Radio transmitter problem

Discussion in 'Wireless & RF Design' started by Mathematics!, Apr 22, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    So I tried to build this radio transmitter

    However when I try to pick up the signal right out side on my car radio. (or any radio for that matter)
    It doesn't work. I have check the connections they all look good.

    The only thing I was having problems was with which leg for the 2N3904
    NPN transistors was the emitter and collector. I see a small line on top of the transistors I am wondering is this the collector or emitter leg? I am assuming with these transistor the base is the middle as the picture on the back of the package shows.

    Also I am using radioshacks part 270-0090 Condensor microphone.
    It has one leg that has 3 green lines is this the leg that goes to ground or power side? ( think I tried both )

    For the antenna I used around 8 to 12 inchs of 22 gage magnetic wire.
    For the inductor I used 10-11 coils around a pencil. I left the pencil in the middle of the inductor coil should I take it out or does it not matter if it's their? I used 22 gage magnetic wire for the inductor as well.

    For the capacitors I used 0.01uF and 0.1uF polyester-film capacitors
    For the resistor I used 1/8 watt carbon-Film resistors radioshack 271-003 500 pack. This had all the size resistors I need in it. I don't think I need more the 1/8 watt power?

    I used a 9volt battery.

    The only thing I didn't have is the varible capacitor and the 4.7 pF capacitor. They just didn't have these. But I have alot of 1, 10 , 15 pF cermic capacitors I just but them in parrell to make desired capacitor variations for the varible capacitor. I think I should still be able to get something.

    For the 4.7pF I just used 5 1pf or 4 1pf is the .7pF or .3pF going to screw the whole thing? I guess their may be some series combination to make it exactly 4.7pF.

    Anyway I don't want to buy any more stuff I have all the resistors and 1/8 and 1/4 watts. I have most of the pF capacitors , I have alot of uf , and some nF. I also have 2N3906 12 pack PNP transistors...etc

    Their must be something I can do to easyly fix this.
    I tunned into different stations around 87 MHz up to 99.3MHz FM
    I tapped the microphone each time but I never got a taping sound out of the car speaker or anything.
    The 9 volt battery is new and I check it with a multimeter to see if it was producing around 9 volts ,it was.
    The car radio is in perfect working conditions so I am at a loss?

    I don't have any bare copper wire if the insulation is screwing up the anttena but I have an old bunny ear TV anntena if that would clear it up.

    I tried keeping the coils of wire in the inductor close together but maybe they are still to far apart?

    For the varible capacitors should I be tunning more to the 40pF or to the 4pf or to the middle just so I know around how much capacitance I need to be at with my capacitors in parrell.

    Thanks for any help

    This is bothering the heck out of me. I should get something weather it be my voice with alot of noise. Anyway maybe I put the transistor or mic in the wrong way but I thought I tried ever combination. AHHHHHHHHHHHH
    Last edited: Apr 22, 2009
  2. leftyretro

    Active Member

    Nov 25, 2008
    Those old analog FM transitter circuits were always hard to get running correctly and very few people would be happy with the results.

    Look for more modern circuits that utilize PLL and a crystal for frequency stablity. Also these days it's much easier and cheaper to buy a commerical audio to FM transmitter module as used for sending ipod and portable cd players over the FM band.

  3. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    I don't care how bad of quality it is. I just want to see it work a little.
    Can anybody answer the collector/emitter legs question above?
    And can anybody answer the mic legs (3 green lines ground or VCC leg ? )

    If I go with more coils will this help?
    Do I need a non-insulated 22gage wire for the anntena and should I use something longer then 12inches to make it better?

    Please any help did anybody actually succeed in building this?

    I don't have these I only have alot of capacitors, resistors , different wire gages , transistors both PNP and NPN , mic , and different voltage batteries.

    Their should be some simple FM or AM transmitter I can build?
    I just don't want to buy any more parts until I can build some transmitter that actually works.
  4. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    The only thing I have is some avr chips could I use these to fix any problems in the circuit ?

    I want to build it from scratch not send away for a kit.

    Anyway maybe their is a better FM or AM radio circuit I can build.
    From my parts?

    I forgot to mention I was building it on a small solderless breadboard.
    Before I made it permanent. But it never worked their so I never made it permanent.

    Maybe the components are to far apart?

    Can anybody give me an RF transmitter circuit that works.
    Without useing a varible capacitor or 4.7 pf , 47pf , 470 pf , 2.2 pF , 22pF , 220pF. Because these are the only things I don't have.
    Note I don't know how precise you need to be could you use 4pF instead of 4.7 ...etc because I can probably get almost exact if I did a series and parallel compo....

    And no I don't have a crystal or a germinum diode but maybe somebody knows how I can do it with an avr Atmega32.

    Note I am not looking for something that is really good quality just something I can transmit to some radio FM or AM frequency so I can here it thru a boom box or car stereo.
  5. deepak007

    Active Member

    Sep 30, 2007
  6. deepak007

    Active Member

    Sep 30, 2007
    i forgot to tell you. use 10pf fixed and 39 pf in parallel in the tank circuit, instead of trimmer. or simply use a 47 pf fixed cap, in parallel with the coil. by the way, where do you live?
  7. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    I don't have 22n or 39 pf capacitors what I do have is this

    For capacitors I have
    0.0001uF = 0.1nF = 100pf
    0.001uF = 1nF = 1000pf
    0.01uF = 10nF = 10 000pf
    0.1uF = 100nF = 100 000pf
    1uF = 1000nF = 1000 000pf
    10uF = 10000nF = 10 000 000 pf

    2.2uF , 3.3uF , 6.8uF , 220uF

    10pF , 150pF , 68 pF , 7pF , 1pf , 560pf , 15pf , 220pf.

    One 4.7nF.

    I will try to see if I can pick up some 47 , 22 , 39 uF pf nf but this might be hard since I am not willing to buy on line and the only store around that sells capacitors is radioshack.

    Anyway I am still wondering if their is a simple way to fix my first attempt at the transmitter I posted. I.e Some modifications or something ?

    For the new bug transmiter given. Is their any others that I can build that Only use the capacitors above?

    Also I am using part 270-0090 radioshacks condensor mic for the mic thats all I have and basically don't want to start buy anything else. Don't know if this will effect the bug circuit since it did specify specific values for that mic?

    You can assume every single resistor value I have.

    For transistors I have both PNP and NPN but no special kinds mostly general purpose . Don't think this is a problem? But your bug circuit specifies exact transistor parts don't know if you can swap them for general purpose ones because I don't know if their is some transistor reaction time that will be screwed up?

    I would love to see a bug circuit actually transmit and I can hear it.
    I have tons of different resistors, capacitors ,... etc I should beable to make some type of transmitter that will work?

    Thanks for your help

  8. deepak007

    Active Member

    Sep 30, 2007
    radio shack microphone is an excellent choice. but change the microphone gain resistor or put a 100k pot instead of a resistor.because different microphones have different signal need to adjust the pot for required sensitivity.the circuit will detect very faint sounds, it's a bug, remember?

    read the whole article, everything is given very clearly.don't build it until you read the whole article twice.the article has four pages.
    you can use 2N2222 or BC547. a list of replacement transistors are given on the third page.

    capacitors? just salvage them from some device. or purchase a 1 OR 2 $ fm will have 47pf and 32 pf. use them.

    good luck.
  9. deepak007

    Active Member

    Sep 30, 2007
    you said that you 10pf and 15 pf caps. right? the circuit uses a 10 pf for feed back and a 47 pf for tank circuit. it means you only require 10pf and 47 pf.

    then use three 15pf's in parallel. it makes 45pf.the frequency will go a little high. squeeze the coil (f decreases) or spread the coil(f increases) for required frequency. and use 10 pf for the feed back cap. ok?
  10. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Ok, but before I try another transmitter circuit.

    I am wondering if I can verify that I am actually building stuff correctly.

    Is it possible to hear a clicking sound from building this

    What frequency will I have to tune into?

    Also is their an easy way to find the emitter from the collector. I am assuming the transistors I have have the middle leg as base.
    Last edited: Apr 24, 2009
  11. deepak007

    Active Member

    Sep 30, 2007
    i have built a variety of transmitters, all fm of course.i have built the circuit you are talking about, works fine.but it's frequency will drift,if you move your hand over it or if you touch the antenna. it is due the loading effect of the,you need one more stage called a buffer amplifier. the circuit i suggested to you is a very stable transmitter and it contains a buffer can be worn on the body too,with out frequency drift.also it has everything very clearly is a good one for beginners.

    take a look at this for transistor pin checking : (read this and tell me if you don't understand.)

    or type in google the name of the transistor and look for the data will find a pin diagram. from that you can know, which pin is which.that's it,don't assume the pins.

    frequency? you can tune it any where on fm band 88-108Mhz.but you need a trimmer for it.if you don't have a trimmer.use a 24gauge wire,and wind 5 turns on the smooth side of a 1/8 inch drill bit.and use a 47pf capacitor across the will get it to work in the fm band.spread or squeeze the coil for the required frequency.
  12. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Do I have to use bare copper wire for the inductor because when I measure the voltage across it. It is always nothing (zero).

    I am wondering if I have to strip off the insulation?

    I cann't even seem to get this to work.

    At the 1/3 tab mark of the inductor must the wire be bare?

    I am think if it has to be bare at the ends and the 1/3 mark then I will just have to use a little smaller gage wire so I can strip it without it falling apart.

    I should hear some kind of interference from the above circuit but nothing yet? This circuit was the one I want to build before moving on to the cool wasp circuit.

    Should the antenna be stripped bare to the copper or just at the ends?

    The transistors that I am using are 2N3904 NPN they say they are in cases TO-92 the picture uses 2N2222 wondering if their is a difference in using different NPN transistors. The picture on the back of the package has base in the middle. I don't know exactly if anybody knows off the top of their head where the emitter/collector legs are. Either way I think I can use my mulitmeter to check them. I will try to figure this one out. But for the inductor question I am still wondering about.
    And Could I substitute a smaller gage wire and use less coils to get the same inductance?
    Same thing but for the antenna.
    Last edited: Apr 25, 2009
  13. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Also my multimeter has transistor tester built in see picture.

    I would like to know how to use this Basically I know how to use everything on my mulitmeter except PNP ,NPN and the thing next to the NPN that looks like a Diode checker. See picture.

    I noticed that for the 2N3904 NPN transistor has on the package Hfe (min) of 100.

    I stuck the 3 legs of the transistor into the first E B C starting from left to right. One way I got 259 then when I flipped the transistor around I got a number 16.
    Don't know what these numbers mean but I am assuming when I got the number 259 > Hfe min this was the right legs in the right holes?

    I am also curious when I try to stick the legs in starting from the right to left E C B it gives me LOBAT what is this mean?

    I noticed that I can also check my leds using the right most C E holes.

    Thanks for any explaination
  14. deepak007

    Active Member

    Sep 30, 2007
    inductor? a copper solid wire contains a insulation coating, which you must remove before soldering the unit to the circuit.just scrape the insulation with a hobby knife,at both the ends.also scrape at the third or whatever winding you need to solder.without doing that you can never get it to work.

    antenna? use a small diameter stranded wire for the antenna.

    pin diagrams? i am uploading images.
    both are general purpose npn transistors.but,they differ in some characteristics. but,both can be used in your circuit,without any problem.

    multimeter. LOBAT? i think it means low battery. REPLACE THE MULTIMETER BATTERY.

    HFE ? if the meter displays HFE the transistor is ok. but it won't show hfe on both sides.
  15. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    Yes , I Striped the ends and if I squeeze the inductor just right and tap the mic. I can hear tapping on the radio receiver. But when I speak into the mic I cann't hear anything out of the radio reciever?

    The mic I am using is radioshacks part 270-0090 Condenser Mic element.
    The electric charactrisitics is
    Supply voltage 1.0 to 10 VDC
    Nominal supply 4.5VDC
    current drain 0.3mA(max)
    signal to noise 60db(min)
    sensitivity -64 +- 2 db
    Output impedance 1k ohm.

    I have seen at radioshack unidirectional microphones mine say's it is omindirectional don't know what the difference is and if it matters. I think it has something to do with the pick up but I don't really know about this either.

    Would it be better to strip the entire inductor coils insulation of? (not just the ends to stick into the breadboard)

    Should I leave the pencil around the inductor? or is this just for making the coils and doesn't need to be around the coil after making the coil.
    Note I have tried it both way's and it doesn't seem to matter maybe it would if I striped it bare the whole entire thing? The pencil has a lead core did this have anything to do with using a pencil? I don't really get the purpose of the pencil.

    How do you determine the antenna length seems like any length will pretty much do ? I am using 10/11inchs of 22 gage wire same wire as inductor.
    Should I strip this totally bare because right now it is just stripped 1/4 in at both ends

    Note I am building
    Which was my original FM transmitter circuit when I get this I will move on to the WASP one.

    What I cann't get is a 4 - 40 pf trimmer capcitor and the 4.7pf capacitor.
    So I have been varying capacitor's in parrell for the trimmer by doing this I can tune into alot of different stations and hear my mic taps. I think when I used 10pf I had to tune into around 105.9 FM.

    I am assuming the formula was 1/2*pi *sqrt( LC ) where L is my 0.1 uH inductor and C is 10pf value. Seems to be the correct formula to use, this is where I can track the taps of the mic to. I have figured it out 40pF gives you an ossilation of over 250MHz and 4pf gives you something lower then 87.1MHZ so you should be able to adjust it to any station you want to transmit on depending on how precise you can adjust this device. (i.e any FM Station)

    For the 4.7pf I am just using 4 or 5 , 1 pf capacitors. Is the .7pf or .3pf
    differences going to blow the whole thing? If so is their anyway I can make 4.7pf from the capacitors I listed earlier? (some series and parrell combo I haven't been able to figure a combo yet for 4.7pf)
    Does the 4.7 pf really matter it isn't really factored into my caculations of the resonanting frequency. Why the need?

    And last but not least what determines the range to be 400meters?
    And is it possible to increase?

    Thanks for any help I am getting closer.
    But obviously the main question is why cann't I hear my voice when I talk into the mic ?
    Last edited: Apr 26, 2009
  16. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    O, I figured out the answered to the antenna question above

    299 792 458 m / s light speed in a vaccum

    say you wanted an optimal antenna for 680 am

    680am frequency is 680 ,000 hz

    This means that the period or one cycle takes
    1/680 000 s = 0.00000147s

    1/4 of this time is the amount of time need for the electrons to reach the peak of the antenna for the first time. So you want the antenna's length to be the distance it will take the speed of light to travel (electrons) to travel 1/4 of a cycle.

    0.00000068 s / 4 = 0.0000003675s to reach the peak the first time.

    Now the simple d = r * t implies
    d = 299 792 458 m/s * 0.0000003675 s
    d = 110.173728315 m = approx 361 feet or 110meters long/high.

    That means for a 680 Hz radio stations they would have a 361 ft antenna for optimal carrier wave.

    If you do the math out you will see
    This means the higher frequencies will need smaller antenna's.
    Which is probably why the antenna's on cell phones can be 3 or 4 inches...etc

    Ofcouse I am using the FM stations but I am not willing to make a 100 ft antenna or so.
    So the 10 or 12 in will have to do.

    But I am still waiting to hear from somebody on the mic problem , and last posted questions thanks.

    I will try tonight or tomarrow to plug my radio into my laptop and record the tapping signal. Maybe that will shed some light on things.
  17. Audioguru


    Dec 20, 2007
    It won't work if it is made on a breadboard becase the inductance ansd capacitance of a breadboard is too high for 100MHz. The wiring between parts must be very short. Use a pcb or a small stripboard.

    The entire circuit must be insulated with a distance of at least 3cm from metal or a person.

    Look at the datasheet of the transistor to see which leg is which.

    The ground pin of the microphone is connected to its metal case. Look at it or use an ohm-meter.
  18. deepak007

    Active Member

    Sep 30, 2007
    audio guru was right. check the mic must have soldered it the other way round. take a multimeter and check the continuity between one of the two pins and the metal body.that will be the ground. it doesn't work then change the mic.
    also check the battery. the battery should have full voltage.
  19. deepak007

    Active Member

    Sep 30, 2007
    don't leave the pencil inside the is a air coil inductor. no need to remove all the coating,just remove where you solder.12inch wire will do,for an antenna.
  20. deepak007

    Active Member

    Sep 30, 2007
    Last edited: Apr 26, 2009