure it is actually transmitting the signal and not just that the receiver is picking up a bogus (noise) signal?

A schematic of your transmit and receive circuits would be helpful for diagnosing
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Offline karmaxTopic starter

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Re: Wireless Emergency stop
« Reply #2 on: April 13, 2008, 12:30:21 AM »
ya it transmits when i press button .but sometimes when i dont press the button it also transmit .I want to know that is it due to noise in enviorment but it doest not so bcoz i am using 315 and 433 MHz rf module.I dont think so in enviorment there is any noise of this high frequency.

Offline karmaxTopic starter

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Re: Wireless Emergency stop
« Reply #3 on: April 13, 2008, 12:33:27 AM »
ya it transmits when i press button .but sometimes when i dont press the button it also transmit .I want to know that is it due to noise in enviorment but it doest not so bcoz i am using 315 and 433 MHz rf module.I dont think so in enviorment there is any noise of this high frequency.

Offline AndrewM

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Re: Wireless Emergency stop
« Reply #4 on: April 13, 2008, 07:26:04 AM »
There is noise in every frequency.  Have you tried running it with the transmitter turned off?  Schematics would still be good to see
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Offline Spoil9

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Re: Wireless Emergency stop
« Reply #5 on: April 13, 2008, 08:11:54 AM »
Not sure where you are at, but I'm finding that there is a lot that operates around 433Mhz.
In Europe, low-power communications is all over that freq,
433Mhz is also in the middle of the 70cm Ham Radio band.

Also, not sure how bad this can effect your project, but 216 is half of 433, and 216Mhz is used for TV transmissions.
866Mhz is twice of 433 and is used for cell phones, cordless phones, etc.

This is called Harmonics. Not sure if it means anything other than some math, but on this note...
Has anyone experienced interference through harmonics? Is this a real problem or just something some "expert" used to scared people with?
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Offline ed1380

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Re: Wireless Emergency stop
« Reply #6 on: April 13, 2008, 08:51:32 AM »
arent emergency stops supposed to be fool proof. ie wired?

if all else fails take a RC cars receiver and use it to disconnect a NC relay
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Offline AndrewM

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Re: Wireless Emergency stop
« Reply #7 on: April 13, 2008, 10:03:18 AM »
Has anyone experienced interference through harmonics? Is this a real problem or just something some "expert" used to scared people with?

Antennas for transmitting and receiving are designed around the wavelength to be transmitted.  You can google the formula for antenna length.  An optimal antenna is one that is exactly the length of one cycle.  From there cutting the wave form in half produces the next most efficient length, and half again, and again until it doesn't really matter.  So an antenna designed for 400MHz will also pick up 200MHz very well, and 800MHz just as well as the 400MHz signal.  But it will also pickup just about every other frequency out there, just to a lesser degree based on the wavelength and transmitting power/location.

That being said, every unshielded conductor with current running through it is a transmitting antenna.  Every unshielded conductor is a receiving antenna.  This is the reason that when you create a PCB you should have the ground trace cover as much surface area of one side of the board as possible, so that it can provide some shielding to the components/leads on the other side.

I ran into major "noise" issues with my home automation system when I setup the house microphones.  The top of the line sound card I was using for input to the computer was noisier than the cheapy sound card which was noisier than the even cheaper sound card built on to the mother board.  The reason was that the higher end sound card had more traces and components, so picked up more noise from both inside the case as well as external.  The motherboard based sound card has all the traces in parallel with the noise generating internal computer components, so picked up less RF from the computer itself, plus it lays parallel to the grounded computer case that absorbed some of the external noise as well.

This is also the reason why network ethernet cable leads have a specific pin out, each wire bleeds over to the next so the pin out is designed with a specific wire twist in mind to have unused (grounded) wires between used wires.
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Offline Admin

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Re: help...... Wireless Emergency stop
« Reply #8 on: April 20, 2008, 03:13:01 PM »
karmax, its really really hard to debug your problem when you don't tell us anything about your electronics setup . . .

As AndrewM said, its probably triggering on electronic noise. You need to design your system both mechanically and electrically to filter out noise.

Noise is typically short pulses, so have your transmitter emit long pulses, and your receiver only accept long pulses.

 


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