- Blog entry posted in 'Menticol's Projects', December 11, 2013.
Hello guys!
Two years after my previous (and funny) failed attempt, I've decided to build again the Audioguru's FM transmitter, this time using slightly better knowledge, tools and techniques. In this post I'm going to share the steps, results and questions raised from the construction.
Well, let's begin. The first step was transferring Audioguru's schematic to EAGLE
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Then was time for creating the board. This time I've followed the recommendations of ETO member Transistor495 about the component placing. I paid special attention to avoiding the usage of unuseful telescopic antennas :p
Despite having bought the the board, transfer paper and ferric acid, at last moment I was afraid and decided to practice with them on another project. In place I used the old friend universal perforated board.
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Now talking about the transmitter components, here's a warning if you live in a country like mine: Local electronic stores will offer any required components just like a first world country, but the the sellers will overprice them if they feel on the mood, specially if the required parts are imported or not very common. For that reason is a good idea to check on your personal junkyard before going out with your wallet!
Lot of stores also have the bad habit of selling the wrong part (like a regular 555 when asked for a precision NE555) or pirated clones that don't work at all. So, if your transmitter (or any other peoject) fails to work:
1. Check your power supply
2. Check all your wiring and component positions
3. Check the value of critical components.
To organize the components sometimes I use the schematic as a map. A bit of overkill but really saves time and frustration.
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Watch out for oversized components. I had some trouble making room for these huge 330 nF capacitors.
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This jungle of pins reminds you why a custom etched board is the proper way. But too late for regrets, let's keep going
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Unused pads must be removed, or their capacitance will interfere with the transmitter operation.
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For the air coils Audioguru advises a 4 mm gap inside the coil. A good pattern is a small screwdriver.
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I was too worried about the size tolerances. Luckily at the end using common telephone wire and the small screwdriver worked just fine.
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The value was confirmed by an inductance meter I borrowed. If your receiver fails to work try squeezing it a little (instead of separating) the coils
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After soldering everything I was ready for firing it up! :D
...But the only thing that "fired" was Q3! :(
After blowing two transistors I found that the LC formed by the C13 and L2 was acting as a short. The suspect was the blue trimpot (remember my warning about electronics stores)
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Using the same inductance meter I found that an old variable capacitor from a radio gave the required capacitance value: 6 to 66 pF. I used it and threw away the blue trimpot.
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After playing with C6 and C13 it magically started to work!
Motivated by my sudden success I decided to swap the small electret microphone for a bigger one, and installing a "Line in" between the base of Q1 and ground to connect an external signal source. Later I discovered these were really bad ideas (see the end of the post), but well, IT STILL WORKS! :D
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How do you know that your transmitter works?
1. First wait until 3 AM or so to make sure you don't disturb anybody listening. Keep in mind that operating this emitter is illegal, so limit your experimentation time as much as possible.
2. Put your FM receiver dial on the lowest point of the dial 88.x or the highest 108.x until you only hear static noise.
3. Apply power on your FM emitter and feed it with your voice, or a pre-recorded sound via the line in. I would have loved to use this numbers station as a source, but feeding static noise to your device under test is somewhat silly.
4. Star turning C6 VERY slowly. At a very small and specific point the static noise on the receiver will suddenly stop. Your transmission will be heard. use C13 for fine tuning.
5. if you feel that Q3 starts to overheat disconnect power immediately (see above), under normal operation this is not supposed to happen.
Happy with the success of the transmitter I decided to get rid of the other trimpot and placing another big variable capacitor.
Maybe Audioguru will correct this, but after the experiment I think that the black wheel (C6 serves as the coarse adjust and the white (C13) fine adjust.
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This success is a good motivation to finally learn how to etch a custom board.
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Well I hope you enjoyed this post. If you have some question I'll do my best with my limited knowledge to answer you.
After this experiment I have some questions too, I hope that a more experienced reader would help me to clear them
1. The received sound is too quiet. I tought that R2 and R3 could be the cause, but removing them didn't made the sound louder. Where's the problem then?
2. The first electret microphone worked fairly well. The new electret also worked, but stopped working when I added the external signal from the computer. Now none of the microphones works, even having desoldered the external signal cable. Why do you think this happened?
3. The huge green 330nF capacitors act as microphones! it happens under certain C6 and C13 values. Tapping on the surface of the workbench or the capacitor itself is perfectly transmitted clear and loud to the FM receiver! I wish the external signal would be transmitted with the same degree of clarity.... How can I get rid of this unwanted microphone?
4. The last question is the most puzzling: Under certain C6 and c13 values the transmitter will SWAP existing legal radio stations! Let me explain: Let's say I tuned a legal radio station on 88.9 on my FM receiver. When the transmitter is powered and certain C6 and C13 positions are selected, the 88.9 station will disappear and in this place I will hear the station located on 96.9.
Why is this happening? How is this transmitter picking up an existing station and re-transmitting it?
Willen, December 11, 2013
Hi Menticol, I also made audioguru's FM Tx, check this picture- [url]http://www.electro-tech-online.com/attachments/my-mod4-bigger-size-jpg.82647/[/url] It is on single side PCB with lots of ground plane I used during board designing on ExpressPCB. But yout board has no ground plane and long wiring, it might cause ustable the Tx and little bad performance. Your few questions I guessed I can solve because I discussed with audioguru lot and lot and asked almost everything :) 1. Sound is not More small, if professional radio has 100% loud then this has 90% loud. But if you didn't compare, it is loud anyway. You tested the loudness with Mic and voice but I tested the loudness of music using cell phone or computer. It has extremely high quality sound which is better than few nonsense FM stations. You cannot feed computer's or cellphone's audio directly to the base of Q1. Audio from computer should be attenuated first then have to feed to C1. Use 27k series and 220 ohms parallel to Gnd of Tx resistor to attenuate headphone OUT audio, then feed this attenuated audio to C1 and lets see its quality. Do not adjust or remove R2 160k and R3 30k resistors, otherwise you will get distortion. If you want to get completely loud audio then you need to use audio peak limiter/compressor circuit before your Tx like this- [url]http://www.sound.westhost.com/project67.htm[/url] 2. Audio from computer is not more harmful to burn FETs inside of Mics. But if you tested touching the earthing then some desktop computer case give powerful shocking. This shocking might burn you mics. If new are also not working then simple check your wiring around preamp and then check your Q1, simply it is solved! 3. Your 330nF capacitor has high voltage rating like 400V so it has extremly large size. Use some small size like mine. If you didn't get any then use 100nF once but after then you will not listen loud bass sound. I think external microphone is caused by inductor vibration. Use tight and thick wife to make wire, like 22 or 23swg. I think you used it, then cause might be back side large jumper wires. Then are acting as a large inductor in 100MHz so you are listening microphonic effect. 4. If you turn on the Rx near of you Tx, then it generally mix other stations. But your Tx is mixing badly I think. Then- - may be your Tx is acting like regenarative radio - only turn on your radio in another room - as I said before your back side wiring acting as inductors so they are causing bad interfere. Better to use Printed Circuit Board to make such VHF project for maximum performance. - close your Tx in metal housing, this will make little stable the Tx and might remove bad interfere. And at last, no need to use bulky gang capacitor anyway. Simply trimmer are enough to tune these tanks. I used only a trimmer for tank around Q3. As an oscillator (Q2) trimmer I used 15pf fix capacitor and I got around 94MHz. I have very blank spaces on FM band so no need to tune every time.
audioguru, December 11, 2013
C6 tunes the RF oscillator to the frequency it transmits on. C13 is tuned for maximum output power (farthest range) but its tuning is very broad. The transmitter signal easily overloads the input of a nearby radio (especially a cheap radio) then stations can get mixed up or this transmitter signal can appear across the entire radio dial. The audio transistor was designed for an electret mic having a signal of only about 5mV. A line level signal severely overloads the audio transistor (an attenuator should have been used) and probably destroyed it. My FM transmitter uses a small electret mic from a cell phone and it is sensitive enough to pick up whispers in another room. I had it in a room where my stereo was playing FM radio softly and its transmission sounded at the same level as the original FM radio station.
Willen, December 11, 2013
May be there in USA/Canada, loudness of broadcast audio limited by FCC, is it? Here, One station has ear-breaking loudness than others'. Here, no strick rules has been stablished, no special RF cops. It is almost 15 yrs ago here we heard FM radios. :) Here, Few yeays ago a gang of terrorist were broadcasting their powerful FM. They continually broadcasted but nobody locate them. After negotiate, they have been included as part of government now.
Menticol, December 12, 2013
Hi Willen! Thank you so much by your very complete answer, I really appreciate your time. Also many thanks to Audioguru by sharing his circuit and the details of it. I'm correcting my construction at this moment, I'll keep you updated of the results
Menticol, December 12, 2013
Hello again guys Here's the corrected board. [url]http://cocodrilabs.files.wordpress.com/2013/12/ag-mk4-board-2.png[/url] Track width is 0.4 mm, keeping an isolation of 0.08 mm I have added the external input and the resistors that Willen kindly advised, together with a 7809 voltage regulator. EAGLE made a mutiny against me and refuses to connect the microphone to ground. I'll correct that with a sharpie once printed. What do you think? it's etching time?
Willen, December 14, 2013
Hi Menticol, Do not etch PCB now without correcting this attenuator- [url]http://www.electro-tech-online.com/attachments/menticol-corrected-gif.82759/[/url] Your attenuator is doing nothing, so again you will destroy your component if you fed audio from PC or cell phone. Look at the picture, Left picture is your mistake. Right picture is corrected. First put 27k resistor in series then ground it with 220 ohms resistor. Then take this audio for your Tx. Now you will get around 5mV suitable attenuated audio. ( this 220 ohms might need to adjust little higher or lower. I made two time the transmitters, Once I had to use 47 ohms only but last time I used 220 for headphone audio of computer or cell phone. Audioguru using 1K for Line Out. Use, listen and decide yourself.) Why you used 2 separate input for 7809 and 7805? Simply you can use output of 7809 as a input of 7805. I felt lot more wiring output from Tx and long audio cable reduces stability. Are you going to use ultra-violet focusing method to make PCB? Then 0.4mm track and 0.08mm spacing from track to ground plane might be normal. But if you have no more soldering experience then chance of bracking tracks is high during soldering and de-soldering. I used 1mm track and 0.5mm spacing. Once I had to chance inductor 2 and few mm track destroyed. I had to used lead of inductor to recover it. I am not an expert, I am just learning simple electronics, so may be sometime you can find me here where I'll asking "what the resistor is doing here?" ;) hehe
Willen, December 14, 2013
Want to add another- You used two pin audio but all headphone or sound system has 3 pins audio- Left, Right and Gnd. It is better to include them all for complete audio. Otherwise some time you will miss some audio played on another channel. So to include both L and R audio, simply use both Use two 27k series resistor (same as my corrected picture) and after 27k connect them together. Then same as before- 220 to Gnd, and to Tx. Did you used 160k and 30k as R2 and R3? You were saying once "160k is not available." then what you did?
Willen, December 14, 2013
Wan to add again few sentences- You are trying to etch PCB with fully improved version, so want to say something again- like this way you can include both L and R audio (3 pin of headphone out) like this way, (but better to use Two-way Switch between Audio IN and Mic in)-- [url]http://www.electro-tech-online.com/attachments/150k-vs-27k-attnuator-png.73252/[/url] You had connected 'music IN' and 'mic' together; But if you connected line out and mic together, then when you will transmit music from PC/cell phone then mic is also working. Then you will hear mic's audio as a background noise may be. So I used external 'two-way switch' to switch 'Music IN' or 'Mic IN' to Tx. When music is broadcasting, Mic has been disconnected and when Mic audio is broadcasting, music IN has been disconnected. It will fully reduce to mix Mic noise in music. Look how I uased two way switch and housing for my transmitter-- [url]http://www.electro-tech-online.com/attachments/my-mod4-housing-jpg.81724/[/url]
Willen, December 14, 2013
*Same post deleted!
Menticol, December 18, 2013
Hello again Rajkumar! I'm so sorry for taking so long in answering this thread, I had an unexpected job interview and I was forced to leave my little radio project for a moment. But here I am again! I really appreciate all of your suggestions, in fact at this moment I'm including your changes (specifically the L and R channel sum). Don't worry if you consider yourself a beginner, I'm light years behind of you. Your advice about the track width was very opportune, as my original track width was too thin and it would never have survived my archaic transfer paper / iron method. Most tutorials recommend the UV method for getting much more predictable results, but I'm not on that league yet :P The reason I used two regulators was to avoid sending a power track straight thought the board. After a couple of days trying finally I decided to use a separate wire. Having a separate power supply for Q1-Q2 and Q3 tempted me to increase the voltage of Q3 (for range increase muahaha) but I found that would only destroy the poor transistor. I wonder if the signal of Q3 could be injected on another more capable fast switching transistor. For the 160K resistor I used two resistors in series, in a somewhat ugly arrangement. Speaking about ugly things I could't find a proper trim capacitor, so sadly I'm sticking with the huge old radio capacitors. I guess I'll need to open some space for them on the board. Thank you again by your time Willen! I'll keep you update of the progress PS: I was spying on your thread about sound compression, your transmitter is getting serious! I didn't knew too much about compressors so I made some research on this site [url]http://www.doctorproaudio.com/doctor/temas/dynamics-processors-compressors_en.shtml[/url] Probably already know most if not all of the terms that they enumerate, but the site has tons of other audio stuff that you may find interesting.
Willen, December 19, 2013
Really I am just learning here Ohms Law. Spy my threads few more days then you will know hehehe ;) - If we are going to use toner paper and iron (toner transfer method) then I felt 1mm track and 0.5mm isolation is better. - Yes, simply we can inject signal of Q3 to another high speed RF power transistor like 2N2219, 2N4427 and it will be known as VHF RF amplifier. It might give 1 to 2 KM on less sensitive radio Rx. - If you didn't get 160k resistor then just use 150k resistor. But if you used 150k as R2 then MUST use 27K resistor as R3 instead of 30k (use 160k+30k OR use 150k+27k). (I was in same problem). Other values might cause distortion!! - If you didn't get trimmer caps then going to use two BULKY gang capacitor??? If your area has no more stations then just use fix capacitor betwwen 10pf to 18pf as a C6 (first trimmer). Make a L1 using 24SWG wire and make little open open windings (see my L1 in brown color/middle of the Tx). The coil can be stretch and compress little to tune output frequency. You can do same with 2nd trimmer and 2nd coil. - You cannot insert 330nF non-polarized capacitors in these narrow holes. May be you have to use little wide lead holes for 330nF capacitor in your board. Or do you have smaller one? At last, I know how compressor limits peaks and makes narrow dynamic range. Then I can inject louder audio into preamp. Peaks had been attenuated so distortion due to peaks will not trouble me lot. But as asked there in my thread I didn't know what is 'attack', 'release', 'threshold' etc... Sure your link will help me lot, thank you so much!
Willen, January 26, 2014
Hi Felipe, I visited your 'Cocodrilo Labs' and became your fan! You are so creative and doing lots of works!!! I guessed your real and academic field and work is not exactly electronics but you feel lots of fun with electronics. Same here, I just finished masters on linguistics but too much interested on electronics! It's my addiction. :)
