555 based AM radio transmitter

Bust out that 555 timer and use it to build your own AM radio transmitter. The circuit that [Rtty21] is using only needs the timer chip, an NPN transistor, three caps, three resistors, and a potentiometer. It generates an amplitude modulation signal around the 600 kHz range which you will be able to pick up with any normal AM radio. From the comments on the article it seems you’ll get around 30-40 feet of range out of the device. We don’t see this as a competitor for the FM spy microphone, but maybe you can use it as a diy baby monitor.

32 thoughts on “555 based AM radio transmitter

  1. Seems like something my airsoft group would enjoy. When ever we can build something rather than flat out buy it, we do. Having a short distance transmission, paired with a radio to pick it up would be a fun little way for us to relay orders from our command posts. Secure? No. Fun? Yes :D

  2. this is hardly a hack, it’s been done before a lot of times. and the result is hardly AM modulation. it’s a mix of AM and FM on a square wave carrier which makes it likely to disturb nearby equipment.

    not to mention it’ll drift all over the place. AM transmission can be much simpler than that. stick to the tried and true method of using a crystal and modulating the power supply. sure, you only have one frequency, but crystals are cheap.

  3. @Th3_uN1Qu3
    I second your comment. This “design” is so wrong in so many aspects, that just using it because it “does someting” (?!?) is very wrong.

    -based on stated component values, it actually oscilates at arround 60kHz. What they hear on the receiver side are harmonics.
    The “good” thing is that it’s easy to find it on AM radi scale (as it will show up every 60kHz, the bad thing is that it’s designed to mess up with everything nearby that requires reception.
    I wouldn’t be surprised if some of (very high) harmonics messes up a nearby wireless (phone, ethernet, …).

    -The transmiter side with transistor that has only collector and base connected, is probably the bast part of this design (sic) – it does not work at all so it limits all the negative aspects mentioned above :)
    If you relay want to messup with this “modulator/transmiter), add resistor leading to the base (1k will do it), add resistor in the collector circuit (depending on transistor, something in 100 Ohm range would do it), connect emoter to the GND (negative suply), and then connect antena (long wire, or wire or coil of any kind) to collector. I bet that will extend the range and mess up with more devices :)
    -another “improvement would be to raise the frequency so you end up using lower harmonic that carries more energy. The frequency is controlled by R1, P1, R3, and C1 (see http://www.royalrife.com/555_calculator.html), and just changing R3 to 1k, or C1 to 100pF, you’ll get to AM band and use the first harmonic (the one that has highest “energy”

    But the best advice would be to google a bit and find a REAL, simple, equally cheap, AM transmiter.

    Radio amaters usually refer to them as QRP, might help in your search.

  4. well this has been around seance the chip was released but nice!

    i suggest running the 555 at 12 volts at such a high frequency i find it to be more stable

    um but you know pin 5 in the modulation pin right?
    just got to make the 555 pulse at the frequency of the radio than send the audio in to pin 5 …. so much simpler if you ask me XD

  5. I agree with Th3_uN1Qu3 and Miroslav. While this is a pretty awful transmitter, it’s probably a great example for teaching fourier transforms to high school students.

  6. @AJR
    Never thought of this that way (“great example for teaching fourier transforms to high school students”), but yes, that would be a clasic one :)

    Now we just need someone to get the FFT gizmo from couple of days ago (http://hackaday.com/2011/01/08/pic-spectrum-analyzer-uses-fast-fourier-transform-routine/) to cover up to 1MHz, and we are talking :)

    To be honest, this approach (square wave modulation) is not that strange or wrong – RF (and audio) power amplifiers in class D (even in class C) actually do the same, but they have tuned filters that allow only wanted harmonic (usually the first) to get out.

  7. @Daid
    not to bad but it looks like your going to get a letter from instructables lawyers soon. The great part about the idea of hackhut.com is that you don’t have to compromise or steal. It’s free to use and best of all READERS DON’T HAVE TO SIGN THE F*** UP JUST TO GET FULL ACCESS TO PICS/CONTENT! and its not jammed with ads of course.

  8. @Paul: Don’t worry:
    -Instructables is in the US. I’m not.
    -The copyright for the articles is with the writes not with instructables. It says so in their terms. So the writers of each instructable has to contact me. And if they do I will take down that instructable from my site. (And block it ofcourse)

    I don’t mind the ads. But the 100 pages for a few pictures with 1 line of instructions per page pisses me off. I tried to keep the ads in actually, but google ads uses so much javascript that I gave up.

    And I’m surprised at the large amount of instructables that are viewed trough my page. And the large amount of brain dead instructables…

    Anyhow. AM transmitter, nice toy. But how about something home build for data transmission instead of audio? 2 microcontrollers with transmitters and recievers for much less the normal radio modules?

  9. @Daid – love you man! x

    @Paul – oh Paul, at least create a few fake accounts and populate with some content, no one wants to be the first penguin.

    @Miroslav & others – thank you for not only pointing out this was not the best approach to something like this, but also providing some detail and google hooks to get an idiot like me on the right path.

    @hackaday – you guys are starting to bore me. If daid keeps his service up and running I’m probably cutting out the middle man (thats you).

  10. I don’t know if I should be angry at Instructables for offering yet another extremely poor design or to HAD for linking to it.
    Seriously, Instructables is a joke. People should refrain from using that site for learning anything. The occasional interesting article there will be (or already has been) published elsewhere anyway.

  11. @tehgringe
    I really didn’t want to do that. fake content sucks. I think hackhut could be useful for real hackers and that someone will be happy to be first. Why don’t you go ahead and do it?

  12. Interesting use of a 555 but I doubt it produces a very clean signal that won’t drift or produce harmonics. I mean, all you need really is a variable cap, a coil (wind it yourself), and a couple of transistors (one for the oscillator and one for the modulator). That would give you something at least tuneable.

  13. Not a good way at all to learn about radio transmitters and what is used is way more than needed.

    All you need for a AM transmitter and a better one than the article is
    1 – transistor (NPN) any 2n variety will work.
    1 – 22k resistor, 1- capacitor .1uf, 9v battery,
    some wire

    It requires some coil making but the end product is superior and can be used for data transmission if you want. Pick up the Forest Mims books from radio shack. Some new comers to the field might be amazed at what you can do with just wire, transistors, and capacitors. If you don’t know who Forest Mims is then learn as he did a lot to make electronics a popular hobby.


  14. @cgmark
    How about scavenging the oscillator coil from a discarded Medium Wave radio to avoid winding one? It comes in the form of a shielded can with a (usually) red tuning screw and has a value of ~107 uH. When used as the frequency determining element in the MW radio, the frequency stability is good enough for the drift to be imperceptible within the +/- 5 kHz IF bandwidth. The frequency can be tuned within a couple of 100 kHz by the internal tuning screw or even wider with a parallel variable cap.

    Direct modulation of the oscillator supply will change the oscillation frequency (hence deviating from pure AM), whether quartz crystal or LC controlled. It is called frequency pushing in RF parlance. Better AM can be had by modulating the RF amplifier following the oscillator.

  15. This is one directly out of the old Radio Shack electronics notebooks, you know that small ones they used to sell.
    I still have a bunch of them.

    Only noteworthy for the fact that nobody at HAD picked up on that.

  16. how nice, censoring comments pointing on faulty circuit and unsuitability of method even for a toy to begin with. only adds to inability of had to recognize it before posting ?

  17. Those who are critical of this hack miss the whole point. This is not about advancing the science of radio communication. I don’t think anyone in their right mind would use this very weak transmitter for any real world applications. It’s one of those disposable projects. It’s a cute DIY project and not much more. Nor will it be taught in universities as an example of sound RF circuit design.
    The person behind the hack is a 15-years-old trying to have some fun who took the time to re purpose the 555 for an interesting application and in the process it motivated so many to build their first RF transmitter ever. That’s the real story.
    The beauty of this hack is also in its ease of replication. This has its appeal for the entry level hacker. If this design works in getting more people to roll their sleeves and do something and hopefully get it to work then that’s wonderful. For many people, the first step is the most difficult one. If such a simple project can get people motivated, and if it builds their self-confidence, then it’s a great project.
    And that’s enough to get it honorable mention.
    You should see the numberof views on the kid’s Instructable page. It’s massive. And the number of folks who tried it is impressive too. I am sure they will quickly realize that it’s a cute project but not very useful from a practical point of view. They will then look for more complex projects and would want to understand the nuts and bolts of AM/FM transmission and so on.
    I built it in a couple of hours but I ended up spending a couple of days after that just reading comments and suggestions of how to make a better AM/FM transmitter. So this 555 transmitter is no masterpiece of science but it’s a masterpiece for easing newbies such as myself into reading and learning more about radio communication.
    So the kid who published this Instructable deserves the proverbial pat on the back for being a great motivator.

    1. Hi TB. This is Rtty21 (the 15 year old kid 3-4 years later). I like what you wrote. You see exactly what I see in the project. It was a starting point. It wasn’t meant to be useful, it was meant to spark some curiosity.


  18. TB is precisely correct in noting that the original post of this simple transmitter has contributed to thousands of curious minds! The seeds of invention come from a starting point somewhere!
    Regarding all the negative posts, I couldn’t help but notice that most of them were written with little regard to spelling or grammar. It led me to ponder how to write a “hack” that inspires interest in learning to write…
    *How to build a speak-and-spell from an old discarded toaster and duck-tape*

  19. This is Rtty21 (Ryan Jensen, the author of the instructable)
    A lot of people are said that my 555 transmitter design is bad. They are completely right. But they are missing the real point of the project. It is for entry level DIY elecctronics hobbyists. I wanted to show some people that it was pretty easy to get something that produced results (albeit very sketchy and poor results).

    I thought it was cool that I could transmit some kind of radio over the airwaves, regardless of how horrible the signal quality was. And I hope I got some other people to experiment with it, have some fun, and maybe learn something in doing so.

    Thanks for all of the comments. I found most all of them very insightful.

    Special Thanks to TB who sees the value in sparking curiosity in the minds of others.

    Jensen (Rtty21)

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