Making of a simple RF morse transmitter/receiver - Page 1
Forum section: electronics / beginners

author: Mutad0r
author_rank: Contributor
timestamp: April 14, 2015, 09:02:25 pm
content: I'm learning about RF stuffs.What I'm trying to achieve is a simple pair of a transmitter and a receiver that can be used to send morse code ( I chose morse code because the both the modulation and demodulation are done by my brain.) For the purpose of expandability to a digital system where the transmitters button is replaced by a GPIO of an MCU and the receivers output is connected to another MCUs GPIO, lets not use a buzzer on the receiver, but instead an LED.So, on the transmitters end I would push a button, and while the button is pressed, a continious sinewave is created at x frequency. This transmitter is very simple, basically all you need is a circuit that makes a sinewave and a button to control whether it's turned on or off. The transmitter part I can actually achieve on my own, depending on the frequency. (I have no idea how near-GHz signals are made, considering that chrystals dont go up that far as much as I know.)The problematic part is the reciever ( to a degree ). I've used filters for audio purposes before, so the first step would be to make a band-pass filter or use a premade filter (from what I understand, a SAW filter can be used for this). The most problematic part for me is turning the received RF signal into a digital one, I found some information on that online ( using the MAX9930, but that doesn't sound like the way that it's usually done.)From experience it's possible that I've used the wrong key-words on google and thus havent found a proper answer to my question, although I have had some close misses.Hopefully I haven't been too confusing while explaining what I'm trying to achieve, but I'm confused so what do you expect?Any and all information that you have to share about a simple on/off signal transmitter/reciever is welcome!

author: Simon
author_rank: Unknown
timestamp: April 14, 2015, 09:11:40 pm
content: ok so what exactly do you want to do ? you can get off the shelf modules

author: VK5RC
author_rank: Unknown
timestamp: April 14, 2015, 11:07:12 pm
content: I would look up ham groups esp QRP (which means low power) they love simple/elegant transmitters /receivers. Re GHz, you usually use a crystal, amplify it, distort it deliberately then filter off the harmonic you want.Edit ;you will need to check with your RF authority re power levels and frequency you can use.

author: skyjumper
author_rank: Regular Contributor
timestamp: April 15, 2015, 03:41:00 am
content: A friend of mine built an entire low power (QRP) 40 meter CW transceiver based on seven 2N2222 transistors and a small number of other components, resistors and capacitors. If anyone is interested, I'll ask his permission to post the schematic. He did a board for it as well. He has a bunch of contacts with it.You would need a ham radio license to use this though.

author: Mutad0r
author_rank: Contributor
timestamp: April 15, 2015, 08:58:11 am
content: Let me clarify that I dont want to make something useful, I want to learn. I could use off the shelf RF chips or modules, but then I wouldn't learn anything.Let's talk about turning that sinewave into a digital signal. Me thinks peak detection can be a way to solve this. I have made a crude drawing to accompany this ( i've added it as an attachment). The red line can be the voltage required to turn on a MOSFET, so when the green line goes over the red line, the mosfet is on and you get a 1/0 output.Again, I appologize for being confusing, but I have soo much going on right now, the worst of which is studying for entrance exams for the university, considering that the education I was given doesn't even cover most of the things in there. Sure, I could concentrate on that and forget about this RF stuff, but unfortunately I have to for reasons too many to explain

author: VK5RC
author_rank: Unknown
timestamp: April 15, 2015, 09:32:32 am
content: Have you thought about getting your ham licence? You have to learn about rf, most hams are members of a local club, meet, have rf talks, swap/sell/buy gear etc. You then get serious access to decent amounts of rf spectrum and power to do some serious experimentation.

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2015, 09:43:30 am
content: Yes you can buy modules but I assume you want to do this yourself.The easiest way to do this is to use 27MHz band which is free.The receiver is fairly simple too. The easiest way is to use a beat frequency oscillator to generate an audible signal from the signal received. The receiver is just an oscillator with a slightly different frequency than the signal you want to receive.

author: Mutad0r
author_rank: Contributor
timestamp: April 15, 2015, 09:45:10 am
content: Not really. I don't know how helpful that would be in Finland. I'm not concerned about making RF devices unlicenced because my goal is to get a signal from 1 meter (So really low power).

author: Simon
author_rank: Unknown
timestamp: April 15, 2015, 10:12:37 am
content: Quote from: Mutad0r on April 15, 2015, 09:45:10 amSo if you tell us more about what you want to to we can bettere help, transmitting RF can be required from metres to thousands of Km

author: AG6QR
author_rank: Frequent Contributor
timestamp: April 15, 2015, 11:07:47 am
content: Regarding peak detection for turning the audio back to on-off digital signal, that sometimes works. But it has problems with weak signals in the presence of noise. A well-trained ear is hard to match for decoding Morse over a long-distance HF path. To come close, you've got to look not only at the volume of the signal, but also at the spectral purity. FFT and DSP can be helpful. Most noise is not as pure a tone as a signal.Regarding the RF side, look up the Rockmite, a very popular and simple tranciever that works amazingly well. But it uses the human ear for decoding.Finally, I agree with those who suggest getting a license and playing with this for real.

author: janoc
author_rank: Super Contributor
timestamp: April 15, 2015, 12:23:57 pm
content: If you want a simple CW transmitter, then Michigan Mighty Mite is a fairly easy project:http://www.qsl.net/wb5ude/kc6wdk/transmitter.htmlReceiver can be a little more complicated, but there are tons of simple receivers around, for example:http://www.qsl.net/wb5ude/kc6wdk/receiver.html- essentially a crystal set with a BFO.However, as mentioned by others already - make sure you have your HAM ticket (and that actually you have HF privileges - in many countries the basic licenses don't include or heavily restrict them!) before you attempt to connect a transmitter to anything even remotely resembling an antenna. Otherwise you will be rather quickly tracked down and fined. HAMs generally don't like interference in their HF bands and are fairly good at triangulating signals.Good luck!

author: deephaven
author_rank: Frequent Contributor
timestamp: April 15, 2015, 02:04:49 pm
content: OP only wants a range of 1 metre, so power will be so low it would be difficult to triangulate him!Did you have a particular frequency in mind? There are a few license exempt bands available.

author: janoc
author_rank: Super Contributor
timestamp: April 15, 2015, 06:00:24 pm
content: Quote from: deephaven on April 15, 2015, 02:04:49 pmDon't underestimate a simple oscillator. The Michigan Mighty Mite that I have posted a link to is a single transistor affair, producing about 500mW of power at 12V. If you happen to connect that to a piece of long wire that will act as an antenna, you can achieve transcontinental contacts with it. It has been done. Even with a much poorer antenna you will easily cause interference - the fact that your primitive receiver is unable to pick the signals up at more than 1m doesn't mean that someone else with a better equipment can't.Unless you are planning to only ever transmit into a dummy load, you should assume that someone may (and will) hear your signal.Quote from: deephaven on April 15, 2015, 02:04:49 pmMost license-exempt bands, such as the various ISM ones or the CB band don't really allow homebrew equipment - the equipment needs to be certified and not user-modifiable (=tamper-evident seals, fixed power limiters etc.) to be able to operate there legally. Not something an amateur is likely going to do.Also, with the exception of CB, most of those are in the upper VHF and UHF, (100MHz+) ranges - not something easily approachable for a beginner wanting to build something from scratch, not using a pre-made module.Pretty much the only bands where you can meaningfully and legally experiment with transmitters are HAM HF and VHF/UHF ones - and for those you do need a license. The exact laws and rules differ between countries, of course - e.g. the popular wireless "bugs" or microphones that one can receive on a normal FM radio are illegal in the most of Europe but supposedly OK in the US (different max. power limits).If the OP is interested in transmitters and radio (it is a wonderful hobby, btw!), I strongly suggest he gets at least a basic HAM radio operator license. Most countries don't require Morse code anymore, the exam is often only a test from the rules and basic operating skills.

author: G0HZU
author_rank: Super Contributor
timestamp: April 15, 2015, 07:01:17 pm
content: The first (and most basic) receiver circuit to try would be a simple envelope detector followed by a comparator. eg a simple schottky diode detector plus a cheap modern equivalent of the LM311 comparator for example. This would give a DC voltage from the envelope detector according to signal strength and you define the threshold of a 0 and a 1 with the comparator threshold voltage.The performance will be relatively poor but it will work OK across a small room and it will produce a digital output from the LM311 if an external 2K2 pullup resistor to 5V is fitted at its output for example.It would take about 15 minutes to build it and it would cost maybe $2 in parts. Once you realise how limited it is in performance you can reuse most of the parts in a more sophisticated receiver design because the comparator can be used again here.

author: janoc
author_rank: Super Contributor
timestamp: April 16, 2015, 09:57:46 am
content: Well, if one is going to use some ICs, then the venerable NE612/SA612 allows to build a simple direct conversion receiver in about the same time and money with much better performance.Use either a crystal for the LO or one of the many cheap AD9851 DDS modules available from eBay if you want variable frequency and don't fancy building an actual VFO (or use the one in the chip itself).Depending on the oscillator/VFO and antenna, this sort of receiver can easily receive AM broadcast signals from all over the world, CW, SSB signals etc. And it is a very good way to learn about the about the basics of oscillators, mixers, filters, etc, especially if you have a scope and can look at the signals directly.Here is a fairly simple circuit:http://www.waveguide.se/?article=ne612-receiver-experimentIt can be simplified even more, dropping the buffer opamp and the automatic gain control if you don't have a strong transmitter nearby (or a very good antenna) that would overload the input.Theoretically, one could substitute the NE612 with something like a diode ring mixer + buffer, but I wouldn't recommend that for a newbie - those things need carefully picked parts and winding transformers to work well.

author: albert22
author_rank: Regular Contributor
timestamp: April 16, 2015, 03:53:36 pm
content: I think that is better to build the receiver first. You will learn a lot and a license is not needed. It will be very rewarding when you start to receive the first signals.There are a lot of projects on the web the magic word to search (as somebody said) is QRP followed by rigs, transmitters or receivers. Although qrp applies only to the transmitted power.The simplest receiver you can build is a regenerative. Look in the link below for "A very simple receiver" (just one JFET)http://www.qsl.net/pa2ohh/Receiving morse code is different from receiving a CW signal for a digital signal or remote control. To hear the beeping sounds of the morse code you need a receiver with a BFO (beat frequency oscillator). The regenerative can do this.An envelope detector will give you the digital signal but you will only hear clicking sounds instead of the morse code beeping. Gating an audio oscillator with the envelope detector will give you the beeps todecodemorse in your brain.For an example of a remote control tx rx:http://electronics.stackexchange.com/questions/42923/what-do-i-need-for-a-basic-rf-circuitThere are unlicensed bands for remote control.

author: G0HZU
author_rank: Super Contributor
timestamp: April 16, 2015, 05:38:22 pm
content: Quote from: albert22 on April 16, 2015, 03:53:36 pmThe OP prefers to decode via an LED indicator and didn't want a buzzer/beeper. But an envelope detector would have worked with either. A basic regenerative receiver using a single JFET would have to be carefully tuned for correct feedback and then and netted into the signal every time it was used (to hear morse code correctly) and it would drift a lot over time. The envelope detector would not need any tuning to just 'work' across a room.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2015, 07:24:40 pm
content: I think it's best to first design a basic transmitter and receiver, before worrying about detecting the tone. A crystal controlled BFO would probably be the most stable way of detecting the signal.