We revisit the telegraph to make one that can transmit wirelessly to a radio!
We're going back to the telegraph this month, with a completely different purpose. This time, we're going to learn about inductors, resonance, and radio transmission. The circuit is nearly the same as last month's, with some tweaks for sound, and then some add-ons to make the radio transmitter.
In the last instalment, we mentioned the use of the radio telegraph as a method of communication where wires were impractical, such as shipping and aircraft. 'Wireless' transmitters on ships came first, before aircraft were invented. The early technology was too big and heavy for aircraft anyway, even if they were around. The old tech was based on valves, which were around long before transistors and while electrically similar, were in physical and practical terms very different.
Technology evolved, got lighter and smaller, and made it to aircraft so that by the time of World War Two, even single-seat fighter planes generally had a radio. These could transmit voice, however. Even so, telegraphy remained an important way to communicate, and wireless transmitters with Morse code were still an official standard for maritime (shipping) operations until 1992, although voice over radio had long been in use alongside.
The transmitter we are building this month generates signals over the Amplitude Modulation band (AM), and can be listened to on any household radio. These themselves are becoming a thing of the past, with many people now just using apps to stream radio stations via the internet rather than having a dedicated radio. As an aside, commercial radio still holds quite a large market share over streaming services such as Spotify, and against podcasts, despite predictions, it would die out. If your house does not own one, most electronics retailers have fairly cheap ones available.
They're actually still a valuable thing to have, because in widespread power failures such as following storms or floods, internet and mobile phone reception is often lost, and radio stations on both FM and AM are part of the emergency information plans of most governments. So, a pocket radio is handy to have in a place like Australia!
SOME HELPFUL HINTS
We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on. You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.
As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.
We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out. You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.
|1 x Solderless Breadboard
|1 x Packet Breadboard Wire Links
|2 x Plug-to-Plug Jumper Wires*
|2 x Plug-to-Socket Jumper Wires*
|1 x 10Ω Resistor*
|1 x 1kΩ Resistor*
|1 x 4.7kΩ Resistor*
|1 x 150kΩ Resistor*
|1 x 470kΩ Resistor*
|1 x 47nF Capactitor*
|1 x 100nF Capacitor*
|1 x 470µF Capacitor*
|1 x BC547 Transistor
|1 x NE555 Timer IC
|1 x Tuning Capacitor 60pF - 160pF
|1 x Aerial Ferrite Rod with Coil
|1 x 4xAA Battery Holder
|4 x AA Batteries
* Quantity used, item may only be available in packs.
The build is very similar to last month's, except for a few details that make a massive difference to how the telegraph is used. We're adding a ferrite antenna this time, and it has fairly fine wires attached to its coils. These don't always go into a breadboard well, so we recommend wrapping them around the offcut of a component leg, and using small pliers to insert this into the board.
We are reusing the telegraph key from last month's build as-is, so if you didn't make that project, you can access the article online for free and follow steps 9 to 14, skipping step 10. We'll also refer to some of last month's 'How It Works' section, rather than repeating it.
Place the breadboard in front of you with the outer red (+) rail furthest away from you, and the outer blue (-) rail closest to you. Add two wire links, one to join the two blue (-) rails and one to join the two red (+) rails
Install an NE555 timer IC with its notch or pin 1 dot facing left. Add the three wire links which connect pin 2 with pin 6, and one more from pin 6 off to the right.
Insert three wire links: One from the upper red (+) rail to pin 8 of the NE555; one from pin 4 to the lower red (+) rail, and one from pin 1 to the lower blue (-) rail.
Place a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) from the upper red (+) rail to pin 7 of the NE555, and a 150kΩ resistor (BROWN BLACK BLACK RED SPACE BROWN) between pin 7 and the other end of the wire link from pin 6.
Insert a 100nF capacitor (marked 104 or 0.1) between the upper blue (-) rail and pin 5 of the IC, then a 4.7nF capacitor (marked 472 or 4n7) between pin 2 and the lower blue (-) rail. We used MKT capacitors but ceramic or greencap types will work.
Install a 10Ω resistor (BROWN BLACK BLACK GOLD SPACE BROWN) between pin 3 of the IC, and a spot off to the right. Add a 47nF capacitor (marked 473 or 47n) from one end of this resistor, off to the right. Also, place a 100µF electrolytic capacitor into the upper power rails with its negative (striped) leg in the upper blue (-) rail and its other leg in the upper red (+) rail.
Place two 4.7kΩ resistors (YELLOW VIOLET BLACK BROWN SPACE BROWN) and a wire link near the 47nF capacitor. One resistor goes between the capacitor leg and the lower blue (-) rail, the other resistor goes next to it from the lower red (+) rail, and the wire link joins this with the other resistor and capacitor leg.
Insert a BC547 NPN transistor with its flat face away from you. Add a 470Ω resistor (YELLOW VIOLET BLACK BLACK SPACE BROWN) from the lower blue (-) rail to a row next to the transistor.
Cut a plug-to-plug jumper wire in half and bare the ends. Wrap one wire around one outer leg of the variable capacitor, and tape it securely. Wrap the other wire around the middle leg and tape that too. As usual, we left ours uncovered for you to see. Plug one wire into the lower red (+) rail and the other into the right-hand leg (collector) of the transistor.
Use cut-off resistor legs to make the ends of the aerial wires into pins. Look carefully and you will see that three of the four wires have dye on them so that one is red, one black, one green, and one plain copper. The dye may not be near the ends of the wire, and may be faint.
Insert the red wire into the row with the 470Ω resistor, and the green wire into the row with the left-hand (emitter) leg of the transistor. The black wire goes into the right-hand (collector) leg of the transistor with the variable capacitor, and the plain copper wire goes to the lower red (+) rail. Our copper wire had a bit of green splashed on it which might look confusing in the photo.
Insert the wires from the Morse Key. One goes to the right-hand leg of the 47nF capacitor, and the other goes to the middle (base) leg of the transistor. Also, plug the red wire of a 4xAA battery pack into the upper red (+) rail and the black wire into the lower blue (-) rail. Install four AA batteries.
TESTING AND USING IT
Tune an AM radio to an unused band around 963kHz. Different stations in different areas transmit on different frequencies, so find an empty space around this number. You should hear only static. Hold down the Morse key, and turn the dial on the variable capacitor until you hear the tone from the generator circuit. Keep turning the dial back and forth until it is the loudest.
To use the Morse transmitter, you can place the radio in a remote location, and use the transmitter to message anyone near the radio. The best option is to have two transmitters and two radios (or more) and be able to talk both ways. Have a read of last month's article for some of the things to think about in designing a communication system. For radio in particular, you might have all transmitters on the same frequency so that every radio hears all stations, and use a unique code to identify which station is transmitting. Or, you could have each transmitter on its own frequency, but then if a receiving radio is on a different frequency, the message will be missed. In some situations, there are ways to manage this, but we'll leave that up to you.
HOW IT WORKS
Radio is a topic that can be hard to simplify. Often, simplification either leaves out details, or is just not completely correct. Apologies in advance, as this simplification will probably do both.
The basic idea of a radio transmitter depends on the fact any electric current flowing through a wire creates a magnetic field around it. If the current varies, so does the magnetic field. This magnetic field travels outward, and although it loses strength the further it goes, it can still travel quite some distance. If a changing magnetic field passes through a wire, then an electric current is created in the wire. That is all the antenna on a radio is: On the transmitter side, it's a piece of wire that has a current passed through it to make a magnetic field. On the receiver side, it's a piece of wire that picks up this magnetic field and has a current at the end of the wire.
There is a bit more to radio than how the signal moves from point to point. The current has to change in order to create a changing magnetic field. If the current is constant, then the magnetic field is constant, and no current is caused in the other conductor. To make the magnetic field, we have to make the current move backwards and forwards, or at least rise and fall. We do this with an oscillator, which is the heart of the circuit. It is also the main learning point of this build, with something called a 'tank circuit', or 'LC' circuit. These are types of 'oscillators', to oscillate meaning to change between two states (backwards and forwards, off and on, and so on) repeatedly.
An inductor is a coil of wire. It may have an 'air core', which means there is nothing in the middle (plastic does not count because it is not electrically or magnetically active), or it may have a core made of some sort of iron-based metal. Most often, that is ferrite, which is a powdered form of iron oxide pressed together and heated until it becomes solid. This ferrite is magnetically active, and so the magnetic field from the current passing through the coil around the metal causes a current to flow in the ferrite, because it behaves the same as a wire in the magnetic field.
This current also has its own magnetic field, and it also 'induces', or creates, a current in the wire that started the whole process in the first place: The coil. However, this new current is opposite the one that created it, so it pushes back. This component is called an 'inductor'. The same thing can happen without a ferrite core. In an air-cored inductor, the magnetic field from one turn of the coil creates a current in the coils next to it. In the ferrite cored inductor, that still happens too, so it's a double effect.
So, what happens when a current passes through an inductor? When the current first starts to flow, the only resistance is from the copper itself. However, the magnetic interactions described above start to happen. The magnetic field builds, which causes the opposing current to build, which slows down the incoming current. Eventually, it stops flowing. Now, there is a magnetic field stored in the inductor.
With no current flowing, there is nothing to hold the magnetic field where it is, so it collapses. As it goes, that creates a change in the magnetic field, which causes a current to flow in the coil again. All of the current and magnetic field directions are reversed this time, though. The same thing happens as this new direction of magnetic and current flow both stabilise.
However, an inductor is not an oscillator on its own. The current needs somewhere to go when the direction changes, among other things. The simplest way to make an oscillator is to pair the inductor with a capacitor. Inductors are represented by the letter 'L' in circuit speak (I is already taken for current, even though we measure it in Amps), and capacitors by the letter 'C'. This leads to the name 'LC circuit', and ours is made by L1 and C5. It is also known as a 'tank circuit'. The addition of the capacitor in parallel changes things.
Capacitors have two plates inside them separated by a gap. The gap can be air, or it can be some material that does not directly conduct. The variable capacitor is actually an air gap, but they are rare today because of the size necessary. There may be lots of plates, too, but they will be connected in groups to form two sets of leaves which are alternately one over the other, to create the effect of two larger plates but in a smaller space. If a voltage is applied to one side, and the other side is connected to ground or a lower voltage, an electric charge builds across the gap.
The capacitor gives the current from the inductor somewhere to go when the magnetic field collapses. It rushes into the capacitor and creates a charge there. Then, when the magnetic field has collapsed, and then reversed, the charge stored in the capacitor is suddenly facing a lower voltage, so it rushes out. The current keeps bouncing from the inductor, to the capacitor, and back.
No component is perfect, so while on paper this process would continue, it does not in real life. There are resistances in both the capacitor and inductor, and also losses through magnetic leakage. Current needs to come into the system, and leave it. If one side of the inductor is not grounded in the first place, there is no reason for a current to flow.
This is provided by the transistor, which connects to ground. The collector of the transistor is connected to the LC circuit, and the emitter to an RL circuit. The RL circuit is formed by a resistor (R), and inductor (L) in series this time. The resistor reduces the current that can flow at one instant through the transistor to ground.
This helps the circuit work but also stops the transistor being overloaded. The inductor plays two roles. When current is flowing from the LC circuit while it is charging, it is also building a magnetic field in L2, the inductor of the RL circuit. This inductor behaves the same as the one above, so the current slows as the inductor builds a magnetic field.
Inductors can be used this way on their own to slow current. They allow a big initial current, then as the current and magnetic fields equalise, current slows. It never stops, because the system reaches a sort of balance (which is why we need the capacitor to make an oscillator) and sometimes, inductors are used this way, as a 'ballast' to allow big surge currents to start something, then settle back to a steady level.
Our inductor L2 does more than this: It builds a current, but when the LC circuit above moves to the phase where current is flowing the other way into the capacitor, the L2 coil's field collapses and shoots some of the current back into the LC system. This helps keep the whole lot oscillating.
Added to this is the fact that the two coils, L1 and L2, are made one on top of the other and are therefore magnetically coupled. This means that what happens in one, happens in the other. If the coils are wound with one opposite the other, what happens in one happens in the other but in the opposite direction. So, pay close attention to those colour coded wires!
The transistor is controlled by the tone generator circuit. Mostly, it's the same as last month: An NE555-based oscillator (a different kind of oscillator). This one generates a frequency much lower than the radio section.
In fact, the component values give it a frequency of about 1000Hz, while the transmitter functions in the ballpark of 960 000Hz. Last month, we fed this tone through a capacitor to block the DC, into a speaker.
This month, we feed it through a much smaller capacitor to the base of the transistor. This turns the transistor on and off, but the capacitor also helps shape the output wave from a square wave into one closer to a triangle. So, the transistor is not turning straight on and off, but rather ramping on and off.
This gives bursts of 960kHz transmission, the intensity of which goes up and down at 1kHz. That's what AM radio is: Amplitude Modulation, where the amplitude (strength) of the carrier frequency is varied to produce the sound. The current to the transistor is turned on and off by the Morse key, so you are actually sending short bursts of radio transmission by turning the transmitter on for a dot or dash, and off after each.
That's a rather long explanation for Kids' Basics, and even then, it overly simplifies the reality or even bypasses it. Hopefully, however, you still have a working understanding of what's going on in this circuit, even if it's not an engineering understanding.
WHERE TO NEXT
As mentioned before, the first thing you will probably do with this circuit is build another, so you can have two transmitters and two receivers for two-way communication. Be sure to go over the 'how it works' section of last month's article, too, because we talked a lot about how to communicate well along with how the circuit worked. So, even if you're someone who isn't ready to learn the details of how a circuit works, there is still plenty for you in that article.
Beyond that, you could try changing component values to alter the sound. Changing R3 and C3 will give different tones, and you could use this as an identifier for different transmitters if they all transmit on the same radio frequency. That way, all radios are on the same frequency and no one will ever miss a message while tuned to another channel.