FM transmission help needed

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Jacob J

Joined: 05 Jul 2009
Posts: 1

05 Jul 2009 17:29 fm transmitter signal range

Hello all

I have a modelship wich I want to keep in the range of 5-10meters from an fake island in a pond/pool. I think I need to build an FM transmitter to place on the island and a receiver to place on my boat. The boat has a constant speed, wich I dont want to change. I was thinking, if I can make the servo-motor in my boat odd, and when the motor is turned on, the servo makes the boat go straight and when the boat get out of the range of the island, the servo turns on, doing so, that the boat turns. When the boat is in the signal range again, the servo switches on, and the boat will sail straight forward again.

I has searched the web and found this circuit for the transmitter. It says that it is a low range circuit, so it should do it. But this circuit is build for microphone input. The question is, can I remove the microphone component and then the transmitter will send out a constant ON signal, or how does it work?

http://www.geocities.com/tomzi.geo/1-transistor/1-transistor.htm

Do you have a nice and simple circuit for a receiver. It only has to control 1 switch and thats the servo-motor.

Hope you can help me, like you did with my PWM controller for my DC motor, it works perfect.

/Jacob J

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KamalS

Joined: 14 Feb 2006
Posts: 47
Helped: 5

05 Jul 2009 23:18 fm transmissioner

Hi, forget about this circuit.. its unstable freq will drive you crazy.

PLEASE bulid http://www.geocities.com/tomzi.geo/2-transistor/2-transistor.htm

After that you need to send the control signal through the connector where the mic would have gone.

You need to build a small FM reciever also.. do you need help in that too?

Audioguru

Joined: 19 Jan 2008
Posts: 1144
Helped: 93
Location: Toronto area of Canada

06 Jul 2009 2:02 what are the limitations of fm transmission

Both FM transmitter circuits are garbage. Their frequency drifts all over the place.

You don't want to receive the signal with an FM radio. You want to use an AM radio with its AGC disabled. Then its audio detector output is the signal strength.

banjo

Joined: 24 Dec 2005
Posts: 644
Helped: 118

06 Jul 2009 13:47 what is fm transmission

Higher frequencies do not really help here. You could use something in the ultrasonic range for the same purpose. Basically, the lower the frequency the easier it is to tune it.

However, your system has some limitations. Lets say that it is all working and the boat is happily circling the island in a CCW direction. Some fool picks up your boat and rotates it 180 degrees. The control algorithm will turn your boat in the wrong direction and sail it out to sea!

KamalS

Joined: 14 Feb 2006
Posts: 47
Helped: 5

07 Jul 2009 3:11 build a fm transmission antenna

Audioguru wrote:

Both FM transmitter circuits are garbage. Their frequency drifts all over the place.

Umm.. did you try and build the schematic I linked to?

If not, would you be able to try and build it?

I built it and I am amzed at how solid it is! Serisouly - do try it out... you won't regret it Smile

Oh - could you share some nice FM mini DIY transmitters you have?

I am hunting one that has pre-empasis although I wonder if that would be an overkill for voice and data transmission (some few bps)..

What do you think?

Audioguru

Joined: 19 Jan 2008
Posts: 1144
Helped: 93
Location: Toronto area of Canada

07 Jul 2009 12:40 diy low power fm transmitter

KamalS wrote:

Audioguru wrote:

Both FM transmitter circuits are garbage. Their frequency drifts all over the place.

A hobbiest on another website built a similar circuit and said it was dead. I looked and saw a bias problem with its preamp transistor so I built it to confirm that the preamp transistor was saturated when the 9V battery was brand new and the transistor was cutoff when the battery voltage dropped a little as it was used. I fixed the biasing but noticed that the frequency changed when anything moved toward the antenna or moved away.

The frequency also changed as the battery voltage dropped.
It sounded horrible without any treble audio frequencies because it was missing pre-emphasis (treble frequencies boost that all FM stations have). It sounded like your stereo with its treble tone control turned all the way down because the de-emphasis in all FM radios cut the high frequencies.

I added an RF transistor to isolate the oscillator's tuned circuit from the antenna, added a low-dropout voltage regulator so that the preamp transistor's bias was stable and so that the frequency was stable, then I added pre-emphasis so that it sounds perfect. I allowed for the different pre-emphasis in the different parts of the world.

Its range is more than 2kM to my very sensitive home stereo tuner and car radio over a huge river valley with nothing in between, 400m to my cheap Sony Walkman radio and across the street to a cheap scanning radio from The Dollar store.

Here is my FM transmitter:

Sorry, but you need login in to view this attachment

vsmGuy

Joined: 26 Jun 2006
Posts: 489
Helped: 28

09 Jul 2009 0:34 miniature fm transmitter audioguru

Audioguru,

I have been reading your posts for some time now and it just shows how involved you are with electronics.

However, we must keep legal issues at mind as well.

According to the United States Federal Communications Commission Title 47 Part 15, "The field strength of any emissions within the permitted 200 KHz
(88 - 108MHz) band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector."

Without the jargon this means "Thou shalt be nice to thy neighbour".

1. I believe the final NPN stage that switches the LC circuit to the antenna can be skipped so that we get a shorter range? Maybe put a diode and resistor?

2. Make the length of the antenna less - something like 10'' maybe

These two would reduce the length of transmission but still I am not sure about FCC (albeit FCC now rarely acts on complaints unless it is from the aviation industry)

This would also reduce overloading of the local FM reciever.

3. I am wondering what the blue boxes are - chicklet caps?

4. Would it be possible for you to send some more detailed/clear shots of the well done stripboard?

5. I am amazed at this schematic - unlike the others, this looks like it is actually worth something.

6. Why didn't you use JFETs instead of the lossy BJTs?

Audioguru

Joined: 19 Jan 2008
Posts: 1144
Helped: 93
Location: Toronto area of Canada

09 Jul 2009 14:27 low power fm transmission in canada

vsmGuy wrote:

According to the United States Federal Communications Commission Title 47 Part 15, "The field strength of any emissions within the permitted 200 KHz (88 - 108MHz) band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector."

You get caught only if you cause interference to somebody. I used my FM transmitter for only 1 hour testing it because it caused interference to a low power foreign language radio station on the other side of my city because my FM dial is full of stations.

Quote:

I believe the final NPN stage that switches the LC circuit to the antenna can be skipped so that we get a shorter range?

The RF amplifier isolates the capacitance of something near the antenna from the tuned circuit of the oscillator.
Its power can be reduced by increasing the values of its emitter and base resistors.

Quote:

This would also reduce overloading of the local FM receiver.

Cheap garbage radio circuits are overloaded just by looking at them.
My home stereo tuner and car radio are never overloaded.

Quote:

I am wondering what the blue boxes are - chicklet caps?

European metalized plastic film capacitors. In the Orient they use "green caps".

Quote:

Would it be possible for you to send some more detailed/clear shots of the well done stripboard?

Sorry, it was made years ago and is gone.

KamalS

Joined: 14 Feb 2006
Posts: 47
Helped: 5

10 Jul 2009 1:54 reducing fm transmitter transmitting range

This is cool..

Audioguru, can we add http://web.telia.com/~u85920178/audio/stereo_enc.htm as the input to your transmitter?

Would it give nice MP3 transmission?

Audioguru

Joined: 19 Jan 2008
Posts: 1144
Helped: 93
Location: Toronto area of Canada

10 Jul 2009 2:13 telia.com fm transmitter

An FM stereo transmitter needs to have an audio bandwidth flat to 53kHz and without any phase shift to 53kHz.
My FM transmitter has pre-emphasis equalization that boosts high audio frequencies then cuts frequencies above 15kHz.
It won't work without de-design.