FM bug - Page 1
Forum section: electronics / beginners

author: victor
author_rank: Regular Contributor
timestamp: October 25, 2014, 07:13:10 pm
content: I built a transmitter based on thishttp://www.sentex.ca/~mec1995/circ/fmt2.htmThis is the actual circuit I built.It's oscillating because I can pick sound when I put it a couple inches from an old fm receiver with analog tunner. So I think it is out of frequency and it is picking harmonics, but I placed it a meter far from the receiver I poke the coil stretch it tighten it, and can't get it in frequency.I tried soldering an output cap in the antenna of 24pF (I don't know how to calculate this, just thought it would be enough), or removing the antenna completely. Tried winding different coils, with 4 5 and 6 turns.I'm using a couple AA batteries to power it (3v). The inductor I used I think AWG23 copper wire from a CAT5 cable, wound on a 3.5mm jack.I know this kind of free running transmitters are not good, but when I was a kid I had one transmitter that had a single transistor, and a PCB spiral coil and a condenser mic and worked out of a single 1.5v battery, and the stability and range was fairly good, unfortunately I don't have it or its schematic anymore.some other inductors I wound and tried.

author: Simon
author_rank: Unknown
timestamp: October 25, 2014, 07:16:05 pm
content: Have you got accurate values and inductor making instructions. It won't take much to throw these too far off to be picked up by an FM receiver. Even the spacing in the coil will make a huge difference. You'd be better off using proper enameled wire so there is no risk or shorts

author: victor
author_rank: Regular Contributor
timestamp: October 25, 2014, 07:23:34 pm
content: I made this layout, I tried to keep everything as neat as I could, and the shortest path possible, because I thought it would make it work better. I will try anything and post results. I tried to wind the coil as beautiful as possible, I even removed the pad under the coil. But judging from the pictures and schematic, What do you think it is wrong?

author: Simon
author_rank: Unknown
timestamp: October 25, 2014, 07:26:11 pm
content: I'm no expert on this sort of circuit, last one i made was a lashup and worked years ago. Check you don't have parts cross talking, maybe output feeding back into input, make sure coil spacings are right

author: Zero999
author_rank: Super Contributor
timestamp: October 25, 2014, 07:35:40 pm
content: There's nothing wrong with the layout.The first thing which strikes me is there's no decoupling. Try connecting a 10nF and 100pF capacitor across the power supply rails, as near as possible to the transistor.

author: Simon
author_rank: Unknown
timestamp: October 25, 2014, 07:39:12 pm
content: Might be worth pushing that transistor down and shortening the legs

author: guitchess
author_rank: Regular Contributor
timestamp: October 25, 2014, 07:45:17 pm
content: I've been making a bunch of different FM transmitters lately and although I'm far from an expert here are some tips I picked up.1. Use an online coil calculator to check your coil size/turns. I likehttp://www.66pacific.com/calculators/coil_calc.aspx2. Use a Hartley/colpits(which ever is applicable)calculator to check the osc. freq. That appears to be a Hartley in your scheme. I likehttp://www.ekswai.com/en_ind3.htmDon't hold either of these as law because they will still vary, but it get you in the ballpark where you are picking up more than harmonics.3. If you have a capacitance meter check the caps in your osc. Even a couple pf can set your osc outside of the FM band.4. Be very, very accurate in your coil dimensions. A half of a mm could throw you out of range.5. Don't try to tune in on a digital tuner. Use a radio with a manual dial.I may think of some more shortly.

author: Simon
author_rank: Unknown
timestamp: October 25, 2014, 07:48:15 pm
content: yes capacitors can vary widely depending on where they came from, some um suppliers will shaft you with +/-50% caps marked as +/-20% which is as good as you usually get on ceramics so that will cause plenty of possible variation.

author: guitchess
author_rank: Regular Contributor
timestamp: October 25, 2014, 08:38:45 pm
content: Also, you will have a little less noise if you build on a ground plane. This may not be an issue since you are making an actual bug. I make my transmitters to broadcast music from my pc to radios around the house/shop.

author: guitchess
author_rank: Regular Contributor
timestamp: October 25, 2014, 09:56:02 pm
content: Just thought of one more.Since you are using non enameled wire for your coil, whatever you are using to stretch/shrink it may be throwing it way out. Try using two pencils and compressing the coil with the erasers. Of course, all bets are off if you compress it enough for turns to touch.

author: victor
author_rank: Regular Contributor
timestamp: October 26, 2014, 01:41:32 am
content: I measured all the capacitors and all in spec. 6pF 25pF and 35pF. I will add some decoupling.But from the formulas it seems that the inductor should have nearly 0.1uH or 0.08uH for a 35pF figure that will roughly oscillate in the middle of the fm band.My inductor is 4mm diameter, 5 turns and 4mm in length, and it seems that it have too much inductance 6uH, and I'm oscillating at much much lower frequency.But I'm not sure if those numbers are correct.But there's also something odd if I increase from 5 to 6 turns my inductance increase but increasing it further to 7 turn the inductance drops 1 order. Is that a glitch in the calculator ??Turns4 = 4.3uH5 = 6.7uH6 = 9.7uH7 = 0.133uHUnfortunately I don't have any tool to measure my inductor besides a ruler.

author: guitchess
author_rank: Regular Contributor
timestamp: October 26, 2014, 03:51:53 am
content: Unless it is me that is misunderstanding the calculator response, that is not 6.7 uH, it is 67 nH. I think that is what the -002 to the right is meaning. .067 uH. When I enter those numbers in the Hartley calc it has a freq of 103Mhz. This is close enough to the top of the band that even microscopic imperfections in your coil could be pushing you outside of the band.Turns4 = .043uH5 = .067uH6 = .097uH7 = 0.133uHI would suggest swapping the cap with a 40 or 45 pf. That would put you closer to the center of the band.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 26, 2014, 04:17:24 am
content: Quote from: guitchess on October 25, 2014, 07:45:17 pmNo,it's not a Hartley--note the single,untapped coil.The base is bypassed to common for RF,so it is a Common Base Oscillator circuit.The only feedback I can see is via C3,(4.7 pf in the original circuit.)I really hate these things,but many people have fond memories of making them,so it must just be me!

author: GEuser
author_rank: Frequent Contributor
timestamp: October 26, 2014, 04:41:21 am
content: QuoteNope it isn't just youI recall making one up and then fitting the output to a radio transceiver input just to see what I could hear on another receiver the bug had maximum gain on the mic , I was really stressed out for days as the bug was picking up something I could not hear (or so I thought) , after days deciding that it was not a oscillation or internal bug noise I finally concluded it was the cement works and very large quarry around 5km or away .It was an inaudible by ear growling grumbling by the fm bug/radio receiver and then I actually started to hear it by ear eventually , the clincher was there was this mechanical "bang/clang" sound sounding like something steel and large just fell over via the fm bug/radio one time and in one ear of the headset and then at the same time I heard it in the other ear direct , but if one was not actually listening for it it was so low that one would have not noticed it at all and it was from that quarry direction .I went around after I could hear that growling/grumbling constantly and was asking people left right and centre if they could hear it , all responses was no , what noise ?.It took me awhile to get used to that noise especially at night when it was all quite in the local environment .

author: janoc
author_rank: Super Contributor
timestamp: October 26, 2014, 02:35:30 pm
content: Just a quick note - I am not sure where are you from, but do make sure to check your local radio regulations!E.g. in many EU countries these types of "bugs" are illegal to use, regardless of output power, because they broadcast in a licensed/regulated band. You could get a really nasty fine if a grumpy neighbor/local airport should you detune it too much/etc complains about interference to the authorities!

author: mikerj
author_rank: Super Contributor
timestamp: October 26, 2014, 03:07:33 pm
content: Quote from: vk6zgo on October 26, 2014, 04:17:24 amThey are rubbish in terms of performance (especially stability) but I can vividly remember how excited my brother an I were to be able to hear our voices on a radio after building one of these. It's a rite of passage for young budding EEs.

author: guitchess
author_rank: Regular Contributor
timestamp: October 26, 2014, 03:15:41 pm
content: As I said, I am no expert, quite the noob actually. It doesn't surprise me that it is not a Hartley. I haven't learned that much, yet. Somehow the Hartley calc that I posted will get you close enough as far as coil and cap sizes though. I learn a lot every time one of you gurus post. Thanks.To the OP, This was the first Tx circuit I tried. It worked, but it is far from the best I've found.This is circuit I am currently using.This is my second favorite.http://www.electroschematics.com/2233/low-power-fm-transmitter/This is the first one that was able to achieve a usable amount of stability and sound quality.http://electronics-diy.com/miniature-fm-transmitter.phpKeep in mind that I use these for broadcasting from headphone outputs. So, how they perform being fed by electrets, I couldn't say.

author: guitchess
author_rank: Regular Contributor
timestamp: October 26, 2014, 03:20:48 pm
content: Please be warned. The first time i tried to alter/test a circuit using a headphone output, I blew the internal amp of my mp3 player because I had a short. I quickly learned the lesson to check the build before connecting it, check it with expendable connections, and then connect in a real world set up.

author: Hydrawerk
author_rank: Super Contributor
timestamp: October 26, 2014, 03:26:32 pm
content: In early 2000s I also built FM bugs like this.I found out that it is better to build a two transistor circuit. It gave me better results.

author: Scrts
author_rank: Frequent Contributor
timestamp: October 26, 2014, 04:14:05 pm
content: Quote from: Hydrawerk on October 26, 2014, 03:26:32 pmYep. This single transistor circuit is extremely difficult to tune and if you touch the inductor a bit - go for tuning it again.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 26, 2014, 04:57:44 pm
content: Quote from: janoc on October 26, 2014, 02:35:30 pmExtremely low-power FM transmitters in the licensed FM broadcast band are legal in the EU since a few years.But you can still get into trouble for violating other regulations, like lack of certification, or interfering with aircraft communication. Or, an extremely nasty one, if there is a local law against bugs, listening devices, surveillance devices etc.

author: Zero999
author_rank: Super Contributor
timestamp: October 26, 2014, 04:57:53 pm
content: The common base configuration makes them good at high frequencies. I built one in the 1GHz region and used it as a beam break sensor for people. It worked quite well. At UHF the collector-emitter feedback capacitor needs to be emitted as the transistor's parasitic capacitance is sufficient.Such a simple single transistor oscillator isn't very good for FM broadcast. The sound quality will be poor because they're no pre-emphasis. To cut the hiss, the treble part of the audio signal is boosted by the station and all modern FM receivers cut the treble to reconstruct the original signal. As a simple FM bug doesn't have treble boost, the high frequencies will be cut by the receiver, making it sound muffled. Add a pre-amplifier with a treble boost filter; the corner frequency depends on the region you live in - look it up.The frequency will drift depending on the load and supply voltage. To get round this, power the oscillator section from a regulated voltage and add a buffer stage to drive the antenna.A loosely wound coil will be microphonic. Wind it on a firm plastic core or embed it in wax or resin.Rather than relying on the audio signal to change the DC bias and therefore the capacitance of the oscillator transistor, use a varactor diode. A simple 1N4001 may give good enough results.Lastly bear in mind that the frequency will always be slightly lower than the calculations indicate. Add about 6pF for to account for the transistor's capacitance.

author: johnboxall
author_rank: Unknown
timestamp: October 27, 2014, 01:20:53 am
content: http://www.talkingelectronics.com/projects/14%20FM%20Bugs/14FMBugsFrontice.html

author: Teledog
author_rank: Regular Contributor
timestamp: October 27, 2014, 01:35:31 am
content: Or maybe grab a cheap VCO (55-100Mhz) off eBay. (read the data sheet of whichever model you decide on!!!)Adjust the f voltage to your desired frequency and then tap in a capacitor to the input/f voltage with your audio input on the other side.Not terribly stable or hifi (regulated f/bias helps a lot!!!).A fun, cheap & dirty way to get a low-power FM Xmitter though! (at any frequency!)Cheers!

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2014, 02:07:55 am
content: Quote from: victor on October 26, 2014, 01:41:32 amNo, it's not a glitch in the calculator. As was mentioned by guitchess you are misreading the output. When the calculated result says "Inductance = 6.788E-002 microhenries" this means "6.788 x 10-2µH" or "0.06788 µH" or "67.88 nH".You can't ignore the "E-002" bit, it is a really important part of the number.Your table is:Turns4 = 0.043 µH5 = 0.067 µH6 = 0.097 µH7 = 0.133 µH

author: victor
author_rank: Regular Contributor
timestamp: October 27, 2014, 10:38:26 am
content: Quote from: IanB on October 27, 2014, 02:07:55 amThanks, I didn't noticed the scientific notation there. I will make some changes, and try again. I found another transistor and a 4~40pF trimmer cap I will use it instead.

author: mikerj
author_rank: Super Contributor
timestamp: October 27, 2014, 11:33:59 am
content: Quote from: Hero999 on October 26, 2014, 04:57:53 pmReduce the coupling capacitor value to provide low frequency cut instead

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2014, 07:48:55 pm
content: Quote from: mikerj on October 27, 2014, 11:33:59 amThat will make it sound even worse than before.

author: janoc
author_rank: Super Contributor
timestamp: October 28, 2014, 02:37:47 pm
content: Quote from: Bored@Work on October 26, 2014, 04:57:44 pmUmm, that made me curious - do you have any pointers?I was under the impression that anything broadcasting in these broadcast bands required a specific license, period. The power output was irrelevant, there was no exception for these low power devices. I am quite sure this is still the case in Slovakia where I am from. On the other hand, Slovak rules tend to be incredibly backwards compared to the rest of the civilized world ...Correction - I think I have found a recent change: It seems that the law was indeed changed in 2012 (implementing an EU resolution 2010/368/EU from 30.6. 2010) and if the ERP < 50nW, license is not required in the FM band. However, there are two gotchas - first, wireless microphones are explicitly banned and second, this sort of "bug" can certainly exceed 50nW without special attention ... Also there are some explicit requirements about what the device needs to satisfy, I am pretty sure that rules out any "homebrew" stuff without the right paperwork.Quote from: Bored@Work on October 26, 2014, 04:57:44 pmI didn't want to get into this, but yes, it is obviously illegal to bug someone or cause interference due to poorly tuned/shielded oscillator (air bands are right next to the FM broadcast band in many places!).To conclude, if you want to play with radio, the best way is to get the basic HAM radio license and then you can play legally in amateur bands. All that takes is passing a simple test and paying the fee for getting the paperwork issued.J.

author: mikerj
author_rank: Super Contributor
timestamp: October 28, 2014, 03:43:38 pm
content: Quote from: Hero999 on October 27, 2014, 07:48:55 pmIt's a 1 transistor radio transmitter, why are you so hung up on the audio quality?

author: nuno
author_rank: Frequent Contributor
timestamp: October 28, 2014, 11:56:37 pm
content: I also built a few when I as a kid, none in a PCB. Used "terminal bridges". The cricuit I used to build is more like this one here:http://www.afiata.com/micro-transmitter-fm/#.VFAtVFcog1IIt uses a high frequency transistor, BF494 and higher supply voltage; apparently it had an open space range of about 100m. The coil was 4 or 5 tuns with 1cm diameter (round them around a pencil or pen). R1 in the schematics is only needed if you have a real eletret mic; I used a piezo so no need for R1.Once tunned (preferably on the radio) I couldn't even approach it too much.