PCS Electronics PCS Electronics - home page PCS Electronics  
RegisterSearchFAQAM & FM Radio transmitters shop UsergroupsLog in
Small, lightweight FM communication system

 
Reply to topic    PCS-Electronics Forum Index » Schematics - help and ideas View previous topic
View next topic
Small, lightweight FM communication system
Author Message
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Small, lightweight FM communication system Reply with quote
I'm looking to design a small, lightweight communication system, and I'm thinking that the FM broadcast band is going to be the easiest, since I can easily get small FM recievers.

However, the transmission is proving more difficult than I originally thought.

I tried small "bug" type transmitter circuits, mainly because of size, but they simply aren't stable enough. I'm fairly new to transmitter circuits, but I'm not new to electronics, but my expertise in magnetics doesn't help much for FM transmission.

So where do I find stability in a small size? I'm looking for something about the size of a 9V battery, and will be powered by a 9V battery. I can get a reviever easily the size of a 9V battery, so the transmitter, reciever, and power source will altogether be roughly the size of three 9V batteries mounted to the back of a motorcycle helmet.
Wed Jun 15, 2005 3:34 am View user's profile Send private message
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Reply with quote
I suppose I should clarify my goal.

I would like a helmet-to-helmet wireless communication system. They already market these things, I know, but they don't work for my style of riding. It cannot be push-to-talk, it must either be voice activated, or constant transmission. However, voice-activated poses significant probems, since my motorcycle and surrounding motorcycles in an off-road environment are usually loud enough to trigger false transmissions.

So I figure I'll use low-power FM transmission. I don't need a licence for under 5W, and recievers are easy, small, and cheap. Each helmet (a grand total of two) will transmit on different FM frequencies, with the FM reciever in each helmet tuned to the other's transmission frequency. No false-triggerring of VOX from other riders or my own engine since they will be constantly transmitting.

I'm not looking for several miles of range. I'm not sure of the relationship between wattage to range (in ideal terrain, of course). If I got a half mile, I'd be thrilled, but even 100yards is probably sufficient. I'm sure I can get that with 1-2W.

I can find a small transmitting circuit, but differences in tempurature, input voltage, and a host of other issues can easily shift the transmitting frequency by several hundred kilohertz, beyond the spectrum that the reciever is tuned to.
Wed Jun 15, 2005 3:51 am View user's profile Send private message
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
Wrong approach.

Get one of those 2way radios (walkie talkie) now sold at 49.99
or so for a pair.

They usually come with chargers and even headphones sometimes.

Set one to constant transmission and you're set to go.



_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Jun 16, 2005 1:36 pm View user's profile Send private message Visit poster's website
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
Wrong approach.

Get one of those 2way radios (walkie talkie) now sold at 49.99
or so for a pair.

They usually come with chargers and even headphones sometimes.

Set one to constant transmission and you're set to go.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Jun 16, 2005 1:37 pm View user's profile Send private message Visit poster's website
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Reply with quote
I haven't seen any of them with constant 2-way transmission. Don't they both send and recieve on the same channel? If so, and I'm in constant transmission mode, how do I recieve?
Sat Jun 18, 2005 4:08 am View user's profile Send private message
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
Do you need a two-way connection?

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Sat Jun 18, 2005 10:16 am View user's profile Send private message Visit poster's website
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Reply with quote
Yes, I'm looking for two-way communication.

I had considered the hand-held radios, like the FRS/GMRS, and utilizing the VOX setting, but I'm afraid I'll get way too much false triggering. My own exhaust is pushing around 96dB at full throttle, some others are well over 102dB. I think any sound-activation is going to be more hassle than it is worth.

If I decrease the sensitivity on the VOX enough to reduce or eliminate the false triggering, I'll have to scream just to trigger a transmission, and I'll be hoarse by the end of the day.

Push-to-talk is not an option, even handlebar mounted. I already have enough going on with the gas, brakes, and clutch. If I have to slow down or divert my attention to a button, I may as well just stop and talk to the other rider directly. That's the whole point of the communication system, so we don't HAVE to stop or slow down to talk.

I'm open to options, but so far a small FM transmitter and reciever on each helmet is looking to be the best option. I can transmit each helmet at a different FM signal, and have the reciever on each helmet tuned to the other's transmitting frequency. It limits me to only two helmets, (or, with two recieves and one transmitter on each helmet, I can have three-way communication) but that's really all I need.

I had a look around the PCS site, I may try the BUG-1. The BUG PRO I looks terrific, but at 400MHz, I can't tune into it with a standard FM reciever.
Sat Jun 18, 2005 7:14 pm View user's profile Send private message
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
BUG-1 is not suitable, it is not PLL locked. Frequency might
drift up or down.

Why not take two sets of 2way radios? Set one on receive
and the other on transmit (permanently).

Set the other set to another frequency, as far away as possible.
Might work and still be cheap.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Sat Jun 18, 2005 11:27 pm View user's profile Send private message Visit poster's website
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Reply with quote
2 sets of radios is an option, but not the preffered one. One radio is inconvenient enough, let along two. Plus, I wouldn't want to mount the radios directly to the helmet, so I would have wires running from a belt-mounted radio up the the helmet. It's not the end of the world, but not really the preferred choice.

I was really hoping for a small system I could mount directly to the helmet itself, the size of a couple 9V batteries. If I could shrink the size enough, I could even make a small cutout in the helmet liner and mount them INSIDE the helmet.

The description on the BUG-1 says it is stable with regards to battery voltage and vicinity of the body. I could shield the unit itself to minimize the effects of stray capacitance on the tank circuit, and encase the antenna in a rubber compound to reduce any effects from changing antenna geometry. If the BUG-1 truly IS stable with battery voltage, my only factor then would be tempurature. Do you still think it would have a significant effect?

Do you think I would have problems with the proximity of the transmitter interering with the reciever that is tuned to the other helmet? Would it help to set them at opposite ends of the FM spectrum?
Sun Jun 19, 2005 9:33 pm View user's profile Send private message
wilkin250r
New registered user
New registered user


Joined: 15 Jun 2005
Posts: 6

Post Reply with quote
This is probably an absurd question, but I'll go ahead and ask it anyways. The BUG PRO uses a crystal-controlled oscillator at 400MHz. Is it possible to change the crystal to operate in the 88-108MHz band? If you tell me the manufacturer, I have no problem asking them directly.
Sun Jun 19, 2005 9:55 pm View user's profile Send private message
pcs
Site master
Site master


Joined: 18 Jan 2002
Posts: 3063
Location: Radio Land

Post Reply with quote
The BUG-1 is more stable than 99% of the simple
free-running bugs out there, but its still just a simple bug.

If you want something like this, suitable for communication,
you need PLL or quartz stabilised oscillators.

You cannot change quartz to make bug pro I work in the
88MHz band.

Another thing, wideband FM is not suitable for this kind of stuff.
It is much better to go the NBFM way, requires less power to get
the same range.

What you want is not a good way to go about it. Sorry. How do you
think a cheap radio receiver will perform with a FM transmitter 5 cm
away from it? You'll have cross modulation, reduced sensitivity,
feedback etc... This might be a problem with 2way radios too, it'd
be best to have them for two different bands.

You also need some kind of audio processing, a limiter or even a
compressor. 2way radios have this already.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Mon Jun 20, 2005 2:28 am View user's profile Send private message Visit poster's website
Display posts from previous:    
Reply to topic    PCS-Electronics Forum Index » Schematics - help and ideas All times are GMT + 1 Hour
Page 1 of 1

 
Jump to: 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group