Single Frequency AM Transmitter

If you’ve tried the oldest radio transmitter, and you’ve demonstrated how sound works, you’re ready to build an AM radio transmitter.

You can build a simple, very low power (that’s critical to keep it legal) AM transmitter for under $5 for every Scout. Battery not included, but some sort of power required. You may need to do a bit of soldering and tinning to prepare parts, or a little more complex wire stripping and crimping.

Only 3 AA or AAA required. Will update  photo when we get new battery case.

To keep it cheap, order the parts from Chinese suppliers. In July 2017 from Canada, you need to allow 8 weeks for delivery. We ordered through AliExpress.com. Be sure to request shipment via China Post, as they hand off to Canada Post for delivery in Canada. Some of the courier companies charge disturbingly high brokerage fees; Canada Post keeps it reasonable, and for low-value packages, sometimes even free.

The design and original idea comes from this page, but we needed to rework a few things.

  • Use a solderless mini breadboard. Learn about them here.
  • Use 22 gauge solid (not stranded) wire, because it clips perfectly into the breadboard and the Arduino board connections
  • Low-cost oscillators now available are 3.3V to 5V. We tested one of these with 9V, and that oscillator has gone on to wherever good oscillators go when they die.
  • For power supply, use 5V and ground pin from an Arduino, or a 3 X AAA battery clip.
A bit of history – first AM broadcast

Check out  and try the simple spark gap transmitter. This is what radio was in the earliest days. The only thing a Morse code radio operator would hear through his headphones is those scratchy clicks. You might want to demonstrate this before starting this project.

Then Canadian Reginald Fessenden invented Amplitude Modulation, a way of adding sound to a single-frequency radio wave.

It’s Chrismas Eve… ships are out at sea, and their lonely Morse code radio operators are listening to the scratchy dots and dashes… then all of a sudden, they hear… a human voice! Then violin music! Read the true story .

How it works

Our transmitter uses two components – and oscillator and an audio transformer.

Oscillator – to create the radio wave
Amplitude modulation. Illustration by Ivan Akira under Creative Commons License.

You can use the picture above, or you can go to this website to visually see and hear it in action. Trust us – it’s worth your time to have a look at it and demo to the youth.

The first wave at the top of the picture is the sound wave we want to transmit. Maybe it’s you talking. Maybe it’s a musical instrument.  It’s a sound wave traveling through air – until it hits a microphone, and that turns it into an electrical wave. Same wave, same frequency, but now its been transferred from the air to a wire. Check out our Teaching about sound waves page to show how this works and to teach about frequency and waves.

The middle wave is an oscillation. That’s a very steady wave at one frequency. For our project, we’re going to use an oscillator that generates a 1 MHz wave. Why 1 MHz? Because it’s in the AM radio range that any AM radio can pick up. 1 MHz = 1000 KHz = 1000 on the AM dial.

AM radio in Canada is allowed in the frequency band ranging from 535 KHz to 1605 KHz. Sound familiar? If not, look at your AM radio dial and check the lowest and highest numbers. Radio station AM 640 is transmiting at 640 KHz (that’s kilohertz, or thousands of cycles per second), and AM1010 is transmitting at 1,010 KHz – or, if you know your math, 1.1 MHz (megahertz, or millions of cycles per second).

Oscillator to generate the carrier signal

In the wave diagram above, the circle with the X and the box marked oscillator are both handled in real life by by a single oscillator.

Bottom of 1 MHz 3.3-5V oscillator. With pins for breadboard connection.

 

This oscillator puts out it’s nice even wave at 1 MHz, and we feed in our audio signal from our MP3 player or portable phone. At the X, it picks up our sound wave, and adds it to the 1 MHz signal.

The third wave at the bottom of the picture shows what happened

  • Notice the separation between the waves – they’re still the same. It’s still a 1 MHz signal.
  • Our signal is changing the height of the waves – that’s the voltage of the wave. If you draw a line across all the peaks , it looks like our original sound wave! That’s the dotted line in the picture.

In fancy radio terminology:

  • The clean 1 MHz wave coming from the oscillator is the carrier, operating at a frequency of 1 MHz
  • The process of adding our signal (the original sound) to the carrier is called modulation.
  • That process changes the amplitude (height or voltage of the waves) to add the signal. That’s why it’s called Amplitude Modulation, or AM radio.

There are many different ways of modulating a signal onto a carrier. AM is just one of them. FM radio (frequency modulation) uses a different method. There are many more. Signals can also be data bits. Some form of modulation is used for every bits and byte going across the Internet. Every different way you get data from the Internet – over the air on a mobile phone, on a black coax cable from a cable modem, on a phone wire over VDSL, over your wifi router – has its own modulation technique that is designed and used to work with whatever wire or device you’re using.

Transformer – to bridge between the audio source and the oscillator
Bottom of 8:1300 ohm audio transformer.

Audio frequencies are low (20 Hz to 20,000 Hz) when compared to our oscillator circuit at 1 MHz. The audio source – or at least the ones we recommend here – will be at a pretty low voltage. In practice, for simple human voice and audio, most of the sound is in the 300-3000 Hz range..

We use an 8 ohm/1300 ohm transformer.  It’s called an audio transformer not because it generates audio, but because it is designed to work at the audio frequency range, and to pass and raise the lower voltage from the audio source to the oscillator side of the transformer.  It’s more complicated than that, but that’s good enough (1).

Our radio circuit diagram

AM transmitter circuit with all our components

Tune to 1000 AM on an AM radio, and you’ll hear static. The radio likes to lock in on a strong signal, but there’s just a mess of tiny insignificant things. That’s the noise you are hearing.

If we simply hook 5V to our oscillator, and then a piece of wire to the antenna pin, we can tune to AM 1000 with the antenna right next to the radio, and we hear… nothing. We have a perfect 1000 MHz wave carrier, but, like the wave diagram above, there’s nothing modulating it. The carrier wave is carrying… nothing!

So we hook up a transformer between the power supply and the oscillator. If we don’t attach an audio input (our microphone) to the other side of the transformer, on the AM radio, we still hear… nothing. The 5 V goes through clean to the oscillator.

When we hook up our audio source and start sending a signal (the top wave in the diagram above), that signal goes through the transformer, and it adds it to the 5V from the power supply. So now, instead of a clean 5V going into the oscillator, it’s constantly changing because of the audio source input being added to the 5V. In turn, our oscillator’s output signal is changing from clean 5V to something varying – the bottom wave in our diagram. It’s still 1 MHz. We’re modulating the 1 MHz carrier’s amplitude – the height or voltage of the waves.

Building our transmitter
Tiny breadboard. Big enough for our project.

Our breadboard is divided by a central gap. There is no connection between the two sides – unless we connect something. That gap is perfectly designed so that components like our transformer, oscillator and other ICs can sit across it.

On the top and bottom side, there are vertical rows of 5 holes. All 5 holes of each  vertical row are connected to each other.

We’ll use these strips for power and ground.

Larger breadboards have long sections with red and white stripes that are traditionally used for power (+ from battery) and ground (- from battery). We’ll use the two right-most strips for this purpose. BUT DON’T HOOK UP BATTERY OR POWER YET. Always get everything assembled and checked first.

To make it clear, we recommend marking the rows with a read and black marker – as you can see in the full radio diagram at the top of the page.

Anything that needs +5V can tap into any one of the 5 holes in the power bus, and anything that needs ground can tap into one of the 5 holes in the ground bus.

 

Oscillator and transformer. They must span the gap, otherwise the vertically-aligned pins are shorted to each other.

Let’s add the first components to show how it looks.

You can’t just place them any old way. Notice how:

  • The lower-left corner of the transformer has a dot molded in the plastic
  • The lower-left corner of the oscillator has a sharp corner. The other corners are rounded. And the lettering is properly oriented.

On the right side, we’ve hooked up the connector for a 3×1.5  volt AAA batteries. We could instead connect the +5V and GND connections from an Arduino board. As always, when building the circuit, nothing should be powered. No battery connected. No power to the Arduino board.

Note: the battery clips usually have stranded wire, and only enough stripped wire at the end for soldering. For connection to the breadboard, the ends will need to be stripped a little more and either tinned or crimped with a pin.

Looking at the battery connections, the bottom row with the black cable is the 0 (zero) volts or ground. Anything that plugs into any one of the 4 holes below the black battery connector will be connected to ground. The fancy name for that row is our ground bus.  The row above and including the red wire is +4.5V (or +5V for Arduino), and anything connected to one of those 4 holes will be at +4.5V relative to the O volts/ground below. That’s our power bus.

Next we need to connect:

  • The oscillator ground pin to 0V/ground
  • The oscillator side of the transformer to 4.5V
  • The other contact of the oscillator side of the transformer to the power pin of the oscillator

At this point, we have nice, smooth 4.5V power going to the oscillator. The transformer has no effect. The oscillator will put out a nice steady 1 MHz signal – but to what? It’s going nowhere.

Only 3 AA or AAA required. Will update when we get new battery case.

So we add an antenna as an oscillator output. What’s the antenna? A piece of wire 8 inches long will work fine. You only need to strip the bit of the wire that plugs into the breadboard, and it goes into the vertical row that includes the output pin of the oscillator.

That is a significant difference between DC and AC. DC going to a wire that goes nowhere pretty much does nothing. AC tends to radiate the power out – and that’s where our radio signal comes from.

Let’s see how it’s working so far – with no input

Tune the AM radio to the frequency of our oscillator – 1000 AM. You should hear some hiss.

  • NOW plug in the power – your battery clip, or connect your Arduino board to the USB
  • Place the antenna wire right next to the AM radio
  • The radio should go dead quiet.

What’s happening?

  • Our 4.5V power is going through the transformer with nothing happening to it. Goes in and comes out 4.5V since there’s nothing connected to the other side.
  • That 4.5Vgoes in nice and steady to the oscillator, so the oscillator puts out a nice, clean 1 MHz sine wave. There’s no sound, because they’re nothing modulating the wave.
Add an audio input

Now let’s hook an audio source to the other side of the transformer. Something with a bit of power, because in this simple circuit, just a microphone won’t have enough power.

So we add a pig tail with a 3.5 mm audio plug on the end to the other side of the transformer, and plug it into your favourite music player. Turn your music player on, crank up the volume… and you should hear it playing on the radio!

What happened?

The voltage from the audio wave is going through the transformer, and messing with that nice, clean 4.5V from the battery. As the voltage now goes up and down, it’s being fed into the oscillator… so now the amplitude of the wave generated by the oscillator is also going up and down. It’s modulated! Amplitude modulated!

If it doesn’t work:

  • Check that the wires are indeed connected to the correct vertical rows to reach the pins. The transformer pins are tricky – they can be pretty far from the edge.
  • Be sure your oscillator is oriented properly. One of the corners is pointed, the others are rounded. The pointed corner should be on the lower, left side. Nothing connected to that pin.
  • Try turning the transformer 180 degrees. Different audio players and headphones can have significantly different audio output levels and impedances. Turning the transformer around could result in a better match.

Building the pig tail

There are lots of options for this that provide opportunities for the youth to learn new skills.

What they all have in common is a 3.5 mm audio plug. That’s the size of plug that fits into your portable phones, MP3 players, laptops, etc.

Old set of headphones

In the illustration, we used an old set of airline headphones. Airlines usually allow you to keep them at the end of the flight. The quality is awful, so they deserve to be sacrificed. The set we used is dual-plug, so the youth had to figure out which wires went to which plug. Whether dual or single plug, they are always stereo, so you use the wire from one side of the headset.

  • You need to learn to use a voltmeter, and especially the continuity test.
  • Figure out which wire is common to both sides (left and right ear) of the headset. That will be the ground wire.
  • Which part of the plug does the ground wire connect to?
  • Which part of the plug does the signal wire (left or right) connect to?

The youth will need to use wire strippers to carefully expose the wires. The wires are small and delicate standed wire, and can’t be plugged into the breadboard. Options include:

  • Crimping on a metal tip
  • Tinning with a soldering iron
  • Using a pin wire and alligator clip from the board.
3.5mm plug extension wire

You can buy extension cords with a 3.5 mm plug at each end from dollar stores. Cut it down the middle, and each end can be given to a Scout or pair of Scouts to strip the wires and connect.

Buy a plug. Solder. Cheapest and fastest for if you need to build a bunch
Since we took their picture, click for link to the store and picture source.

If you are going to do this for a large group and/or and you’re time-constrained, buy the plugs and solder them directly to 22 AWG solid (not stranded) wire that will plug directly into the breadboard.

Challenge question

The 3.5 mm plugs on most headphones have a tip, and usually two or three metal rings.

  • Why the different number of rings?
  • Which one is the ground? Without looking it up, how can you test for it?

Bigger challenge: some have 1 or 4 rings?

Potential enhancements:

Scouts/Venturers

Use a circuit board and learn to solder the components to the board.

Venturers/Rovers

Challenge 1: Adding a transistor for input amplification. The design requires input stronger than just a microphone – that’s why it uses an iPod/MP3 player as input. We’re working on adding a transistor so we can use a simple crystal or other microphone.  We’ll document it here when we get it right.

Challenge 2: Adding a transistor for output amplification. Get more power and distance from your unit. There’s a critical difference in the transistor/amplifier required, as Challenge 1 works at audio frequencies and Challenge 2 at radio frequencies.

STEM STUFF

In this lab, we’ve introduced a lot of new things

Electrical circuit diagrams

  • Electrical circuit diagrams. We identified all these components
    • Power supply
    • Ground
    • Transformer
    • Microphone
    • Antenna

We didn’t use a formal oscillator component in the diagram because there are a lot of different ways of doing it, and we want to show the exact connection to our real-life oscillator.

We used a general power supply diagram. Formally, a battery looks like this

Author placeholders:

  • will a piezo work for mike?
  • When we add a mike, ould we add a switch and option to make it a pre-amp with direct audio output?

 

(1) It’s really an impedance matching issue. You can read up on impedance of simple microphones and iphone/ipod/smartphone/mp3 player impedances. When you figure it all out, send us a detailed, comprehensive explanation and we’ll use it here.

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