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Help with FM Transmitter

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j.vimal

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Hi
I would like some help regarding the construction of a simple FM transmitter. I have assembled (not yet soldered) the circuit as follows:
**broken link removed**

First i would like to know if the circuit is correct!
Also, i would like to know, what cold be done to improve its performance.
Also, what would be the range ? I am going to use a 9V battery (or eliminator).

Finally, what precautions should be taken so that it works perfectly?
I have a variable capacitor, but i havent shown it in the picture.

Thanks!
 
We can't see if the circuit is correct without a schematic. Frequently, schematics have errors that we can see, you can check your own wiring.

The parts are much too far apart to work properly at the very high radio frequency of FM. Kits have this cicuit the size of a stamp or less.

It is an extremely simple circuit, a toy that is not very good. It will drift its radio frequency if something (including you) gets near it, temperature changes including when it is warming up for a couple minutes and battery voltage changes as the battery runs down.
An expert reviewed kits like this, laughed at them and had his 6 year old daughter build them for him! He showed that circuits with a few more parts have better range and much reduced frequency drifting.

Its range? Without seeing a schematic how can we see? Most of these things transmit about 200 feet to an excellent analog FM tuner, they drift their frequency too much for a digital tuner. Maybe across the street to an ordinary radio or Walkman and maybe room to room in a small house to a clock radio.

The sound? Not very good. Distorted, muffled and certainly not stereo. OK for voice, lousy for music.
 
Hi j.vimal

I recognize that circuit, ive probably built it before, just a different variation. Wire it as I have in the picture.

I don't see a microphone though.

I'm guessing that you don't have the exact value of resistor @ the emitter of the transistor to the left, so you are using a parallel combination to achieve the correct value.

To improve range, add an antenna where I have, start out with like a 10 inch piece of wire and then clip it until you get the best range.

And yeah, the components are waay to far apart, you should make it as small as possible.

Range isn't all too impressive. With a similar design, I could walk to the other side of the street with my clock radio with the transmitter inside of my house. Or if the transmitter was outside, I could walk about 100 meters away.

These bugs are fun to build and then put somewhere in the house and then listen to everybody from your room. The 9v battery will not last very long. I built a very similar tranmitter that works off two AA batteries and it will last for about 5 days continously.

BTW, could you post the schematic?
 
Thanks a lot for the replies!

Now, again, i have few questions.
Why do the parts need to be very close for them to work properly?
Should I shield the inductor from electrical interference? So that it even works when I move towards it. :)
So, two AA batteries, i.e. 3V, works for 5 days continuously? Wow, Or Am I going wrong at predicting the voltage value?? :)

BTW, to avoid draining my pockets, I guess I could use an eliminator. :D or at least, rechargeable batteries!

I have attached the schematic...

Oh sorry, I forgot to include the mic :) But i am using the mic which my headphone has. And I have bought a male to female convertor too for that.

Thanks. I will put the parts close together...
 
Hi J.V.
The parts must be close together because the length of a wire determines its inductance, which is substantial at the very high frequency of 100MHz.
Shielding the coil won't make much difference because the antenna is connected directly to the coil (without an isolating transistor between) and therefore the antenna and anything near it become part of the circuit's tuned frequency.

This exact circuit is posted on this and many other forums.
At my first glance at it I saw its problems:
1) Transistor Q1 is supposed to be an audio amplifier for the microphone, but instead the circuit's poor design makes it a thermometer and battery voltage indicator.
If you have a brand new 9V battery, its voltage is 9.4V. R2 and R3 divide it by 11 to 0.855V. It is actually a little less due to Q1's base current, mentioned later.
Assuming a collector current of 1.8mA, the transistor's Vbe is 0.67V (see datasheet graph attached). Therefore the voltage across R5 is 0.185V.
With 0.185V across R5, its current is 1.85mA.
R5's current also flows through Q1 and R4. therefore the voltage across R4 is 18.5V. From a 9V battery? No, the transistor is saturated and can't amplify! Looking at the graph, just cool the transistor or reduce the battery voltage and it will begin to amplify!

Calculating the actual voltage at the junction of R2 and R3 can be done this way:
With the transistor saturated, the voltage across it is about only 0.1V and forms a voltage divider of the battery with R4 and R5. With 9.3V across R4 and R5 in series, Their current is 0.92mA and therefore the emitter voltage is 0.092V. Adding the 0.67V of Vbe, the base voltage is 0.76V.

A 9V battery is considered to be dead when its voltage drops to 6.0V. A similar calculation as above can be done with a 7.0V battery, and it will show that the transistor is cutoff and therefore also can't amplify.

Somewhere in between having a battery voltage of 7.0V and 9.4V the transistor will work. But looking at the graph it will stop if heated or cooled. Bad design.
 
Audioguru, If the given circuit is bad, could you post a better circuit? That would be helpful! Because, I havent yet started off.

Vimal
 
Hi Vimal,
It is easy to fix, I was hoping you would do it.
Before we fix it, there are other problems that should be investigated:
1) Muffled sound #1
The extremely high value (0.01uF) of C3 combined with R4 and R6 effectively in parallel, cuts treble audio frequencies above only 3.2KHz.
Replace C3 with a 0.001uF (102) ceramic disc capacitor.
2) Muffled sound #2
FM radio stations and TV boost treble audio frequencies when transmitting, called pre-emphasis. FM radios, tuners and TVs cut the treble boost back down to normal, at the same time reducing hiss.
This simple FM transmitter doesn't have pre-emphasis so its received sound will have treble audio frequencies cut. We will fix it in a moment.

Let's fix Q1 so it isn't just a thermometer and battery voltage indicator:
1) We need to increase the value of R5 to add negative feedback for stability. Change the value of R5 to 1K.
2) Let's add audio voltage gain so it is almost the same as before. Add a 22uF (C7) capacitor in series with a 220 ohm resistor (R8), and connect them across R5.
3) Let's add pre-emphasis. Connect a 220nF (0.22uF) capacitor (C6) for europe and Australia, or a 330nF (0.33uF) capacitor (C6) for North America across the new 220 ohm resistor.
4) Let's make a stable voltage divider to feed the base of Q1.
Change R2 to 200K and change R3 to 33K.
5) Let's add supply bypassing. Connect a 0.001uF (102) ceramic disc capacitor (C8) from the base of Q2 to ground, and a 100uF capacitor (C9) across the battery.
6) Let's give the transmitter some more range. Change the value of R7 to 220 ohms.

Build it this way and let me know how it works. Leave some room on your circuit board to add an antenna isolating and booster transistor beside Q2 later.
Check this out, Zach. :lol:
 
Now that you have a descent Basic Transmitter, Why don't you show him how to add a Final RF Stage to help stop Frequency drift from Hand capacitance and antenna changes?
 
Thanks!

I guess, I would start building this later. Because, I have my exams coming up! The final exams! So, I'd be better studying...

Please post more suggestions. Those would be most welcome. Or, If any of you had already built one, please DO post the schematic AND the actual image of your setup! That would be even more helpful.

Thanks. Once again...
 
thats perdy nice lookin audio :lol:

would i be able to transmit music decently over that? just curious?
 
Here's the final circuit that I think will sound so good, and have such a far range and fairly good RF frequency stability, that I might even build the darn thingy!
Soon I'll post a little RF detector to assist tuning its output.
Later I'll convert it for a 3V supply by changing a few resistors, and see how much its range shrinks.

My FM dial is very crowded with many stations so I will have difficulty finding an RF frequency for it. I might even have to get a license and get an existing station kicked off!
 

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Thanks Audioguru.

Well, I just dont know how and where to start! I am a 'newbie' to this field. Sorry, I think I should have mentioned it earlier itself...
Anyway, these things would be useful when I take Electronics Engineering at my college.

Also, Audioguru, could you suggest a circuit that would be simple and easy to build for a beginner? i.e. What was the first circuit you had ever built?

I am thinking of building an AM receiver, the simplest I have seen so far. The version in my textbook says: Only, an antenna, a resistor, a capacitor, a diode, and a headphone are needed.

I have the schematic.

Also, apart from all these, I keep worrying whether it would work. Was your very first circuit successful? I know, that one has to keep trying all the time. But, Now, I just dont have that much time!

Anyway, Would the receiver be a good idea to start with?

Also, I would like to know: What means of communication is effective for long range communication? Say: Communicate to your friends who are living about 10 km away. Is InfraRed a good one? (I think IR wouldnt even come close! But I am just curious!)

Actually, I had planned the FM receiver so as to reduce my telephone costs, which are soaring up high, due to my frequent conversation with my friends. But, I gues, building a transmitter would cost even more with the batteries! But, as I had mentioned, I would rather use an eliminator.

Thanks Audioguru for the help!

And, please help me find an effective means of communication with my friends! Hey, any means by which I can speak country-country??? Lol :)
 
Hi Vimal,
Build this transmitter, the simple one I corrected or the deluxe one, as your 1st project. One of my 1st projects was an FM transmitter similar to the one you started. I had lots of fun transmitting to the kit FM tuner I made earlier. I had a speaker in the open window and walked down the street hearing my own echo. But when I came back, it made an acousical feedback squeal that blew my tweeter!
I made lots of kits when I was young. The FM tuner, stereo amplifiers, an oscilloscope and a multi-meter. In those days there were kits for everything, including a colour TV. Checkout kits today, they are good to learn how to solder.

Don't make a super-simple AM crystal radio, they were for the olden days when there were only a few stations. Now, they pickup all the stations at the same time because they have only one tuned circuit. Real radios have many tuned circuits to pickup just one station and attenuate the rest.

Forget about Infrared. How far does your TV remote transmit? Only across the room.

I don't know where you are, but most places have laws about transmitting, so you don't cause interference to your neighbours. I have never done it but Ham Radio people communicate over long distances.

Are you charged for every telephone call? Where I am (Canada), I can make as many calls as I want, for as long as I want, at no charge, within about 20km. I do pay a small monthly fee whether I use the phone or not.
They tried to charge an extra fee for TouchTone like most people have, but I insisted on keeping my free clickety-click-dialer phones which I switch to tones for banking and automated routing after the call is answered.
I am charged a lot for long-distance calls but there are ways around it if not too far: I simply call a beer company or newspaper office, hear a short advertisment, then dial my long-distance call for free using their inter-office network. Frequently they are busy so I have to try them all or try again later.

Good luck with your exams!
 
I am charged a lot for long-distance calls but there are ways around it if not too far: I simply call a beer company or newspaper office, hear a short advertisment, then dial my long-distance call for free using their inter-office network. Frequently they are busy so I have to try them all or try again later

AHAHAHHHAHAH AAHAHAHA HAAAA!!!! thats the funniest hting ive heard al day!

Actually, I had planned the FM receiver so as to reduce my telephone costs, which are soaring up high, due to my frequent conversation with my friends. But, I gues, building a transmitter would cost even more with the batteries! But, as I had mentioned, I would rather use an eliminator.

forget about using even the audio's latest circuit for communication! unless your friend lives a block or two away. You would need a REEEEEEEEALY sensitive receiver to hear that thing 10km away. For that long of distance, you're gonna need a couple watts of transmitter power . . . which is probably highly illegal where you live :lol:

And, please help me find an effective means of communication with my friends! Hey, any means by which I can speak country-country??? Lol

You might look into some of those two way radio/walkie talkies. I've seen some that have like 10 mile radius (more than 10km). You wouldn't have to worry about batteries because you could very simply adapt it to work with a wallwart power supply. One of those oversized plugs that has a transformer in it.

Depending on where you live, you would probably have to get a license for a ham radio, not only you but your friend. Those walkie talkies require no license.
 
It's true, Zach. I use the networks of beer companies and newspapers to make free long distance phone calls.

I think it's very funny that in the UK every TV is licenced for a fee. Is it like that down under?

It's also true that these little FM transmitters don't go very far. Down the street about 100m.

I have two "walkie-talkies" that operate on about 900MHz with 1W output each and I think they are narrow-band FM. No licence is required but even with 38 channels, sometimes it is difficult to find an unoccupied one. When i am communicating, other people barge-in sometimes. I haven't tried distance but they are supposed to go 3km.
I thought they would be good to find my wifey in our huge food stores. We enter together then she shops while I go the other way looking for and looking at all the other pretty girls there. With my new tri-focal glasses with the latest high-tech coating I can see through ....., you know. Fantastic glasses!
To find my wifey after a while, I thought these talkies would work great. But because I joked around so much and transmitted embarrasing things to her she refuses to use them anymore. :lol:
 
It's also true that these little FM transmitters don't go very far. Down the street about 100m.

------------------------
actually, with an rf amplifier they can go as far as you wish
 
Hi Mastertech,
Here's a broadcast band FM transmitter kit with a 4W output. It draws 450mA at 18V. It has a driver RF amp and an output amp. Look at its heatsink. I wonder how far it goes.
 
lol audio, you sound like quite the character :lol:

I think it's very funny that in the UK every TV is licenced for a fee. Is it like that down under?

You mean you have to have a license to own a TV? No, its not like that here, btw, i live in the US

Do you live in the UK or Canada?

With my new tri-focal glasses with the latest high-tech coating I can see through ....., you know. Fantastic glasses!

Are you, ahem, kidding me!? right?
 
zachtheterrible said:
You mean you have to have a license to own a TV? No, its not like that here, btw, i live in the US

The TV licence in the UK fee funds the BBC, it means that BBC channels don't have any adverts on them.

Many countries throughout the world also have TV licences, but they generally have adverts as well.
 
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