PicAxe based AM Transmitter

While looking at a site I found this nice little project: http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html. I was thinking of somehow using this information to build my self something useful, like an r/c plane beacon or PongSat micro-transmitting satellite (no, we don't have this kind of experiments here...). Searching through the site I managed to find this page which looks even more interesting: http://sci-toys.com/scitoys/scitoys/computers/radio/computer_controlled_transmitter.html. So, now I started thinking of how to interface the PicAxe with this transmitter module. Actually, it's kinda straight forward. Searching through the web I finally got this page: http://www.mcmanis.com/chuck/robotics/projects/eelb.html

I replicated the setup using a 08M chip. Everything looked OK. I was able to oscillate the chip using PWMout. The actual code used is this:

The radio receiver is able to tune at 650KHz and I can here the tone. The problem is that I need to have the radio receiver attached to the antenna since the radio signal seems to be pretty weak. Does anyone have any suggestions?


Edited by - steliosm on 23/07/2007 23:30:24

Transmitting a few cm is fine, but once you want to go beyond that you need proper tank circuits and transistor driven output stages and long antennas eg 75 metres for a 1/4 wave at 1Mhz. There are lots of AM Tx circuits on the internet - just type in AM Transmitter into google.

I looked at these in quite some detail a few months back, but in the end went for 1W FM modules at either 315 or 430Mhz. At $20 for the Tx and $4 for the Rx it is hard to justify the expense of trying to build a unit from scratch. 75m of aerial wire will cost more than a 315/430Mhz Tx unit.

I know you have ham radio people crowding the 430mhz range, but I wonder if the 315mhz range is clear? It would be a very simple experiment to buy a 315Mhz Rx unit for $4 and connect it to a xtal earpiece and see what is out there.

If you do want to go for AM, at least retune the frequency of the Tx/Rx so you are a bit above or below the AM broadcast band.

This little hack is not related to my previous project. It's just a nice and simple way for an AM transmitter, although it could use more power to extend it's rate.

I've seen quite a few AM transmitter circuits but none of them was using a PicAxe ;-)

It might be worth investigating the type of low power ‘baby monitoring’ transmitters that can be used with commercial FM receivers. There are lots of schematics around that could be modified to take a digital input. In the following example www.zen22142.zen.co.uk/Circuits/rf/2bjttx.htm the mic amplifier stage could be left out and replaced by a 500 Ohm pot between the PICAXE output and ground, with the wiper connected via a capacitor to the base of the rf output transistor. Then the pot adjusted for maximum usable range (about 50m).

The problem with using audio type communication channels is that their ac-coupling circuitry produces dc output levels that are dependant upon the duty cycle of the characters transmitted. A simple solution would be to transmit each character twice but with the second inverted. This doubles the bandwidth required but then so does Manchester encoding and most other modulation techniques.

The output from the radio’s headphone socket will have to be cleaned up by a single comparator ‘data slicer’ circuit before being presented to the receiving PICAXE. ( I really do wish we could post our own schematics on this forum!).

This is the simplest transmitter I've found ( <A href='http://www.sciencebuddies.org/mentoring/project_ideas/Elec_img039.jpg' Target=_Blank>External Web Link</a> ) and I'm sure it could be modified to work with a PICAXE.

You might be surprised at what one of these little fellows can do, in good conditions and if you hold your mouth right.

Years ago, when I was an active ham operator, I worked a fellow on 7 MHz CW one evening. He was about 1000 miles away from me, and was putting out a very readable, if not very strong, signal.

As we chatted, I found that he was using a 7400-series hex inverter as his transmitter. A part of his chip was the crystal oscillator, with the rest being his &quot;power amplifier.&quot;

I was amazed that he was able to key it with no discernible frequency pulling - &quot;chirp.&quot; He promised to send me a sketch of his circuit, but he never did, so I have no idea how he was able to key it so cleanly.

Of course, you can't expect that sort of range at frequencies below 1 MHz, but with suitable antenna coupling, you should be able to have solid communications with friends around your neighborhood, even with nothing more than a chunk of hookup wire as your antenna. Properly coupling to that chunk of wire is the key.

Tom

Hippy this is the setup I'm actually copying to work with a PicAxe.
The same Transmitter can be found in the first link I gave at the first post.
I was also thinking of PWMout on a very FAST transistor that will be connected to a 12V rail and to the antenna. Do you think this will work?

Sorry, hadn't spotted it was the same circuit.

I can't comment on RF as it's not my field. There's a slightly more complicated setup here ( http://ludens.cl/Electron/amtx/amtx.htm ) which might be better. My feelin is that anything which puts more <i>oomph </i> up the wire is better, but I'm sure it's more complicated than that.

Edited by - hippy on 24/07/2007 18:08:35

Never the less I though it was a nice little hack for the PicAxe. Unfortunately I only got a BC548B which is not suitable for high freq switching.

Did someone mention transmitters? I've been a radio ham for decades &amp; recall rustling up my first semiconductor based AM transmitter ~1965,in an era when transistors cost a few hours wages EACH. I must have supervised a zillion FM versions since,although stability at 88-108MHz can be <b>hugely </b> annoying. A further issue relates to today's super crowded FM bands usually swamping your weak signal. Amazingly breadboarded FM Tx versions are possible - see Andrew's latest 08M version =&gt; <A href='http://www.picaxe.orcon.net.nz/fmtx1.jpg' Target=_Blank>External Web Link</a>. He's intending this for &quot;drive by&quot; solar powered WiFi battery status data insights,simply switching to the freq. on his car radio when driving thru' that rural region.

Good old low freq AM is certainly appealing &amp; dead easy to generate with a 1MHz crystal =&gt;<A href='http://scitoys.com/scitoys/scitoys/radio/computer/computer_controlled_transmitter.html ' Target=_Blank>External Web Link</a>,but &quot;bare bones&quot; RC approaches using logic ICs are very stable as well,&amp; very attractive for interfacing to Picaxe generated beeps,tones etc. The HC logic family versions of NANDs &amp; inverters are the general approach for the later (numerous offerings- Google),&amp; being square wave genes. offer rich harmonic output (resonant tunable)even up to the FM band.

Footnote: Most handy radios are now FM only &amp; finding a MW set can be a hassle! A recent straw poll of students revealed not single AM set in the class (although NUMEROUS FM sets on MP3 players,cell phones &amp; even laptops etc),&amp; we had to retire to the carpark to tune on an AM car radio! Marconi would have been amused... Stan (ZL2AJZ)



Edited by - manuka on 25/07/2007 00:00:21

Hippy,
That actually is a very nice little cct. Not the most elegant but considering what the guy had available it is pretty good.

The important thing to consider with low power RF circuitry is correct impedance matching to the antenna and correctly modulating the carrier. The caps across the antenna coil help tune it and also affect the impedance matching. The better the transmitter is matched to the antenna, the more power is coupled and then radiated out from the antenna. It's much more critical in low power transmitters than the higher powered ones.
In the case where these ccts use a TTL can oscillator, which has a square wave output that feeds the antenna, it's preferable to first turn the square wave into a sine wave. That helps control harmonics plus a whole bunch of other stuff mainly related to interference. Low power it won't matter because you can only Tx a few dozen meters or so but any further and you may interfere with other AM transmissions, so the lower the harmonics produced the better.

 

I'm a little confused-- Are you using the Scitoys circuit and driving it with a Picaxe? Or are you using the McManis circuit?

I used the Scitoys circuit (PC software version) and got pretty good range =&gt; +300 ft, with just a 1 meter antenna. I didn't have a 1 MHz xtal lying around, so I used 1.6 MHz --still in the AM band

Seems to me the main difference is the voltage that the transmitter is being driven at-- I used a 9v can to drive mine. I used a transistor interface to a PIcaxe 18X, because its what I took on Holiday. The Mcmanis circuit is driven at under 5v.

You ought to be abe to get good range with a long enough antenna. There have been (illicit) reports of trans-continental contact late at night. It seems to me staying legal is a harder problem.

I got into a minimalist mode while on holiday in june and built a xtal receiver to go with the transmitter and was able to do OK over a city block.


remember the PC software expects 500Hz for LOW and 1000Hz for HIGH. I suppose you could change that and recompile. (info at link Stan gave)

The SOUND command will also modulate the signal just fine

Larry, put some schematics online, I'm curious about your implementation, especially the part with the transistor.
Actually I'm replicating the McManis circuit which is a uC driven Scitoys circuit.
Hippy, Andrew's circuit looks very interesting. Does he have an AM implementation?

You can even make a transmitter using a LM78XX regulator, search the web.

A few years back a faulty regulator in a wallwart/plugpack of a PC Printer took down one of the main television stations in Melbourne by crashing some of the main systems several times. Took them nearly an hour to systematically shut down every single electrical device they had before they found the culprit.

Have you tried a higher carrier frequency and duty ?

I don't pretend to know much about radio--I'm kinda like hippy thinking it's just putting Oomph up the line.

There's not much to add in the schematic department, and ASCII art is better done by others, but here goes---

I used a Picaxe to drive a crystal like the Scitoys circuit. I did NOT use the internal oscillator of the Picaxe like the McManis circuit.

Note that on the first Scitoys link you mentioned, The crystal is driven by a 9v battery and modulated by the transformer. The second Scitoys link shows two connections to the crystal and pins 4 and 5 of the DB9 connector. Pin 5 is ground, pin 4 is DTR. DTR line from the PC drives both the carrier and the tone frequency as it turns on and off.

Connect the DTR connection to a picaxe pin instead and drive it with a pwm or sound command. I don't know much about crystals and what it takes to drive them, or if they vary in that regard, so I just used a 220 ohm resistor to protect the pin. I got my crystal out of the junk heap and the only markings on it was the frequency. The resistor may not be necessary.

That worked OK for me, so then I decided to try the higher voltage method and used a 9v source for the crystal and turn it on and off with TIP 120 transistor (again, what I had with me on holiday) with a 1k resistor to its base.

But you may not even need to do that. if your application is short range, and if you crystal is different, driving off the Picaxe pin may be enough.

One thing to note is that the Mcmanis circuit is doing something different that the Scitoys one.

The Mcmanis circuit sends out the carrier all the time and turns the modulation tone on and off for high and low.

The Scitoys PC circuit turns the carrier AND the modulated tone on and off, and modulates the audible tone on two frequencies, one for high (1000 Hz) and one for low (500 Hz) . you should always be sending out one of the audio frequencies for it to be sending the carrier. Same when you use it with the Picaxe.

Tom2000 does that look like the Transmitter you described?

<A href='http://www.webx.dk/oz2cpu/radios/137kcw.htm' Target=_Blank>External Web Link</a>



Edited by - steliosm on 25/07/2007 21:01:47

Ouch-not a pretty sight! Predictably I'm not a fan of ugly &quot;rats nest&quot; layouts,even at such low freqs as this 137kHz,as fault finding alone can be a NIGHTMARE.

This version is on the long wave band,so I doubt if this was the logic gate tx previously mentioned- propagation is principally ground wave at these &quot;DC band&quot; low freqs of course.

A significant AM band issue relates to being able to readily RECEIVE signals. As mentioned above,classic 550-1600kHz AM MW receivers are now thin on the ground,although readily bought new for ~US$5-10 here in NZ anyway. The MW band is full of impulse style ignition noises too,which usually means weak signals will be swamped if you are near a road. Somewhat in recognition of this most MW sets have very poor sensitivity,although a DIY RF stage or long wire aerial can be added. The antenna doesn't need to be costly copper, as even cheap &amp; strong galvanised steel wire will do. I'm always conscious of such random lengths being innocently strung near power lines however...

It may be a suitable time to more widely consider ALL possible Picaxable wireless technologies (laser,ZigBee,433 MHz, 150MHz, 470MHz UHF CB,IR,FM,MW etc),weighing up costs,range,data speeds,antenna,spectrum spots,band noise/interference,legalities, power &amp; receiver issues. The old 27MHz AM CB band (&amp; a spot ~49 MHz) legally finds use for RC planes &amp; model cars &amp; the 100mW power allows great ranges- thru' many obstructions as well. See OFCOM's UK regs. =&gt;<A href='http://www.ofcom.org.uk/radiocomms/ifi/licensing/classes/rlans/technical/ofw311/' Target=_Blank>External Web Link</a>. This low VHF spectrum is additionally very quiet &amp; antenna easy to rustle up, &amp; mega ranges are possible when ionospheric &quot;skip&quot; obliges! (Radio hams have long had a global network of ~28MHz &quot;10m&quot; beacons specifically alerting to such band openings). Stan

Manuka,

A really good guides to through air optical communications is the following free download handbook&#8230;.

www.imagineeringezine.com/files/air-bk2.html

<i>&quot;Tom2000 does that look like the Transmitter you described?&quot; </i>

Howdy, Steliosm,

Well, as I mentioned, the guy never sent me a sketch. But from his over-the-air description, I think he was using a single hex inverter chip.

&lt;break&gt;

Hey, Stan, since &quot;you showed yours,&quot; I guess I can show you mine:

Very 73 de AB9B!

There was such a unit published in QST many years ago - I once had a 1200 mile CW contact with 15mw total power input and a Windom antenna on 40m... when conditions are right you can go a long way! 73, AJ0J