Calculate Windings on Coil?

[manuka]

tobyw: Time you were in bed mate! Check the innnards of a typical canned RF coil. This exploded coil former is ex. Andrew Hornblow's PICAXEd FM TX. Low freq ones in AM radios even have the capacitor inbuilt too.

Such FM TX audio bugs on the 88-108 MHz band have long been standard fare for newbies, & kits abound. Many schools/polytechs make them for exactly the sort of educational insights you're hankering for. Aside from their wandering frequency,their biggest hassle usually relates to the transmission, as powerful urban FM stations usually now swamp the band. If you're interested in FM bugs (& all beginners seem to have made one) check Colin Mitchell's site(s)

PLEASE START AT LOWER FREQUENCIES -with your apparent "newbie" skill level it's asking far too much to consider 433 MHz. Stan.

 

[Paix]

In fact Srnet was giving you good advice.

The legal position in the UK for at least the last 10 years is that anyone making a transmitter requires to have it type approved. A lengthy and expensive process, of which the likes of Srnet, with his wealth of experience will be well aware and hence his advice to you.

This fact, along with the CE certification was responsible for a number of radio kit producers in the UK shutting up shop in the last 15 to 20 years. The cost of compliance was just too onerous to be worth carrying on.

The only people permitted by law to make and operate transmitters without having to seek type approval are licensed Radio Amateurs. Seen in that light, having to take a relatively simple examination to get you on the ladder is well worth it. You don't get to operate anything other than off-the shelf kit until you have passed the Intermediate licence examination however.

Many people have already mentioned the 433MHz units that can be purchased. Their simplicity, economy and type approval are all major selling points that allow relatively small businesses and individuals to get on the airwaves and produce final equipments at a worthwhile price point.

On your own, you are unlikely to be able to manufacture a transmitter and receiver pair for use in the 433MHz band. Trying to find a home grown transmitter with a communications receiver might be problem enough and when eventually found outside the allowable band, assuming that you were a suitably licencsd AR operator, then you would have technically and legally been in breach of your license and at risk of having it revoked.

It is much easier to make equipment for the lower HF bands, but there the range of just a few milliwatts of signal can be be international. Miles per milliwatt http://pa1b-qrp.blogspot.co.uk/2012/04/k3ww-with-100000-miles-per-watt.html. 36mw all the way between the Netherlands and the USA. Not typical perhaps, but it just goes to show. That was between Radio Amateurs, before the guys with the government ears, such as NSA, GCHQ and others. http://stakeholders.ofcom.org.uk/binaries/enforcement/spectrum-enforcement/baldock.pdf

Now, when you decide that your signal won't be seen, consider that this technology is available to all internet users, provided by a University Amateur Radio Club. What do you think the professionals might have available ? http://websdr.ewi.utwente.nl:8901/

As a licensed RA operator you can at least have a dabble legally and gain the cooperation of friends to help you sort out issues.

At VHF and beyond I recommend that you stick with the modules that are available. there are many different technologies and some are a lot better than the basic ones, but come with greater entry level programming requirements.

Sorry if you feel that I too am a bit of a damp squib, but it's best that you understand that the radio spectrum is increasingly regulated - apparently despite deregulation - due to the increasing demands made upon it. Of course in the 433MHz ISM band, you could still get blown away by a not so close by AR op running between 50W and his legal 400W antenna input, so choice of spot frequency would be best researched for anything you might consider essential.

 

[alhoop]

Low power(mw) transmitters are legal in the US under FCC rules and regulations part 15. I understand the same is true for Canada.
Is there a similar part in the UK rules and regulations.
Al

 

[geoff07]

Sadly, no. Just the license-exempt 'telemetry' bands are available. Of course, there is a fine distinction between an oscillator and a transmitter.

 

[tobyw]

In fact Srnet was giving you good advice.

Not in his original reply. He seems to have edited it after I responded to add the line about doing a course - which others had already suggested anyway. The original post I was responding to simply said 'if you don't know these things then you shouldn't be trying'. It was a pretty unhelpful comment, and I guess the fact it was edited later suggests he realised that. Maybe he was just having a bad day or something. But it really puts people like me off asking questions when you get responses like that - I was always taught that the only stupid question is the one you don't ask. And I can't believe that the experts on here didn't learn at some point by trying something which was too ambitious and failing.

Anyway - thanks for your post. My main aim wasn't really to make anything much - just to understand how it works and maybe try an experiment or two. But it looks like I am better off sticking with the prebuilt devices for transmitting. I can at least experiment with trying to make receivers.

 

[eclectic]

Toby.

Use your 433 transmitter with a simple wire antenna.
(Not Yagi. See EiRP regs.)

"Play" with different receiver antenna, made from
free bits of string, sticks and wire.

You'll get some fascinating insights.

e

 

[manuka]

But it looks like I am better off sticking with the prebuilt devices for transmitting. I can at least experiment with trying to make receivers.

Are you still talking 433 MHz here? If so then DIY receivers at this freq. are again NOT learner material I'm afraid. PLEASE START LOWER - MAXIMUM 100 MHz !! Even here you'll have issues since most signals are FM requiring quite complex receivers.

Helpful hint: Please specify your UK location,resources,facilities, background, skills level, budget, age(?), actual location etc so Forum members can better consider suggestions more in context.

FWIW I spent ~5 years as a keen as mustard teen before I even ventured as high as 144 MHz (the 2m ham band), & did most of my early wireless learning & constructing below 15 MHz, especially on the 3.5-3.8MHz (80m) ham band. (This was before 27Mhz CB was rolled out in NZ, & freqs. up at 30MHz-50MHz were considered rocket science stuff). Somewaht as a result of my rural NZ isolation I probably spent 10 hours reading about circuits & applications for every hour spent building.

Do you have any good reference books? Want suggestions? Regards-Stan.

 

[Goeytex]

@Toby

RF stuff is difficult, especially at higher frequencies. This is not beginner electronics. That being said, and in spite of the well-intentioned discouragement you may have received here, I would suggest that you go ahead and give it a try. Read up on RF communications theory as much as you can. Maybe build something from a kit to get started. You will need a good scope or spectrum analyzer if you want to design your own stuff.

As far as legal concerns ( I am not a Lawyer), do the best you can to avoid breaking regulations, keep the power levels low( <10 dBm ). But also understand that the many of the Chinese manufactured modules commonly promoted and referred to here & elsewhere are generally NOT CE or FCC compliant ( No CE or FCC Mark) and simply buying one or turning one on is technical violation of regulations. One must wonder then why some are so adamant about warning you about legal issues with home built low power RF. I have not yet heard of anyone going to RF prison for experimenting with low power RF stuff. However, being in the UK Nanny State, with a surveillance camera on every street corner, it may be possible there. Here in the USA regulations allow us to operate up to 5 home made RF transmitter devices without permits/ certifications as long as we stay in approved frequency bands and do our best to comply with power level restrictions, harmonic noise limits, etc. Check your local/ regional rules & regulations to see if there are similar hobbyist regulations.

 

[John West]

As I read it. what tobyw wants to do is indeed "beginner electronics." A low frequency oscillator on one end of the kitchen table coupling to a tuned circuit on the other doesn't constitute a "transmitter" in any common-sense definition of the word. If frequency, power levels are kept very low, and oscillator circuit size is kept very small so that the detectable range is little more than the length of the kitchen table, I say go ahead and play, and consider it all to be a simple induction pick-up circuit.

In the USA one might run such a circuit at 8 kHz and feed it all the power one wished. I don't know what the lower frequency limit is in the UK for what they call RF, but just going down to a low enough frequency for experiments is enough to get outside the USA RF radiation laws.

As for coils and capacitors, the first chapter of the ARRL Handbook for Radio Amateurs has the graph for determining a resonant frequency for inductor/capacitor combinations. Wind the same number of turns (a lot, to get as far down in frequency as possible) of wire on 2 oatmeal boxes, and put 2 identical air variable capacitors in the circuits to resonate and match the frequency of the transmit and receive circuits, and you don't even need to have any frequency measuring equipment in order to match the 2 circuits.

While RF radiation is always a concern, and is therefore legally regulated, any number of unlicensed legally home-built oscillators are in use around the world. Any PICAXE circuit with a ceramic resonator is an example of a home-built oscillator that is radiating RF to one degree or another, legally.

 

[eclectic]

Horse's mouth.
2013 bandplan

page 11 onwards for the frequencies

http://stakeholders.ofcom.org.uk/binaries/spectrum/spectrum-information/UKFAT_2013.pdf

 

[John West]

There you go, tobyw. Stay beneath 8.3 kHz with your resonant circuit, and you're good to go. The only word of warning is that oscillators can (and do) generate harmonic signals at multiples of the fundamental frequency. Down at 8 kHz I don't expect you to have any problem with that, however. Just keep your power down by driving your resonant circuit with something like a simple op-amp and you should do just fine.

EDIT: You can use the PICAXE "COUNT" command to read the frequency of your oscillator. Read more on the command to determine what frequency range it will cover with your PICAXE clock frequency.

 

[eclectic]

Just a thought experiment...

Use Pwmout for the basis of the oscillator.

e

 

[tobyw]

Are you still talking 433 MHz here? If so then DIY receivers at this freq. are again NOT learner material I'm afraid. PLEASE START LOWER - MAXIMUM 100 MHz !! Even here you'll have issues since most signals are FM requiring quite complex receivers.

Hi Stan

No I just meant sticking with the shop bought transmitter AND receiver pair I've already got from techsupplies, see link here. So I don't need to make any kind of receiving circuit. The module just outputs a logic signal which I can read using a simple serin. They are very simple to use, but where's the fun in simple? For actually making a circuit, I'm planning to follow your advice and go for low frequencies.

Please specify your UK location,resources,facilities, background, skills level, budget, age(?), actual location etc so Forum members can better consider suggestions more in context.

About me - 43, very late starter in electronics, based in Banbury UK. Main interest is just learning for the fun of learning. My background is computers and programming so that side of things is reasonably straightforward for me - but it's the electronic theory that I really struggle with. Hence my desire to do things from 'ground up' sometimes.

I'm definitely going to follow your and others advice and stick to the low frequencies. I only mentioned 433Mhz because of the fact that it seemed to be 'exempt' going by the tech supplies website. But sticking to low frequencies suits me fine to understand the principles. Now that you gave me those formulas I have knocked up a spreadsheet with calcs in it - and already it gives me a great insight into the relationship between number of turns, coil width, inductance, and frequency without even needing to grab a breadboard.

I probably spent 10 hours reading about circuits & applications for every hour spent building.

I'm lucky to be learning post internet. No idea how anyone managed before it was around.

Do you have any good reference books? Want suggestions?

I do have a few, but not specifically on this topic, so I would definitely like suggestions yes please!

 

[John West]

Just a thought experiment...

Use Pwmout for the basis of the oscillator.

e

That's OK for a "thought experiment," but I'd hesitate to use it as an actual signal source due to the harmonic content of the square waves generated by the PICAXE, in keeping with the letter of the law. Then again, we are talking about milliwatts at audio frequencies being put into a circuit that has a radiation efficiency approximating zero.

 

[AllyCat]

That's OK for a "thought experiment," but I'd hesitate to use it [PICaxe PWM output] as an actual signal source due to the harmonic content of the square waves

Hi,

Yes, I'm horrified by some of the "projects" in this forum which involve generating 20+ watts of "PWM" (aka a wideband radio signal) onto a long string of LEDs (aka an antenna). However, IMHO the dam was burst when they legalised "mainslink" for broadband communications.

But it's possible to get even the 433 MHz ISM band "wrong". Again, often there are posts here where the constructor has got the polarity of their serial comms incorrect, so instead of transmitting with the (required) 1% or 0.1% duty cycle, they're transmitting for 99% or even 99.9% of the time. Arguably "illegal", but certainly against the "spirit" of the "Industrial, Scientific and Medical" unlicensed band.

Cheers, Alan.

 

[John West]

This might be a good point to remind experimenters that a simple RC filter or inductor can go a long way to reducing RF noise generated by their projects, and that the FFT function on even some inexpensive digital scope modules can detect those harmonics and display them, making it relatively easy for the experimenter to quash them.

 

[manuka]

Again, often there are posts here where the constructor has got the polarity of their serial comms incorrect, so instead of transmitting with the (required) 1% or 0.1% duty cycle, they're transmitting for 99% or even 99.9% of the time.

This probably needs some explaining -here's something I once penned

With serial data directly sourced from a PICAXE pin, configuring as &#8220;t2400&#8221; results in a wasteful TX(transmitter) current drain, even when the data signal is absent! This relates to the &#8220;idle high&#8221; nature of the t2400 (t =true) setting, & can be remedied by coding instead as n2400 (n = inverted) for both the TX & RX. The &#8220;n&#8221; setting idles low, so with no data output there will be no transmitter current drain.

UHF "Birdies" (spurious outputs) can now be readily spotted with the likes of a budget handheld RF Explorer. I've recently written an enthusiastic article (the completion of which became prolonged due to the evaluation unit continually being borrowed!), & found it's displays extremely revealing for 433 MHz transmissions. Most were in fact gratifyingly very clean.

 

[eclectic]

"budget"?

£222

http://www.coolcomponents.co.uk/catalog/explorer-combo-p-1041.html

 

[John West]

Yep. You'll need to budget to get one.

 

[manuka]

That £222 is for their most full featured model, which covers near DC-light! (Well 15MHz - 2.7GHz anyway).
RF Explorer models start at US$99,but their superior WSUB1G wide band UHF (~200-950MHz) is US$129.

I've recently reviewed this later unit favourably,and am about to purchase one myself- orders are freight free directly from Shenzhen (China) based Seeed Studio. No freebies at this level I'm afraid, especially since the unist virtually sell themselves! Stan.

(FWIW - this is of course now well OT, but Seeed Studio use a business model that may well appeal for short run PICAXE based projects)

 

[John West]

Still OT, manuka, but I've yet to receive from Chinese vendors any electronics modules that didn't require at least some rework, on every module. Same with some custom commercial work being done for a company I worked for earlier in the year. I realize companies like Apple and Motorola can get excellent work done there, but I'm not a big enough business to monitor and control the production process that far from home. Neither was the business I worked for, so they had to pay me to QC and repair 800 SMT modules for them.

When all was said and done, (and I was paid,) they could have gotten the work done correctly, and sooner, right here in the US for about the same money.

China can be good and China can be cheap, but expecting both good and cheap is still a gamble.

 

[manuka]

Fair comment- the RF Explorer is of course the brain child of Spaniard Ariel Rocholl & Seeed apparently only follow his designs. We're very much in a global village these days!

FWIW -since the early 1960s I've been importing hobbyist & commercial electronic gear/parts into NZ from UK,EU,USA,Japan,Taiwan,Australia & (only recently) mainland China. In that 50 year period I recall almost every source has had products that at some time were buggy.To their credit most however are right on the nail & extremely good value.

FYI- in total contrast to our present near free trade,NZ for decades (~1960-1990) had tightly controlled regulations, leading to VERY high markups on almost anything imported. As a result the good old days WERE NOT hobbyist friendly locally-I particularly recall importing Clive Sinclair's sourced MAT121 bipolars (Micro Alloy Transistors)from ~1964 that took 6+ weeks to arrive & cost me the equivalent of US$20 each in todays money.This price however was then about 1/3 of inferior local offerings...

Although some sourcing woes have been trivial,others were potentially costly (both in time & money), especially when imported for R&D in my work. Memorable were Heathkit part omissions, a $$$ US made energy meter that was seriously miswired at the mains level, incorrectly labled 7812 (they turned out to be 7815 but the 5 was inverted & "looked" like 2!), incorrectly connected polarised electrolytics which failed explosively after 5 minutes power up, frozen rotary switches, Basic Stamp-1 kits with suspected ESD damage, dry joints (MUCH more of a problem in the hand soldering days), corroded battery terminals, solar PVs that went open circuit when hot etc etc

With today's thin profit margins & white hot rate of change I'm reasonably tolerant about QC, and accept that some products may well be substandard. One does ones footwork before ordering of course - increasingly most are of exceptional build. UK sourced PICAXE gear has been consistently top quality- I've never had a lemon in 10 years from Rev.Ed! Stan.

 

[John West]

The primary problem I've run into is with SMT's that are "under-soldered." As just about everything is built with SMT's these days, that means problems with just about everything. Under-soldered SMT's are a particularly ugly problem, as the circuit itself can check out just fine one moment, and fail the next, or be intermittent due to flexing or simply thermal changes. The darn things are all so small anymore that it's nearly impossible to be truly certain that every pad is properly soldered.

The last batch of Chinese modules Premelec and I purchased together were sorted out and placed in Zip-loc bags to store them. When I pulled the bags out of storage to actually use one of them I noticed 2 tiny SMT components lying in the bottom of the bag, an LED had fallen off one bd, a resistor had fallen off another. That's when I went through all of the modules, carefully eying all connections and wiggling each larger component, such as inductors. More than one inductor lifted one leg right off the pad when I did that.

It was no big deal, really, as I only needed a couple of minutes to re-solder most of the components on each module, and the modules were extremely inexpensive (step-down switchers,) but still and all, the work did have to be done, or there would have been the devil to pay if I'd powered any of them up to drive valuable circuitry.

Just a reminder to folks to put some time into QC'ing everything they mail-order, especially power supply modules.

I apologize for the severely OT nature of this, but it's an important point to be made, and this is where it lead to.

P.S. Everything I've received from Rev. Ed. has been top-notch. And the sum total of advice I've received from this forum has been worth far, far more than what I've ever spent for the materials. Oh, and everything Technical posts, I read twice.

 

[boriz]

Simplest I can think of...

RX is a single MOSFET with the Gate connected only to a length of wire (aerial) and the Source connected to Earth. TX is an ordinary 12v relay wired as a buzzer (interrupts it's own coil current) with a similar aerial on one end of the coil and the other end connected to Earth.

Muahuahuahua! *Rubs hands like villain*

 

[geoff07]

Hertz managed it without the mosfet!

 

[John West]

But Hertz cheated. He understood physics.

 

[BeanieBots]

Simplest I can think of...

RX is a single MOSFET with the Gate connected only to a length of wire (aerial) and the Source connected to Earth. TX is an ordinary 12v relay wired as a buzzer (interrupts it's own coil current) with a similar aerial on one end of the coil and the other end connected to Earth.

Muahuahuahua! *Rubs hands like villain*

I know the UK is a "Nanny State" but even the relaxed laws in US and AUS state "Spark sending apperatus is strickly forbidden".
It would be VERY irresponsible to do something like that.

 

[John West]

We prefer "relaxation oscillators."

 

[boriz]

We prefer "relaxation oscillators."

Like you get from the 'ladies shop'?