Calculate Windings on Coil?

[tobyw]

Hello all

I've had some fun using bought transmitters receiver pairs to get two picaxes communicate. But I'd really like to try and make my own transmitter and receiver - but I'm struggling to find information to help.

I wonder if anyone can help with a few questions?

1. Is it a realistic thing to do? I'm not looking to create a walkie talkie with 1km range or anything - just want to build something which I can send a '1' or a '2' from one end of the kitchen table to the other.
2. How do I go about 'adding' my signal to the carrier wave? I know I could theoretically connect a 1mHz crystal to a dc power source with a switch, and every time I switch it on, I will create a frequency which a tuned radio will hear. But how would I go about modulating that frequency with a crystal? Or am I completely off track?
3. For the receiver - how would I calculate the number of windings I need and the capacitor value required to create a circuit which will resonate to the frequency of crystal I use? I presume it is possible to calculate what the resonant frequency would be of a particular size of coil and capacitor..but I can't find anything on Google about this. I've made a crystal radio before using a fixed coil and a variable capacitor, but I'd prefer to be able to build a circuit specifically aimed at a certain frequency, with a small trimmer presumably to fine tune.

If anyone could shed some light on this process, or point me to some links where I can learn more to help me work it out myself, I'd be grateful.

Many thanks
Toby

 

[fernando_g]

I'm not an expert on radio-communications, but I do know that frequency bands are allocated for different services, and one must respect those to avoid interference to/from other devices or services.

For instance, the 1 Mhz frequency is right in the middle of AM radio.
You would have to use a frequency assigned to "free" services.
There is also limits on the amount of radiated power and how long one can transmit.

For such a short range of comms that you require, probably Infrared is a better way to go.

Again, I'm not an expert on radio communications.

 

[BeanieBots]

....I'd really like to try and make my own transmitter and receiver .. Is it a realistic thing to do?

Not if you don't know what you are doing and/or don't already have good electronic design knowledge.
As tobyw points out, frequencies and power are allocated BY LAW and especially so in the UK.
Infringement can result in prison sentances even if accidental so it's not something to 'play with' lightly.
Please bear this in mind if you do start to play. Also, there can be real safety consequences with illegitimate RF transmissions.

That aside, it's not a topic for the faint hearted because when you get up to RF frequencies things that you took for granted at DC become completly different components. For example, you can make a transformer by running a couple of PCB tracks next to each other. A length of wire will have different impedance depending on far away it is from another peice of wire (characteristic impedance). You can get reflections comming back down a cable which can blow up output stages. Powering up with no aerial (or badly tuned) can cause damage to a transmitter.

Manuka and a few others are our resident RF experts. Hopefully they will be able to point you in the right direction.

 

[eclectic]

One possible direction:


http://www.rsgb.org/clubsandtraining/

Do a course, get training and a knowledge of the law.

Pass, then legally make and use your creations.

e

 

[mrburnette]

@tobyw:

May I suggest material along this line: http://www.arrl.org/ham-radio-license-manual first for basic understanding of electronics and rules/regulations associated with operation of a transmitter. Many countries allow low-powered (flea-power) unlicensed and unregistered transmissions in specific bands. This will be country-specific.

The ARRL and the Ham Radio community are your friends in such endeavors. There are many on this forum and will likely speak-up.

Your goal is easily obtainable, but I would recommend a methodical approach in learning exactly how the transmitter works. The math of winding a coil with a specific inductance at a particular frequency is very simple. Implementing a legal transmitter within your home is another matter.

- Ray

FOR THE FUN OF IT, here is the online link for a javascript calculator... http://www.daycounter.com/Calculators/Coil-Physical-Properties-Calculator.phtml

 

[tobyw]

Thanks for the warnings.

I didn't really think I was going into ham radio territory - I was hoping just to make a very low-powered transmitter receiver pair using a 3v battery to send binary data from one picace to another across a distance of a few feet (kitchen table) in the amateur band - 433MHz for example?

"The math of winding a coil with a specific inductance at a particular frequency is very simple."

This was what I was after. But I'm struggling to find any information on that math. I've tried various search terms but I can't find anything with the equations / measurements.

"Implementing a legal transmitter within your home is another matter."

I'm amazed it's possible to be illegal with a 3v battery and a simple coil! You live and learn. But I guess you guys know best. I will just have to stick to buying them from tech supplies instead of making my own.

Thanks

 

[hippy]

I'm genuinely surprised by all the nay sayers here.

On the legal side, no one is likely going to say do something illegal. Not in public or where anyone can hear them say that.

On actually doing what you want; it's more is it worth the effort ? For fun or for the having done it then maybe, but just to get a signal from A to B it's a lot of work when you can buy what you need and know that will work.

 

[tobyw]

I didn't mean it in that way hippy. I certainly don't want to get up to anything dodgy. I just meant - I genuinely thought there would be a fairly broad amateur band which it would be easy to work within, and I would be able to choose an appropriate crystal to use. And with such low powers involved I also naively thought it be unlikely to cause any real issues. The legal stuff was an eye opener - I thought it only applied to powerful transmitters. I've edited my post to avoid confusion.

On actually doing what you want; it's more is it worth the effort ? For fun or for the having done it then maybe, but just to get a signal from A to B it's a lot of work when you can buy what you need and know that will work.

Well that's the thing though, with electronics - isn't it? I mean, I can make a picaxe flash an led very easily. But it's more satisfying when you do it with a 555. And if I was more competent, I would probably try making my own monostable vibrator using logic gates. It's like the difference between climbing Everest, and taking a helicopter up to the top. Although to be honest, climbing Everest would be easier than understanding some of the calculations involved in electronics!

 

[eclectic]

@Toby.

Even after you've wound the coils,
you'll still need to check them.

Here's just one way:

http://www.peakelec.co.uk/acatalog/jz_lcr40.html

e

 

[tobyw]

@Toby.

Even after you've wound the coils,
you'll still need to check them.

Words cannot express how much I want one of those devices eclectic. Unfortunately I just spend the best part of a 50 quid on 2.54mm crimping equipment (surprisingly expensive) and a SP0256A-AL2 speech chip. The wife would not be pleased if I forked out on one of those gizmos just yet! Thanks for the link though..it's on my shopping list.

 

[techElder]

Tobyw, learn to calculate what you need for tank circuits and the like for your RF projects, but go the middle ground and buy "coils" and transformers from parts suppliers. You can buy a range of components and see what effects they have on your circuit. Tons better than getting into all the intricacies of bobbin dimensions and wire sizes!

 

[rossko57]

I genuinely thought there would be a fairly broad amateur band which it would be easy to work within,

Another consideration would be that you're going to be sharing that band with other amateurs ... they wouldn't appreciate something dirty spiking their projects. Likewise, the neighbours may have wireless oil tank senders or WiFi or or ...
Not nay-saying, just pointing out the need for homework in what should be done, where, and how it might go wrong.

I would have thought stuff like these would cover the calculations
http://www.midnightscience.com/formulas-calculators.html
http://www.rfcafe.com/references/electrical/inductance.htm

 

[tobyw]

Another consideration would be that you're going to be sharing that band with other amateurs ... they wouldn't appreciate something dirty spiking their projects. Likewise, the neighbours may have wireless oil tank senders or WiFi or or ...
Not nay-saying, just pointing out the need for homework in what should be done, where, and how it might go wrong.

I do understand your point. I would have thought that things like wifi and oiltank senders operate in non-amateur bands though? But most importantly, I was sort of assuming that in an amateur band, where there is conflict, that's why you need decoders and encoders?

Otherwise, surely my wifi could just interfere with their wifi without any help from me? What happens when you have two people using radio controlled cars near each other?

 

[rossko57]

I dunno what band you have in mind or what eBay wireless weather stations use, that's the point - homework required.

First mention of encoding (which is a good plan), we started with a go/nogo transmission requirement. Encoding is good to stop interference being misinterpreted as valid data, but it doesn't stop interference .. well, interfering.
I think the RF engineers are worrying that homebrew kit can radiate in unexpected bands etc. even when it appears to do what you expect. It's difficult to find that out without test gear.
It _is_ all do-able, but will be more complicated than winding a coil to do it right. It's easy enough to reproduce Marconi's spark transmitter on the kitchen table but it just ain't socially acceptable anymore

 

[srnet]

I would have thought that things like wifi and oiltank senders operate in non-amateur bands though? But most importantly, I was sort of assuming that in an amateur band, where there is conflict, that's why you need decoders and encoders? Otherwise, surely my wifi could just interfere with their wifi without any help from me? What happens when you have two people using radio controlled cars near each other?

Well, if you dont know the answers to those questions, your not really in a good position to start building your own radio transmitters.

Do a Radio Amateurs License course, covers the basics and is available in lots of areas.

 

[tobyw]

Well, if you dont know the answers to those questions, your not really in a good position to start building your own radio transmitters.

Why just post snide comments? I've already clearly said that I'm not going to, following the useful information here. But I'm still having a discussion with rossko which is helping me learn about the subject. What are you contributing? I have always found this a really helpful forum - where did you come from?

 

[JimPerry]

I do understand your point. I would have thought that things like wifi and oiltank senders operate in non-amateur bands though? But most importantly, I was sort of assuming that in an amateur band, where there is conflict, that's why you need decoders and encoders?

Otherwise, surely my wifi could just interfere with their wifi without any help from me? What happens when you have two people using radio controlled cars near each other?

WiFi is in a regulated "unregulated use" spectrum and is divided up into about 10 bands - your WiFi scans for a band not being used and then uses it. Model cars and planes use specific bands allocated by the marshalls in charge (when in competition) and all aerials/ models have a coloured pennant to indicated what they are using - to avoid crashes etc.

The early comment about infrared is probably apt - get it working and you are infact using line-of-sight low-frequency wireless. The world has changed since I built an electric-shock machine, using surplus GPO relay (and transformer wired backwards)s. as a project before "O" levels 45+ years ago

 

[tobyw]

I dunno what band you have in mind

Me neither. I was hoping to find these things out. But it looks like that's not going to be easy, so it's best to stay well away.

I bought a transmitter/receiver pair from tech supplies. They operate at 433MHz and say they are license exempt. So that was what made me think there would be frequencies which were ok (and also from what I have read about the legal allocation of the spectrum already - I know for example that connecting a 1MHz crystal to a 9 volt battery and hanging it out the window would not be legal)

I'm wondering now whether even using this transmitter/receiver pair would be considered a no-go? It obviously would interfere with anything else operating on that frequency. If I just hook up the receiver to a picaxe and read the incoming values I can 'see' how much traffic there is. But picaxe handily allows you to look for pre-transmit values, so I found it relatively simple to create a protocol to filter out unwanted noise. I would have assumed that any other wireless device operating in any band would have to do the same, because there would be too many devices to insist they all use a different channel.

 

[tobyw]

The early comment about infrared is probably apt - get it working and you are infact using line-of-sight low-frequency wireless

Good point. I have used infrared quite a lot, including for remote control, but it obviously has some drawbacks too. But the wireless thing was really about learning, rather than practical application. I just wanted to understand what is happening at a really primitive level, instead of just using prebuilt equipment which is very complex to try and understand. 'Tis not to be though.

The world has changed since I built an electric-shock machine, using surplus GPO relays. as a project before "O" levels 45+ years ago

I'll say it has! I wouldn't want to be marking your exam

 

[JimPerry]

It was shocking
Try http://electronics.howstuffworks.com/radio6.htm

 

[tobyw]

It was shocking

Groan.

So on the point about wifi using bands... what is the definition of a 'band'. Is it a more specific frequency within the wider frequency? And how 'wide' is it? I presume that even so, devices still need to have their own encoding/decoding methods to filter out other devices which might be using the same band? I mean, for things like a wifi device. For rc cars, I would imagine they don't, hence the reason you can get interference. But for data devices, is it the case that if I were to stand next to it firing off an intermittent signal at the exact same frequency, it would corrupt the data being received? What about parity bytes and all that stuff?

 

[JimPerry]

I'm probably out-of-date with the precise terminology - but to me a band is a small set of frequencies in a part-spectrum designated for a purpose. RC cars used to use the spectrum allocated to CB with channels (bands) within the CB spectrum - with very little error correction.

Modern use of stuff has moved to digital and lots of error correction etc.

It's 30+ years since I worked as an editor for 73 Magazine (American Ham Radio, long since defunct)

 

[tobyw]

Thanks Jim for all the info. I have a lot of reading to do.

Would you mind answering one last question - this transmitter/receiver pair I bought from tech supplies... am I ok to be using that? I was also hoping to just make a little LC circuit with a battery and see how the current flows with an LED in the circuit..but even that I'm beginning to wonder if someone's going to come and tell me their garage door keeps opening and closing....

 

[mrburnette]

@tobyw:

I'm an old fart so thinking back to my pre-teens is not always clear, but I do remember building in the late 50's and early 60's crystal receivers... the kind where you have to wind your own coils and build (or buy) the capacitor. I also remember my fascination with radio through my teens and the numerous shortwave receivers I built.

I know you are looking for some specific and easy to absorb information, but if you want to take a look at some really old history on how radio used to be, and also give you some insight into how basic receivers work, you may find these links of interest: crystal radio
and Marconi wireless. One of the great fights in the 20th century was the awarding of the patent for radio to Tesla and not to Marconi but you can find numerous articles on that without a direct link.

Good luck on this... I know many of the forum members are knowledgeable in this area; but regulations being weird across the globe makes it difficult to point you to a specific source other than local Ham operators.

- Ray

 

[g6ejd]

Low Power Device Radios are modulated in FM with an output power of no-more-than 10 mW. There are many allocated channels for these, they don't need a licence. At 433Mhz about 70 channels.

As @hippy has already said, building equipment at these or any other frequencies is a real challenge unless you have a lot of background knowledge and skills to match. I know of just a few people who could get any 2.4GHz equipment (Wi-Fi band) to work, if that gives you an indicator of the challenge. The techniques for building at these or other frequencies require you to understand all the device specs, losses, capacitive effects and to a lesser extent inductive effects.

I could definitely build a Tx and Rx for 433MHz, but I would not bother, because there are so few suppliers of the needed components in the market place, certainly not e.g. Maplin, they don't even come close. Farnell yes, but now your into spending a small fortune when you can buy for £2.50 a complete Tx-Rx matched pair that work. I just got a pair of 433 units off ebay and they work great at 19200kBps.

I'm saying all this in the context of being a Radio Amateur with 30-years of experience, let alone my acedemic knowledge and skills and having the essential test equipment to do this - just trying to bring a practical view to the debate.

 

[eclectic]

@Toby

http://www.rsgb.org/committees/spectrumforum/band-plans.php

e

 

[geoff07]

There are license-exempt bands, which you are allowed to use, on the basis that you do not exceed a (very low) power limit, and do not cause and do accept interference. 433 MHz is one of these. But you are not permitted to operate outside these bands unless you have specific authority, such as have qualified and licensed radio amateurs. When an amateur operates within the amateur bands he/she is required to have the equipment to ensure that they are not exceeding the (higher) power limits and are not transmitting energy outside the bands allowed (e.g. through harmonics). So building a transmitter, though an interesting exercise, has a lot of baggage with it.

However, it is not difficult to pass the radio amateurs exam (I passed when I was 16) and that would enable you to do all sorts of interesting things. Meanwhile, it is hard to beat the 433MHz modules if you want to use short range radio. As already pointed out, making tuned circuits for VHF is no joke when you have no means of seeing or measuring the signal. The tx modules generally don't use tuned circuits, they just squirt the oscillator signal straight into an antenna, relying on the resonator for frequency compliance and the oscillator design for input power compliance.

 

[JimPerry]

Thanks Jim for all the info. I have a lot of reading to do.

Would you mind answering one last question - this transmitter/receiver pair I bought from tech supplies... am I ok to be using that? I was also hoping to just make a little LC circuit with a battery and see how the current flows with an LED in the circuit..but even that I'm beginning to wonder if someone's going to come and tell me their garage door keeps opening and closing....

Should be fine - for real-world use tech-Ed supply a AXE213 connect kit http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/AXE213 which uses a couple of chips to Manchester encode/decode the signal

 

[hippy]

Would you mind answering one last question - this transmitter/receiver pair I bought from tech supplies... am I ok to be using that?

Absolutely. They are designed to be compliant with the requirements of license exempt use.

If you can build your own modules and make them equally compliant you can use those. The difficulty is in making / proving your own modules are compliant as well as knowing how you would go about making such modules in the first place.

 

[tobyw]

The difficulty is in making / proving your own modules are compliant

That kind of brings us full circle, because it was the information I was looking for in the first place But I guess I am not going to find out easily.

I've found an amateur radio association near me, so I will see about doing a foundation course and maybe someone there will be able to help. Thanks all

 

[JimPerry]

http://www.crystalradio.net/cal/indcal2.shtml Perfect?

 

[geoff07]

but even that I'm beginning to wonder if someone's going to come and tell me their garage door keeps opening and closing

But it works both ways - if my garage door opens because someone else is using a 433MHz tx nearby I have to accept that and deal with it at my end (e.g. by coding). I have no grounds for getting cross! That won't stop me from doing so, of course.

If you have access to a spectrum analyzer, tune it to 433.9 MHz with a whip aerial and see how much is going on. Round here, quite a lot!

 

[manuka]

tobyw: I'm sure a bright spark like you is up with the classic LC resonance formula that relates freq, capacitance and inductance. Even a casual look at both this & the empirical Wheeler coil formula indicates 433 MHz LC combos are low value (µ/nH and pF) & hence really too fiddly to organise yourself.

Pretty much the limit of kitchen table DIY capacitor coil winding for transmitters is the 88-108 MHz FM band- check Andrew Hornblow's breadboard approach -scroll down ~½ way to 3 pix. This TX can be PICAXE tone modulated for Morse style information transfer to receive on any FM radio. ALERT: The FM band is now so cluttered with high power stations that your weak signal may be near impossible to tune (unless you live in a very remote region).

But why bother? PICAXEs & 433 MHZ devices are made for each other, & both are dirt cheap. Perhaps put your DIY zeal into range extending antenna instead? If admirably it's LC coil winding experience you hanker for then perhaps look at a MW band receiving resonant loop
Stan. (Radio ham ZL2APS -licenced 1967)

 

[RexLan]

You guys are pretty rough ... the poor guy was just looking to make something simple

 

[manuka]

Which is why I've suggested lower frequencies! You can have a lot of LC learning down in the "DC bands". As a teen (& pre calculators...) I recall calculating & winding a multiturn 455 kHz BFO coil for add on SSB resolving to an AM only Panasonic comms. receiver. This taught me a LOT & I was gratified that it worked out well. Amazingly it's still at hand nearby in the same radio!

FWIW my approach as an educator has been to ensure learners are initially enthused - they may otherwise become demoralised and abandon the whole thing. This was/is a major reason why PICAXEs were/are so satisfactory- I'd known folks previously that'd spent weeks just trying to flash an LED...

 

[tobyw]

You guys are pretty rough ... the poor guy was just looking to make something simple

To be fair most of the people replying are offering lots of useful information - only a couple of people leapt in without reading the question and not really offering much constructive advice. Otherwise the answers have been very helpful, as they usually are here.

 

[tobyw]

tobyw: I'm sure a bright spark like you is up with the classic LC resonance formula that relates freq, capacitance and inductance

Not bright enough I guess. That's exactly what I wanted to know - so THANK YOU for that equation!

This TX can be PICAXE tone modulated for Morse style information transfer to receive on any FM radio

But wouldn't this fall into the category of instant imprisonment, as some of the early responses implied?

The other thing is, is it possible to use a crystal to create the receiving resonator? Looking at the 433MHz receiver I got from tech supplies, there doesn't seem to be anything looking like an inductor on there. I was wondering how it works...if you can use a crystal to create the transmitter, can't you use one to detec the frequency too?

 

[eclectic]

snip
Looking at the 433MHz receiver I got from tech supplies,
there doesn't seem to be anything looking like an inductor on there.

http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/RFA001

White cubic jobbie, with green stuff covering the
adjusting screw.

e

 

[tobyw]

http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/RFA001

White cubic jobbie, with green stuff covering the
adjusting screw.

e

Ah got it. I thought that was a trimmer of some sort. Should have put my glasses on... thanks

 

[geoff07]

.if you can use a crystal to create the transmitter, can't you use one to detec the frequency too?

Well, yes you can. You are verging on the mysterious subject of modulation and demodulation and some potentially serious maths if you choose to go into depth. Perhaps the simplest detector using a crystal would be a beat frequency oscillator (bfo). You receive the incoming signal, mix it with a signal from a crystal of a frequency say 1kHz higher (or lower). The result is a sum and a difference frequency. Forget the sum, which is not much help to us, but the difference is of course 1kHz. Which is audio and is how morse code is detected. Or perhaps I should say 'was' i.e. wartime radios etc as today a synthesizer would be used rather than a crystal.

If you are interested in radio, I suggest investigating SDR (software-defined radio) - probably the easiest way to get started. A simple receiver plus a good quality (high sampling rate) sound card and a pc are all you need. One look at a waterfall display and you will be amazed.

http://gnuradio.org/redmine/projects/gnuradio/wiki