FM transmitter -reallly need help-

hello...

Im building a FM transmitter but it just doesn`t work well.ive been building about 4-5 diff types just to at least get the working one.ive got all the parts correctly but only for the inductance (which have to be hand made) and the elecret mic.

the problems:

1-1st circuit : can transmit but gone in just 1m.(the circuit should be about 100m)

2- 2nd circuit : nothing (no hawling sound and no voice)

3- 3rd circuit : got howling sound but no voice (claimed to be 200m-500m by pcs-electronics web)


questions:

1. since all the parts and connection been triple check i assumed it should be the coil for the inductance.what are the diff between the AWG and SWG copper magnet wire should be used to wind as the inductance.i couldn`t get the AWG wire but just the SWG. is it the AWG for the US and SWG for asia? (im in asia)

2. is that copper wire that should be use is the same as in the dc motor winding? can i just use the normal single core wire as the inductance? will that affact the circuit?

3.how to actually solder the copper wire to the vero_board since i found it couldn`t be soldered (im using the SWG). can it be replace with the fix one stated 1uH or anything similar the the value of henry given by the coil by using the henry equation.how does the diameter of the copper wire affect the henry value since i found no diameter needed in the henry equation.

4. how to actually wind the copper wire.should it be a gap between the loop.can i just subs the copper wire using the normal single core wire by stripping the insulator.

5.how does actually the electret mic looks like?is it a small one with a size of a button of a shirt diameter?how will the internal resistance of the electret mic affect the circuit .ive got a few of them but with diff internal resistance (by taping bot mic`s pin with the multimeter and check for resistance).

maybe some of the method i used to check the components was wrong and please anybody correct me.im no electrical nor electronics student but machanical.i really interested in electronics hobby and i`ve built quite a few in the pass but this is the 1st time regarding transmiter and wave.

i really need some help or comment/advice from u guys since ive been working on it for everyday for a week till now.

really appeciate it.thanks.

Hi Manz,
The FM transmitter circuits you built that made howling noise worked! :lol:
The howling noise is called acoustical feedback, and is caused when the microphone can hear the radio which amplifies it, its sound goes through the air back to the microphone and it is transmitted to the radio again that amplifies it more and around and around the sound goes making the howling sound. Use earphones on the radio and keep its volume down to stop the howling.

Motor winding wire is insulated with a thin coating of varnish or enamel. It must be removed from the ends of the wire by scraping with a sharp knife or with sandpaper, before the bare copper wire can be soldered.

Many cheap radios are overloaded by the transmitter when they are close together and you might be tuning it to an overload interference point on the dial instead of the main strong frequency, which makes the transmission seem like a short distance. My cheap Walkman radio has a local/dx switch and is not sensitive to overload when switched to local and I can tune it to my transmitter, then I switch it to dx where it is very sensitive for long range reception. :lol:

thanks for the info. for the copper wire : the problem solved, that is to solder the wire to the v`board. and it should be due to the coating tht it seem like not conductive (multimeter).

ok now for the howling sound : it seems like there`s a transmitter and reciever feedback.that `woooo` sound itsn`t it? if i tune the variable caps to some point i`ll get the sound but i can`t speak through the mic.there`s no sound been transmit.even if i change with other electret mic (in case the one i`m using is not in good condition). is it because the mic`s resistance dind`t match the circuit. is there such a thing?

Hi Manz,
Please attach the schematic with parts values and coil construction details of your FM transmitter. It sounds like the original defective design that I fixed, then improved into my design. :lol:
The defective circuit's microphone preamp worked only when its battery voltage was a certain amount. With a new battery it didn't work. With an older battery it didn't work.

wait i had trouble posting the image

how to actualy put the image here?is it with the `add attachment` button?

manz said:

how to actualy put the image here?is it with the `add attachment` button?

Click to expand...

Yes!, post the image as a GIF, JPG's are too large and too poor a quality for diagrams - but for pictures use JPG.

i really really dont know wether is it allowed to do this.

https://www.pcs-electronics.com/schematics/Small_Fm.jpg


thus anybody know where the `C7 (4.7)` should be attached to? is it to the ground (-ve).

manz said:

i really really dont know wether is it allowed to do this.

https://www.pcs-electronics.com/schematics/Small_Fm.jpg


thus anybody know where the `C7 (4.7)` should be attached to? is it to the ground (-ve).

Click to expand...

Yes, it's a ground symbol, the -ve of the supply in this case.

dear Nigel Goodwin,thanks a lot.

whut do u think wit the circuit??(about the distance and clearity).i dont mine if it just mono. is there anything wrong with it? ( i suppost it shouldn`t be any) so thats mean there is something wrong with mine.i just couldn`t find why mine doesn`t work. it got the howling sound like there`s a transmitter-receiver feedback but it just couldn`t hear my voice trough the radio.. is it due to the mic?? how does the mic`s resistance affect this circuit. i`ve got a few (electret mic) with different resistance. (by tapping both pins with the multimeter).for your info, im using a bread board but i kept everythings close together including the LC wit the transistor.

please, i realy need help.i`m so close already to make it work.i guess.
https://www.pcs-electronics.com/schematics/Small_Fm.jpg

manz said:

dear Nigel Goodwin,thanks a lot.

whut do u think wit the circuit??(about the distance and clearity).i dont mine if it just mono. is there anything wrong with it? ( i suppost it shouldn`t be any) so thats mean there is something wrong with mine.i just couldn`t find why mine doesn`t work. it got the howling sound like there`s a transmitter-receiver feedback but it just couldn`t hear my voice trough the radio.. is it due to the mic?? how does the mic`s resistance affect this circuit. i`ve got a few (electret mic) with different resistance. (by tapping both pins with the multimeter).for your info, im using a bread board but i kept everythings close together including the LC wit the transistor.

Click to expand...

You CAN'T build VHF radio circuits on a breadboard, it's completely unsuitable - see the many similar posts in the past!. You would be better off building Audioguru's improved version (which fixes many of the problems with the original circuits) - he also has posted pictures of the veroboard layout he used.

As for range: Range will depend also on the type of receiver that you have. Obviously a shower radio will not have as much range as a high end radio :lol:

Also, it sounds like maybe you have the circuit sitting next to the radio working and then you go to move it farther away from the radio. Your hand coming close to the antenna and/or inductor will cause a shift in the frequency its transmitting at. Try moving the radio instead of the transmitter.

Hi Manz,
The instructions for your extremely simple FM transmitter circuit says to use a "mik", which is probably a dynamic microphone (coil and magnet), not an electret one that needs a resistor to the positive power supply to power its FET transistor inside. Try it modified for an electret microphone like this:

hi audioguru..

thanks a lot for the info.. i`ll try with the modified circuit... i`ll let u know how it goes in a while... anyway i`ve look to your fm transmitter circuit mod3 (told by Nigel Goodwin-tq).. if everything is working well with what i`m working now, i`ll for sure proceed with your enhanced circuit. i just dont have the confidence yet to proceed so that`s why i try with the simpler one first..

for the mean time i`m working it on a veroboard. is it ok on the veroboard. i`ll keep everything closed togeter though..

thanks to all.

Hi Manz,
On my Verobaord I cut all tracks to minimum length. My mod4 version is much better because it uses a low-dropout voltage regulator for its audio preamp and RF oscillator. Its audio is very clear and its RF frequency drifts very little compared to the loss of treble, high distortion and high amount of drift with simple circuits. :lol:

hi audioguru..

thanks alot for the circuit.it works pretty good.also have a minimal frequency change even if i get near the transmitter. with the existing antena it could go ubout 100++m. i just dont know how long the wire for antena should be.i use about 7 inch of normal single core wire (with the insulator).is that enough?? or should i changed to 12 inch wire???

now i will proceed with your mod4 transmitter but i have a few question about the circuit :

1.what is actually LM2931A5.0 ? is it a transistor as well ? because in the pic you post for the mod4 , i could see there`s 4 transistor but in the schematic there`s only 3 of the 2N3904.

2. whut is actually the `I and 0`symbol on it. if it is transistor how to determine the B/C/E? (if the shape is diff than the normal shape)

3.can i use 12dc power supply to it.what are the change should i make in order to do that.

4. should the antena really have to be 30inch or wire.(or it just for max perfomance)

5. since i`m in asia, what value of C4 i have to use (100nF or 150nF).

anyway THANKS A LOT to you and everybody.i`ll proceed with the mod4 tomorrow..(if i know and can get all the parts).if not,i`ll just can proceed with mod3 only.i may still need help from all of you to complete that too :wink:

Hi Manz,
Try an antenna about 30 inches long oriented broadside and in the same direction as the same-length one in the FM radio. Then most FM transmitters will have best long range. With your very simple one its frequency will drift if you or anything conductive comes near it since the antenna and anything capacitively coupled to it affects the oscillator's tuned frequency.

An LM2931A5.0 is a low-dropout 5V/100mA voltage regulator in the same small case as a small transistor. It regulates very well when its I = input voltage is as low as 5.6V, since a 9V battery will drop to 6V or less over its life. An ordinary 78L05 voltage regulator needs an input voltage of no less than about 8V for good regulation, and a 9V battery would still be like new at 8V.
Most semiconductor manufacturers make low-dropout voltage regulators.

My mod4 transmitter has a range of more than 1km to a good FM radio with a fresh 9V battery. Its RF output power is well over the legal limit. With 12V it would probably go 2km but its output transistor might melt or smoke. :lol:

ok..here`s the situation....

im planning to use it in a car so that i dont have to get a batt everytime...the circuit i`m using now can be used in a car since the input voltage state there is untill 12v dc.i`ve tested it just now and it range can be untill 250m or so..

the quetion is:

1.should i reduce the voltage to feed the mod4 circuit?? is yes, how??

2.or is there any part should be added/change to the mod4 circuit so that it can be used in the car by using car batt and not the 9V batt.

3.or do u thing the circuit mod3 is more suitable for me since im a newbie? every parts in both circuit i know/familiar but just the voltage regulator.(i`ll try to search for it tomorrow but if i couldn`t get one, i`ll most probably proceed with the mod3 circuit.)what is your opinion??

4.is there any major different between those two circuit (mod3 and mod4) in term of range,stability, and most importantly, frequency drift.hor thus the absence of the voltage regulator in mod3 circuit affect the overall performance interm of stability and range??

Those are the things that I`m confused with and I realy really hope that you or anyone else could at least give your opinion about the situation.
but most importantly how can i used the 12V dc in the circuit.is that a realy hard thing to do caused i really dont now much about it.just asking and hopefullly it could be done.I`ll be looking for the parts ealry in the morning tomorrow.

thanks for helping the newbie.thanks to all.

Hi Manz,
You could add a resistor to ground in series with the emitter of the output transistor in my mod4 FM transmitter and it will operate fine from a car battery. I think a 75 ohm to 100 ohm resistor will do the trick.

A 78L05 voltage regulator IC will work fine when powered by a car battery. A 78M05 or ordinary 7805 will also be fine but are bigger. :lol:

ok thanks a lot audioguru..

i`ll going to get the parts and hopefully everything goes fine with it...
wish me luck :lol: i`ll let you know how it goes then.