Some modern shortcuts revive an old technology to teach a very valuable concept.
BUILD TIME: An afternoon
DIFFICULTY RATING: BEGINNER
The Telegraph has a varied history that is tied to the development of urbanisation, and internal and international trade. In some cases, it has a very negative impact in indigenous people in previously uncolonised areas of both the United States of America, and Australia, and we wouldn’t write this article without acknowledging that. However, our focus is on the Telegraph as a piece of communication technology, and how some of the challenges it experienced are still relevant to even the most advanced computer systems today.
We’ll launch into the building of the project first, then explore how it works and the systems lessons to take away from it. We will also take a very compressed look at the history of the telegraph, so you can see where this version sits in the overall development of the technology, and what is often called a telegraph that in fact is not.
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.
Tools & Material Parts Required:
Hot Melt Glue Gun & Glue Sticks
Officeworks or Spotlight
2 Thumb Tacks
1 x Ice Cream Stick
Officeworks or Spotlight
Corrugated Cardboard OR Foam Core Board,
A breadboard & prototyping hardware
Jaycar or Altronics
|1 x Solderless Breadboard||-||PB8820||P1002||DF-FIT0096|
|1 x Packet Breadboard Wire Links||-||PB8850||P1014A||SS110990044|
|2 x Plug-to-Plug Jumper Wires*||-||WC6024||P1016||SS110990029|
|2 x Plug-to-Socket Jumper Wires*||-||WC6028||P1021||ADA1954|
|1 x 10Ω Resistor*||R3||RR0524||R7510||SS110990043|
|1 x 100Ω Resistor*||R1||RR0548||R7534||SS110990043|
|1 x 10kΩ Resistor*||R2||RR0596||R7582||SS110990043|
|1 x 68nF Capactitor*||C1||RM7115||R3023B||-|
|1 x 100nF Capacitor*||C2||RM7125||R3025B||DF-FIT0118|
|1 x 100µF Capacitor*||C3||RE6130||R5123||DF-FIT0117|
|1 X 1N4004 Diode*||D1||ZR1004||Z0109||DF-FIT0323|
|1 x NE555 Timer IC||IC1||ZL3555||Z2755||-|
|1 x Small Speaker||SPKR||AS3000||C0610||ADA4227|
|10m Light-duty Speaker Wire*||-||WB1702||W2100||-|
|1 x 9V Battery Snap||-||PH9232||P0455||DF-FIT0111|
|1 x 9V Battery||-||SB2423||S4970B||PAKR-A0113|
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.
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 100Ω resistor (BROWN BLACK BLACK BLACK SPACE BROWN) from the upper red (+) rail to pin 7 of the NE555, and a 10kΩ resistor (BROWN BLACK BLACK RED SPACE BROWN) between pin 7 and a spot off to the right of the IC.
Add a wire link from the end of the 10kΩ resistor back to pin 6. Also install a 10Ω resistor (BROWN BLACK BLACK GOLD SPACE BROWN) between pin 3 of the IC, and a spot off to the right.
Insert a 100nF capacitor (marked 104 or 0.1) between the upper blue (-) rail and pin 5 of the IC, then a 68nF capacitor (marked 683 or 68n) between pin 2 and the lower blue (-) rail. We used MKT capacitors but ceramic or greencap types will work.
Install a 100µF Electrolytic capacitor with its positive (unmarked) leg to the end of the 10Ω resistor from pin 3, and its negative (striped) leg in a vacant row to the right. Also place a rectifier diode with its striped cathode end in the upper red (+) rail and its unmarked anode end in an empty row below. Any rectifier diode will do (we used a 1N4004) but don’t use signal diodes like 1N4148.
Cut a plug-to-plug jumper wire in half and bare the cut ends. Twist one half through each terminal of a small speaker, and make sure the connections are tight. Tape over the ends to keep them secure and prevent shorts, but we left ours uncovered so you can see. Plug one end into the lower blue (-) rail, and the other into the negative (striped) end of the 100µF capacitor.
Cut 40cm off a length of speaker cable (see next step) and bare one end. Cut a plug-to-plug jumper wire in half, and bare the ends of that. Twist both halves to the end of the speaker wire, and tape the joins. Bare more of the other end of the speaker wire, about 4cm, and twist each around a thumb tack.
Cut two plug-to-socket jumper wires in half, and bare the ends. Also bare the ends of the remaining speaker cable. The length of cable depends on where you want the speaker placed, but read the ‘using it’ section for that. The parts list says 10m. Twist the two plug halves onto one end of the speaker cable, and the two sockets on the other, forming an extension cable. Tape the joins.
Cut two pieces of cardboard or foam core board 12cm long. One is 7cm wide, the other is 3cm wide. Mark the centres of each, and hot melt glue the narrow one on top of the wider one so they meet on the centre line.
Cut a slice of card the same width as an ice cream stick. Cut into four pieces: Two at 1cm long, and two at 3cm long. Glue the two longer ones together.
Push one of the thumb tacks through the ice cream stick at one end. Glue one of the 1cm pieces of card on top. You might need a little extra glue to cover the end in the pin depending on the thickness of the card. Add glue to the underside of the other thumb tack and push it into the other 1cm piece of card.
Glue the small piece of card with the thumb tack in it to the end of the base section, and the two pieces stuck together in step 12 to the underside of the ice cream stick. When that glue is firm, glue the assembly down to the base so that the thumb tacks face each other. Also use drops of glue to secure the speaker wire.
Plug the black wire of a 9V battery snap into the lower blue (-) rail of the breadboard. Plug the red wire into the upper red (+) rail and connect a 9V battery. You should hear sound from the speaker straight away. If you don’t, remove the battery and check connections. If you get sound, disconnect the battery and unplug the red wire.
Take the wires from the ice cream stick assembly and plug one side into the same row as the end of the diode. Plug the other wire into a vacant row beside it, with one row as a gap if there is space. Now, plug the red wire into this row. Connect a battery to the snap and press on the end of the ice cream stick. When the thumb tacks touch, you should hear a sound. If not, check connections and the direction of the diode.
HOW IT WORKS
The circuit itself is one we have used before in Kids’ Basics. It’s just an NE555 run in Astable mode, at a frequency that the human ear can hear. The take-home lesson from this project is actually in how you use it, covered next. However, it would be very arrogant of us to assume that everyone has read all the previous Kids’ Basics. Some people are new, and not everyone reads everything every month anyway. So, we will describe the circuit operation first. A really detailed description of the NE555 and its operation has been presented in Kids' Basics Issue 42.
Ignoring the morse key made from card and the ice cream stick, and instead assuming power is always applied, current at first flows through the 100Ω resistor and the 10kΩ resistor and into the 68nF capacitor. This charges up with current limited by these two resistors together, meaning they set the charge time. This capacitor is connected to both pins 2 and 6 by wire links. Pin 2 is the trigger pin, and pin 6 is the threshold pin. Pin 7, where the 100Ω and 10kΩ resistors meet, is the discharge pin, but in the first phase it is not doing anything. The threshold pin 6 is connected to the internal circuitry of the NE555 and monitors the voltage at the capacitor, acting when the voltage reaches two thirds (2/3) of the supply voltage.
Pin 6 is connected internally to a device called a ‘flip flop’, which changes the output and some internal connections. While the capacitor is charging up, pin 3, the output pin, is high, or on. This means it is supplying current and will show a voltage if measured. As soon as the ⅔ point is reached at pin 6, the flip flop ‘flops’ the other way. Now, pin 3 is low, or off. It will not show a voltage if measured on its own (nothing else connected to it) but it is sinking current. This means current can go through it to ground, unlike some ICs where low is just ‘off’ or disconnected.
Also at the same time, the internal transistor at the discharge pin 7 turns on, connecting that pin to ground internally. The capacitor can now discharge current through the 10kΩ resistor alone, and the voltage across the capacitor falls as this happens. The 10kΩ resistor is responsible for the discharge time. By making the resistor between the supply rail and pin 7 so small (100Ω) compared to the 10kΩ resistor between pin 7 and 6, we make sure that the charge and discharge times are almost the same. This is because the capacitor charges up through both resistors but discharges only through one of them. In other cases, you might want different charge and discharge times, but not in this case.
As the capacitor discharges, the voltage is being monitored by the trigger pin, pin 2. This looks for the voltage falling to one third (1/3) of the supply voltage. When it reaches this point, the internal flip flop ‘flips’, turning on the output and turning off the discharge transistor. Now, the output is ‘sourcing’ current again, and the capacitor can charge again. There is no switching of current on and off to the capacitor. Rather, when the discharge transistor is on and connecting the pin to ground, the path through it is easier than the path through the 10kΩ resistor and the capacitor’s own internal resistance, so the current through the 100Ω resistor from the supply rail goes straight to ground instead, and the current stored in the capacitor can discharge as well.
The output is connected via a 10Ω resistor to one side of a 100µF capacitor, which has its other side connected to the speaker, which in turn connects to ground. As the output turns on, this capacitor charges up via the 10Ω resistor. However, current will only flow through the speaker while the capacitor voltage is rising. When it has charged, the current stops. When the pin 3 output goes low, the capacitor discharges through it.
Speakers are designed to handle alternating current (AC). When the voltage is constant (direct current, or DC), the heat generated is greater and the coil in the speaker can be damaged. The capacitor ensures that no DC can pass through to the speaker. At the frequency chosen for this circuit, this would likely not be an issue anyway, unless you are changing components while the circuit is working. The charge and discharge times of the capacitor and 10Ω resistor are longer (at the supplied current) than the cycle of the circuit. However, better safe than sorry.
At the values given, the circuit has almost identical on and off times. This is very nearly a square wave. The frequency is about 1023Hz (1.023kHz). The Hertz (Hz) is the unit of measure for frequency and describes one complete cycle, from the beginning of on, right through to the end of off right before on happens again. Or, any other two identical points you choose to measure. In a square wave, the beginning of on or off is a sensible point to describe. In a sine wave, which is a smooth continuous wave like waves in water, you might have to find a different point like the peaks, or the exact middle of the highest and lowest points.
A 1kHz square wave just happens to have a lot of harmonics going on in it which, without getting into an acoustics lesson, means it sounds quite rich to the human ear. You can change either the 68nF capacitor value, or the 10kΩ resistor value, to vary the sound. If you change by a long way, like adding a 1MΩ resistor, and you don;t hear anything, the circuit is probably still working. Human hearing is from around 20Hz to 20 000Hz (20kHz) when you are born, dropping to about 50Hz to 16kHz by your mid-teens, and then from about 70Hz to 13kHz by your 30s for most people. When many people hear a mains hum (which should be 50Hz) it is often actually a 100Hz hum caused by DC equipment being affected by both halves of the 50Hz mains wave.
As a side-note, there is a little quirk with astable operation of the NE555. It will be unnoticeable at this frequency but clear at visual frequencies, like the two-LED flasher configuration. Under normal operation, the charge and discharge occurs with the capacitor voltage at between 1/3 and 2/3 of the supply voltage. However, when the circuit is first powered on, the capacitor has no charge and is at 0V. The first charge cycle will be longer, because it is charging to ⅔ Vcc (the designation for supply voltage) from 0V rather than from 1/3 Vcc. At slower frequencies, this may be visible.
The key to using this device is, literally, the key. That’s the name given to the device we made from cardboard or foam core, and an ice cream stick. Real telegraph keys looked more complicated for a variety of functional reasons, but they did the same job. All are just a momentary-on, push-to-make switch. In our case, the switch powers the circuit only while the thumb tacks are touching, resulting in bursts of sound.
In a system which communicates entirely by sound, some sort of code must exist in order for the sound to have any meaning. In an alarm, for example, a distinctive sound may mean that a device is failing, or an evacuation is needed. Think of a fire alarm. A washing machine may use a series of beeps to indicate when it is finished, or overloaded, or the water is turned off. All of these cases require the user to know what the sound means.
The telegraph was no different. One of the earliest ideas stuck, and it came from Samuel Morse. This American invented a system where different combinations of short and long sounds represented different letters and numbers. His system was quickly modified to suit the rest of the world, and International Morse Code (the previous system being renamed American Morse Code) was adopted. We have a chart showing this available for download from our website.
So, pressing the key quickly was a ‘dot’ (or ‘dit’, in some places) and a long press was a dash (or ‘dah’). The user pressed the key to spell out words. In International Morse Code, a dot is one unit long, a dash is three units long, and three units of silence separate letters in a word. Seven units of silence separate words. We were unable to find a standard for the time length of a unit, and information suggests there is not one. In fact, it was a point of pride for operators to be able to reach a certain number of characters or words per minute, which implies that the unit size varies. What is important is consistency across a message.
Of course, you can spell out whole sentences like typing a text message, which is exactly what many telegrams (text sent via telegraph and written down at the other end then delivered like mail) were. However, it is slow. In many situations, codes existed. The most famous is SOS, the international distress signal. Contrary to popular belief, this does not stand for “Save Our Souls”, nor does the code it replaced, CQD, stand for “Come Quickly, Distress”. We have a link to a Wikipedia article on that if you’re interested.
These codes are designed to be sent quickly and interpreted even more quickly. Codes were situational, not universal. For instance, many militaries had their own codes around operational and administrative needs, as did many shipping companies. When using your telegraph for anything, you might like to come up with a similar agreed code. This could be anything from a code that means ‘come to dinner’, to a code from you to your sibling asking if it’s safe to message without anyone hearing.
THE LESSON IN COMMUNICATION
Using Morse code, or any other telegraphic system, has some things in common with modern information systems. The first thing to think about is identification. If you have
one telegraph line between you and your sibling’s room, it is pretty obvious where the message is coming from. However, in the days of overland telegraphs, many stations were connected to each other. Having an identifying code for each station was essential. When dealing with early wired telegraphs, usually one wire went to one station, in a point-to-point network. This explains the huge number of wires on old telegraph poles.
In radio telegraphy, however, every station within range heard each transmission. Sender and receiver identification codes were essential. Radios with separate frequencies came later, but that was only good for sending a message. There was, and still is on modern radios, a call channel where a sending station calls the station they want to send to, and agrees on a channel to use. This often happens in computer systems. If you are good enough at coding to have played with wireless Arduino or Raspberry Pi, you will have encountered this. As soon as there are more than two devices, there must be a way for them to identify each other, and a way for the system to decide what message needs to go where. This can be as simple as which light to turn off in a remote controlled system. You might use a combination of letters or numbers to identify each unit, and your code will have a function that looks at this code and decides what to do at that point.
The second major lesson is probably the bigger one: Confirmation. It is no good sending a message, with no idea if it got there, or was heard when it arrived. In telegraph terms, the operator may not be there at the receiving end. This is understandable if that’s your sibling’s bedroom. In telegraph days, the operator’s job was serious and disciplined, and someone had to be there all the time. The line could still be damaged without anyone knowing. In radio telegraphy, however, the signal may be affected by other things like weather, or range in terms of ships and planes. There is no solid, permanent (aside from damage) link like there is with a wired telegraph. Nor is there a guarantee that anyone is in range, as senders and receivers move, unlike wired telegraphs.
So, the first thing that needs to occur is that the sender needs to send a message to the intended receiver, and ask if they are there and ready to receive a message. If you get no response, the receiver may not be there. If they are there but busy, this is where short-cut codes from earlier become useful. A quick and easily-sent code for, “busy, try again soon”, makes this very clear. Yes, your message got through, but the receiver won’t be able to hear it yet. The same thing happens in modern information systems. In our recent rocket launcher project, both halves of the wireless system talk to each other so that messages are not sent blindly. Only when both sides agree that they are ready is a message sent.
The other thing that needs to be confirmed is that a message arrived in good condition. In telegraph terms, this can be a code for ‘received’, or a repeat of the whole message. Repeating the message was not regular practice in commercial telegraph services, but it was often a feature of military or railway communications where mistakes could cost people their lives. This way, if any words were missed, the problem gets picked up. Things like ‘yes, send the train down this line’ or ‘no, do not send the train down this line’ could be easily confused if the first part of the message does not arrive. To this point, careful planning of your message so that things that are opposite cannot sound the same is good practice. Something like “Clear to proceed on ‘Up’ line”, compared to “Wait, do not send”.
These things also factor into modern communication. Have you ever received a text message, then the exact same one later? Your phone, when it receives a text message, sends back a confirmation message. This is internal and you never see it, but it tells the sender than the message arrived and was not corrupted. In bad weather, or busy network times, or poor reception, sometimes the message arrives but the confirmation never gets back. So, the system tries again.
To make sure the message gets there the way it was meant to, modern systems often include some sort of coding. This might be a pre-planned set of codes where the bits of information also perform some maths (remember, it’s not actually letters and numbers underneath what you see when dealing with computers and phones, it’s 1s and 0s). If the maths gives the wrong answer when the pre-arranged sum or equation is applied to it, the message must be wrong. Again, this was harder to implement in telegraphs, but the fact a word did not make sense would usually tell the receiver that bits were missing. Computers, however, are not using words as we know them.
One final point on confirmation: The Two Generals problem. This one is a basic information technology lesson when you study that subject. Basically, it means that even when you send a confirmation message, you have no way of knowing that the confirmation message got through. So, your receiver can send a confirmation message to your confirmation message, but what if that doesn’t get through? There is a great video explaining this (and why it’s called the Two generals problem) in the “Reading and Resources’ section. The short version is, you will have to decide how much confirmation is needed, and at what point you have to just hope things are ok.
WHERE TO NEXT?
Right now, you have a battery-powered key-operated noise maker. To turn it into a usable telegraph, you need two, and they need to be separated. That’s why we made the extension cable earlier out of the longer bit of speaker wire. Unplug the speaker from the board, plug in the extension lead, and plug the speaker back into the socket end of the lead. Now, you can put the speaker elsewhere, like your sibling’s room. The length of the wire will be determined by this.
You will also need another identical copy of the whole device, so that your sibling has a key and circuit (the transmitter) and you have a speaker. If you’re feeling adventurous, you can have multiple units going to different parts of the house. If you do, you will face problems similar to real telegraphs in terms of how to run so many wires, and how to handle multiple transmitters and receivers while keeping track of which one is which.
One thing you have to think about that original telegraph builders did not (but should have) is to ask the property owner. In this case, that’s the adults you live with.
If you want to know more about how real telegraphs worked with so many different lines and stations, we have linked to the Wikipedia article on the Electrical Telegraph in the ‘Reading and Resources’ section.
A VERY COMPRESSED HISTORY OF THE TELEGRAPH
The earliest telegraphs were invented before any reliable source of electricity was invented. They were powered by static electricity stored in Leyden Jars, a very early forerunner to the capacitor. These early machines only worked within a building, and were experimental rather than serious commercial attempts. Most had one wire for each letter and number, and used static electricity to move some small object in response, for each letter.
With the invention of the battery, however, more serious machines came along. Soon, battery power was used to transmit messages longer distances over cables. Different machines were invented for this, including an interesting one by Cooke and Wheatstone. Following that, the single-wire coded system came to be the accepted method.
A single wire between two stations, with the ground as a return, carried an electrical signal from a chemical battery, controlled by a telegraph key.
At first, the idea was to use the current to operate an electromagnetic device which would make marks on a roll of paper tape as it moved through the machine. These dots and dashes represented the Morse Code for each letter, number, and space. It was not long, however, that operators realised they could easily hear and understand the clicks from these electromagnetic machines, convert the Morse Code in their heads, and write the message in plain language. So, technology gravitated towards devices meant to make sound via electromagnetic activity.
These devices all relied on wires, which had its challenges even where it was the best idea, but was unsuitable for ships and later, aircraft. Along came Radio. Early radio was not suitable for transmitting voice, even if the rest of the technology to do so had been invented yet. It was also very broad, with transmitters working on huge slices of the available frequencies. Instead of today, where you can tune into decimal points of a certain frequency, these early radios operated over pretty much the whole RF spectrum or huge parts of it. However, they could easily carry the dot-and-dash morse code, and could do so while reaching any station that could hear them, unlike wired telegraphs. This was perfect for shipping, which was the industry to first adopt the technology as mainstream. At first, the telegraph key simply turned the transmitter on and off. Early radio technology had a warm-up time, however, so fast switching was impossible. Rather, the key interrupted an internal part of the circuit. Soon, however, the transmission of a tone was the preferred way to go. In this way, the radio can stay on and transmitting, and just the sound be turned on and off.
This technology stayed until voice communication became more successful and gradually took over. However, in shipping to a degree and in the military in particular, Morse-based radio telegraphy stayed in use much longer, as extensive systems for very clear, reliable communication, including standard codes for nearly every procedure you can think of, had been invented and were in widespread use. In fact, the SOS transmitted in Morse Code stayed the international maritime distress signal until 1991, when it was replaced by satellite and radio emergency beacons. It was, however, one of several emergency options once voice communication became more viable.
Our project is a combination of these two systems. The tone generator dates from radio times, but wired communication came first and was still the more common on land. However, the tone system is more suitable to Kids’ Basics readers than trying to listen for the clicks of a relay or electromagnet and try to tell which is the ‘on’ click and which is the ‘off’ click.