[Moved] Help with AM radio (or morse) transmitter

[kodiak]

Hello. I'm very inexperient with eletronics, but as a class project I've been working on a low power and low range AM transmitter. Unfortunately, I could not finish this project because I could not find the necessary material in the country I live (even in the internet). I could basically modulate an audio signal using a signal gerator as a carrier, but, my oscilator module can't achieve more than 400kHz. I've already tried to use a Wien's Bridge, and other Opamp based circuits, but they all have similar limits. I'm using a TL072CP, and I see that people can achieve the desired results using an TL072BC, but the second one is not sold here. I've discussed this with my professor, and he told me to do a Morse transmitter and receiver in the frequency I can achieve. But I'm afraid there won't be enough time to do it from the begining (I can't make the transmitter by my own, i would not be able to finish both of them in one week). So I'm in a position in which I really need your help.

I can still make a normal AM radio frequencies, but I wont be able to amplify nothing but the Audio signal with my opamps. What would you suggest me to use as a guaranteed functional oscilator? (I couldn't find 1MHz crystal oscilators here). I'm a little bit afraid of working with indutors (because of the need of coiling the wires and it's lack of precision), and that's why I was searching for opamp based oscilatiors.

Or do you think I should make the morse transmitter and receiver?

This were some of the circuits I've been testing:
(modulator)

(wien's bridge)
And this was my most recent hope (the oscilator I saw in an internet video)
The project is not meant to be an comercially acceptable transmitter (or/and receiver), it just have to transmit anything that can be understood as a signal transmitted in very short ranges (some centimeters are enough).
So thank you guys, you have no idea how worried I am because of this project, I really need your opinion and suggestions.

 

[BradtheRad]

To broadcast morse code, you just switch your transmitter on and off.

The idea is to adjust your frequency so it is close to the frequency of a local AM station. If you are lucky, the frequencies will combine to create an audible beat frequency. You would hear the morse beeps coming from your AM radio.

As for how high a frequency you can achieve... Suppose you can only manage to get 600 kHz from your transmitter. Are there any AM stations in that part of the band? Then you're in luck.

Continue to experiment with sinewave oscillators. 600 kHz is within the capability of cmos IC's (4000 series). These are easy to use.

Next thing, you'll try attaching one end of a wire to different points in the circuit, looking for the position which creates a satisfactory antenna.

 

[G4BCH]

Do a Google search for regenerative receiver and look at the images there are a lot of ideas there for very simple receivers that can be made with only a few components that you will be able to adapt to what ever you have readily available.
The regenerative receiver can be made to oscillate. The idea is that for AM receprion you operate it just before it oscilltes to get maximum gain. If you let it oscillate it becomes your transmitter, it can also receive more code transmissions.
If you build two circuits, one without the audio amplifier, then you will have a transmitter and receiver that are compatible, or if you build tow identical circuits you will have a pair of trancsivers that can comunicate with each other. YOu woill need to add a means of starting and stopping the osciullation to make your morse code, buthtat is just a switch in series with a suitable resistance.
Don't be worried about winding coils there is plenty of guidance available for that, and your circuits will be more stable than if ou use RC time constants. If you want to continue with the Wein bridge idea then one of the circuits in that came up in the search might be of interest to you.
https://eesite.bitbucket.org/html/electronics/wien_regen/wien_regen.html
For your transmitter you could use a logic gate oscillator, not ver stable butit works and is easy to build. I have seen many yerars ago, but cant' find now a complete transceiver made from a 7400 IC.
For orther ideas serch for simple cw transmitter or transceiver, you may wnat to add the term QRP to that search ( it is an abreviation used to mean low power), again there are a lot of ideas ot there that use only a few components and can be put together in a very short time. Most use crystals, if you can find two that are on the same frequency than use one in the receiver and one in the transmitter, the two will then work together.

 

[betwixt]

... and do not be afraid of inductors! Everything you ask for can be done with a single transistor if you use an inductor in the oscillator. The construction is only critical if you need to make an exact inductance value, in your case if you use a domestic AM radio receiver you can tune it to your transmitter over a wide range so even if you are a few turns out on the winding or the diameter isn't exact, it makes no difference.

Brian.

 

[vfone]

I found that even some RF engineers are afraid building inductors. Is something like doing the scariest thing in their career. And I am not joking.
Years ago when I was younger, the highest number of inductors that I ever built (wound) for a project were about 200 in just one day.

For about $25 you can buy a pretty accurate inductance meter that gives you confidence making inductors.

**broken link removed**