A Simple Science Fair AM Transmitter

A crystal radio is a common enough science fair project, but the problem is, there isn’t much on anymore. The answer is, of course, obvious: build your own AM transmitter, too. AM modulation isn’t that hard to do and [Science Buddies] has plans for how to build one with a canned oscillator and an audio transformer.

We don’t imagine the quality of this would be so good, but for a kid’s science project it might be worth a shot. Maybe something like “What kind of materials block radio waves?” would be a good project statement.

The site is aimed more at kids and has a good list of materials (mostly from Jameco). They do credit a site that has a more detailed explanation of how the thing works, though. Both sites have a lot of other projects that would be suitable for a young hacker’s science project.

If you look at the wiring, you’ll see that the transformer actually influences the power to the oscillator. This is a crude way of getting amplitude modulation, but it works. A better way is to let the oscillator run and modulate an amplifying stage as the signal goes to the antenna. However, this transmitter is low power and low fidelity, so the simple set up is more than adequate.

This isn’t the first time we’ve looked at AM transmitters. Any of these would make a nice pairing with a simple crystal AM receiver.

18 thoughts on “A Simple Science Fair AM Transmitter

  1. Hmm. Let’s seem them reduce it to a tapped coil of wire and a transistor and two capacitors and two resistors. And of course a single cell. That idea comes from a book by R. Stuart Mackay. It was written up in the March 1968 issue of Scientific American.

      1. I built a ZN414 radio with a single transistor audio amp, and listened to the radio during school. Put my hand against my ear so the teacher couldn’t see the earpiece. Even during tests/exams. The idea came to be of putting a transmitter outside with a tape-recording of study material from the books, so I could cheat during tests, but never went that far.

  2. In early 1971 I built an MCW transmitter for the AM band. It was in an Elementary Electronics magazine that collected projects. It was about three montys after I found the hobby electronic magazines at tge newsstand.

    It caused me to find an electronic store out of the Yelliw Pages ( it turned out a good choice, I later learned there was a cluster of such stores in the area). I copied out the parts list and got a ride to the store.

    It wasn’t complicated but none of this new age simplicity. An audio oscillator that was keyed, feeding an RF oscillator. So any AM broadcast receiver couod receive it, no BFO needed.

  3. As long as the oscillator module remains stable with the varying voltage the quality should be quiet good. Just good raw wideband AM.
    This is not too far removed from the way 27Mhz CB’s used to generate AM. the audio transformer was in just in series with the power to the final output amplifier transistor.

    It is not uncommon for some amateur radio operators to use just an oscillator module and a Morse key as a transmitter and see how far they can get on just about no power.

  4. I used to build low power foxhunt transmitters using a canned oscillator, 9v battery and a wire antenna. Fun for kids.

    Put a small yagi antenna on a receiver, the kids can hear a signal when the antenna is pointed at the transmitter.

    1. In the markets I have lived in, the AM broadcast band no longer has anything that would interest the typical kid. There isn’t a lot of AM on the ham bands anymore, although some. There are only a few shortwave broadcasters left, but there are some. Also a lot of odd content there, though. Aircraft, yes, but your crystal set for 100MHz is going to be tougher to build.

      1. At night the AM broadcast band is still full. You just can’t hear them in the day due to crappy propagation and noise. There use to be far more local stations that broadcast in the day and shutdown at night to make way for long range stations.

        Last year at a ham fest I seen a guy demonstrating a neat little antenna that could pickup distant AM stations in the middle of the day. It was exceptionally directional and required tuning as you went up and down the band. It was a bit surreal.

        Broadcast radio in general has nothing that would interest a kid or even most adults. It’s mostly freaking religious stations left on shortwave. Once in a while BBC or some other world news broadcast pops up on shortwave. Even on FM it’s hard to find a decent music station. They have all gone adult contemporary or country.

  5. Yikes! That poor oscillator.
    At the least, maybe a biased shunt diode (to ground) across the output of the oscillator (with modulation applied through an audio capacitor to the shunt bias) and leave a constant power supply to the oscillator module?

  6. Years ago I remember someone used to sell AM radio transmitter “kits” on eBay that were exactly this, one of these oscillator cans, a transformer and a sheet of instructions in a plastic baggie.

    They also sold a CW transmitter, the same thing without the transformer.

    It’s a time-tested idea but I do wish the article at least mentioned the need for filtering, the fact that this is actually a square wave and that means it is also transmitting on all the odd harmonics at the same time. Your 1MHz transmitter is also a 3, 5, 7MHz… etc.. transmitter. It’s probably too short range for anyone to notice but you can never be sure.

    Even a simple 2-component RC filter would be better than nothing at these power levels.

  7. The project is certainly a quick way to have a basic working AM transmitter. My only small quibble with it is from an educational standpoint – the beginner doesn’t learn that much about oscillators and RF from a silver box with 4 leads. This is unlike the humble crystal radio, which in it’s simplicity, still demonstrates the concepts of antennas, a tuned circuit, demodulation and converting an electrical signal to sound.

  8. Realtors still us these things to transmit information about a house being visited. You can get one on EBAY for around $40 USD, but click on my name and youll will be brought to TALKING HOUSE website and can buy one new for $95 USD. Its a PART 15 AM broadcast transmitter that has an endless loop cassette player attached to it. You can use them at conventions to broadcast repeatable information on the brooadcast band. People would have to bring a portable AM radoi to hear it. Or dial into it in the parking garage. You just need to post the frequency on signage around the convention hall.

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