An adaptable noisemaker for gameplay, a unique alarm, or even amateur film.
BUILD TIME: 90 MINUTES
DIFFICULTY RATING: BEGINNER
Anyone who has ever watched a war film involving naval ships from around World War II onwards has probably encountered the sound effect used to send the crew to battle stations. It’s a harsh-sounding electromechanical buzzer sound, pulsing on and off. It appears in animated films and cartoons too, and similar sounds are used for warnings in many other situations from fictional space settings to films set in military bases. The sound itself varies wildly, both in tone, the length of the pulse of sound, and the gap between them. Similar alarms are used when a submarine dives, or in damage control incidents.
Our build is adaptable in that you can set the characteristics yourself and experiment until you’re happy. We’ve set the sound a certain way but even while researching we found great differences even in the sound used on naval ships. For example, the sound used on Royal Navy ships is very different to that used on United States Navy ships, and within both navies, the sound varies depending on when the ship went into service. With this circuit, you can change the sound so much that it will even sound like an aircraft cockpit warning sound, also a common trope in films.
INSTRUCTIONS AND ADVICE
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're building on a solderless breadboard. We avoid soldering to make Kids' Basics 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 image 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 & MATERIALS (See Text for details): |
|---|
| Carboard box |
| Scissors |
| Sharp Knife |
| Thick Paper |
| Coloured Paper |
| Fabric |
| Pencil |
| Double Sided Tape |
| Blu Tack |
| Glue Stick |
| Electronics Parts Required: | ID | Jaycar | Altronics | Core Electronics |
|---|---|---|---|---|
| 1 x Solderless Breadboard | - | PB8820 | P1002 | CE05102 |
| 1 x Pack of Breadboard Wire Links | - | PB8850 | P1014A | CE05631 |
| 5 x Plug-to-Socket Jumper Leads * | - | WC6028 | P1021 | PRT-12794 |
| 1 x 1kΩ Resistor * | R3 | RR0572 | R7558 | FIT0119# |
| 2 x 8.2kΩ Resistors * | R4, R5 | RR0594 | R7580 | FIT0119# |
| 1 x 56kΩ Resistor * | R1 | RR0614 | R7600 | FIT0119# |
| 1 x 240kΩ Resistors * | R2 | RR0629 | R7615 | CE07449 |
| 2 x 100nF Capacitors | C2, C4 | RM7125 | R3013B | CE05188 |
| 1 x 470nF Capacitor | C3 | RM7165 | R3033B | CE07453 |
| 1 x 4.7μF Electrolytic Capacitor * | C1 | RE6060 | R5048 | CE05130 * |
| 1 x 100μF Electrolytic Capacitor * | C5 | RE6130 | R5123 | CE05258 |
| 2 x NE555 Timer IC | IC1, IC2 | ZL3555 | Z2755 | COM-16473 |
| 1 x 1N4004 1A SSR Diode * | D1 | ZR1004 | Z0109 | CE05272 |
| 1 x Amplifier Module | - | XC3744 | - | - |
| 1 x Toggle Switch | SW1 | ST0335 | S1310 | ADA3221 |
| 1 x 4AA Battery Holder | - | PH9200 | S5031 + P0455 | PRT-12083 |
| 4 x AA Batteries | - | SB2425 | S4955B | CE07559 |
* Quantity shown, may be sold in packs. # Use nearest values and adjust capacitor value for sound.
The Electronics Build:
When using jumper wires off the breadboard, try to choose light colours for positive or signal voltages, and dark colours for ground connections. The short wire links on the board are colour-coded for length, so you don’t have options there.
Step 1:
Place the breadboard in front of you with the outer red (+) rail facing away and the outer blue (-) rail closest to you. Insert the wires to join the power rails and the two NE555 ICs, making sure the notch or pin 1 dot faces your left.
Step 2:
Install the 56kΩ resistor (green-blue-black-red- -brown) between the upper red (+) rail and pin 7 of IC1. Install the 240kΩ resistor (red-yellow-black-orange- -brown) from pin 6 if IC1, off to the left. Add the four wire links visible.
Step 3:
Insert an 8.2kΩ resistor (grey-red-black-brown- -brown) between the upper red (+) rail and pin 7 of IC2, and another from pin 6 off to the right. Add the four wire links shown
Step 4:
Add two 100nF (0.1 or 104) capacitors one between the upper blue (-) rail and pin 5 of each IC. We used MKT capacitors but any 100nF capacitor will do. Also, add a 100μF electrolytic capacitor across the upper red (+) with the stripe to the blue (-) rail.
Step 5:
Place the six wire links shown. Note that there is no link between pin 4 and the lower red (+) rail for IC. Instead, the long link from pin 3 of IC1 ends at pin 4 of IC2. Double check this and the previous pictures now to make sure all the wire links are in the correct holes.
Step 6:
Install a 4.7μF electrolytic capacitor between the lower blue (-) rail and pin 2 of IC2. Also, install a 470nF (0.47 or 474) capacitor between the lower blue (-) rail and pin 2 of IC2, and a 1kΩ resistor (brown-black-black-brown- -brown) from pin 3 of IC2 off to the right.
Step 7:
Take three plug-to-socket jumper wires and plug the amplifier module into them. Take note of which wire colour goes to V (power) G (ground), and S (signal).
Step 8:
Insert the 1N4004 diode between the upper red (+) rail and a vacant row. Make sure the silver stripe is closest to you. Plug the V lead of the module into the end of the diode. Plug the G lead into the upper blue (-) rail and the S lead into the end of the 1kΩ resistor.
Step 9:
Take two plug-to-socket jumper leads (or any other type you have spare) and cut the sockets off. Bare the ends of the wires and twist them through the terminals of the toggle switch (we use the middle terminal and one of the outer ones). Tape the joins, but we left ours exposed so you can see.
Step 10:
Plug one switch wire into the upper red (+) rail and the other into an empty row. Push the red wire of the battery pack into the same row. Push the black wire of the battery pack into the lower blue (-) rail. This keeps it a bit further away from the red wire so they can’t touch if either comes loose.
SET UP AND TESTING
Start by turning the dial of the amplifier module all the way anti-clockwise, which should mean rotating it to the left. Then, advance it clockwise again about a quarter of a turn. Install the batteries into the holder, checking before you insert the last one that the wires have not pulled out of the breadboard and touched a component (yes, that is from experience).
Turn on the toggle switch, and listen. You should hear sound straight away from the speaker. If you don’t, turn the dial a bit more. If you get half way and can’t hear anything, something’s wrong. Check that the switch wasn’t already on and you just turned it off, then pull out a battery and start checking connections. The most likely issue will be a wire link in the wrong row, often one hole on either side of where it should be.
How Does It Work?
We could almost rename Kids’ Basics to ‘555 circuits, and a few others because we use the versatile 555 timer so often. This time, we’re using two NE555 integrated circuits (ICs) to do the job. There is such a thing as an NE556, which is two 555s in one package sharing common Vcc and Gnd pins, but not only do many makers already have a couple of 555s in the parts box, but the wiring was actually easier to lay out neatly with two separate 555s for this particular circuit. Regular readers are familiar with the 555 in astable mode by now, but there are still lots of occasional and new readers.
The NE555 timer IC can run in different ways, but the most common configurations are ‘monostable’, or ‘one-shot’; and ‘astable’, or free-running. Both of these ICs are set up in astable mode. First, IC1. This has its timing period controlled by a combination of R1, R2, and C1. The NE555 contains a device inside it called a flip-flop. This controls the output, and it is normally set to turn off at two thirds the supply voltage, and on again at one third. This is sensed by pin 2, the trigger pin, and pin 6, the threshold pin. Because the voltage at pin 2 is zero at the start, the flip-flop flips, and the output at pin 3 turns on. Meanwhile, current flows from the supply rail (Vcc) through resistors R1 and R2, to charge the capacitor C1 connected to pins 2 and 6. When power is first applied, this capacitor is at 0V.
When C1 has charged to two thirds of Vcc, the threshold input pin 6 senses this with an internal device called a ‘comparator’. This compares two signals: the pin 6 input and a reference that is set to give a value of two thirds of the supply voltage. The comparator changes its output state based on whether the voltage at pin 6 is less than or greater than the reference. As soon as the pin 6 voltage rises above the two thirds Vcc reference, the comparator’s output changes and the flip flop ‘flops’ so that the output pin 3 (which is connected to the flip flop by internal transistors) turns off.
When the flip flop ‘flops’, pin 7 becomes active. This is not an output, but an input. It now connects to ground, and the capacitor can discharge through it, hence its name: the Discharge pin. Between the discharge pin 7 and the capacitor is the resistor R2. This resistor slows the current leaving the capacitor C1, which is part of what gives us a time delay between the flip flop’s on and off times. However, pin 2 and pin 6 are connected together, so the capacitor voltage is presented to both.
Pin 2 now comes into play. It has another comparator connected to it and is watching for the supply voltage to reach the one third point. When it does the internal flip flop ‘flops’, and turns the output on. The discharge pin 7 also turns off, so the current can no longer leave the capacitor C1 to ground via this path. Now, the capacitor charges again, but it does so through both R1 and R2. This means the on and off times cannot be exactly the same with this arrangement, but we don’t want them to be. This cycle continues as long as power is applied.
The output pin, pin 3, is able to both source (supply) and sink (drain to ground) current. It is ‘high’, or on, when the trigger pin’s comparator has set the flip flop to be on, and off when the threshold pin 6’s comparator has set the flip flop to be ‘low’, or off. The words ‘high’ and ‘low’ can also be used to describe the output from pin 3. As an aside, because the capacitor C1 starts at 0V when power is applied, the voltage on it has to rise to two thirds Vcc from 0, not from one third Vcc as is the case for the rest of the operation. This is why at first the output stays on for a longer pulse than for every pulse after.
That’s IC1. IC2 operates nearly the same, except for two important factors. On IC1, as most astable NE555 circuits, the reset pin 4 is tied to Vcc. When the reset pin is connected to ground, the IC’s operation is stopped and the output set to low no matter where it was in the timing cycle. The reset pin must be held high for the IC to work. If left unconnected, the pin can pick up stray signals and noise, and operation can become unreliable. This is called ‘floating’ in electronics. The really good news is, because the NE555’s output is either sourcing when high or sinking when low, it never floats. Some ICs’ outputs just float when they’re low, unconnected to anything. By connecting the output of IC1 to the reset pin of IC2, IC2 only operates when IC1’s output is high, and is reliably held at reset when IC1’s output is low. This means we can give pulses of astable operation, followed by periods of nothing.
The other major difference is IC2’s timing components R4, R5, and C3. The capacitor is a tenth the size of the one for IC1, and one of the resistors is much smaller, too. One is slightly larger than its IC1 equivalent but not by a lot. The effect of this is a much shorter timing period. Short enough, in fact, to be audible if a speaker is connected to the output. The size of these components determines the sound, but so does their relationship. For example, even though the two 82kΩ resistors add up to 164kΩ, using a 130kΩ and a 33kΩ to give 163kΩ would give a very different sound, much more so than the 1kΩ difference might suggest. Changing the values of these two resistors and the capacitor C3 will allow you to tailor the sound to your preference. The effect of all this is that IC2 makes sound while IC1 is high, and there is silence when IC1 is low.
There are two 100nF capacitors, C2 and C4, which stop stray signals affecting pin 5. There is also a 100μF capacitor across the supply rails, which we added after unreliability issues during development. It helps cope with the sudden current draw. We used a sound module meant for Arduino to avoid the needless complexity of building an amplifier. It has a 1kΩ resistor on its input as ‘insurance’ (circuit protection), though its input is probably able to be driven directly. The NE555 can source 200mA, but this is too much for most small speakers and well and truly exceeds their power ratings. It is powered by the supply rails through a 1N4004 diode, which has between 0.5 and 0.7V drop across it. This takes just enough off the battery voltage to avoid frying the 5.8V maximum-limited module.
The amplifier module allows us to get a decent volume with minimal connections. It’s not that there’s any more power, but the speaker on the module is designed with good sensitivity, meaning it will make more noise for a given input power. This can’t be said for most of the small speakers that we could find for individual sale, which are general purpose types of higher (but still small) power levels and less sensitivity. We tried one of these first and it just wouldn’t cut it. You can get lots of volume with these speakers by using a small resistor in series with the speaker, or even no resistor, but this exceeds the power rating of the speaker. It will work for a while. Maybe a few hours, maybe a few weeks. Eventually, however, the speaker coil will burn.
The Craft Build:
To house the circuit in a more authentic-looking way, we’re going to house the circuit in a cardboard box dressed up to look like the speaker units often depicted in films and animations featuring the devices. You can use a gift box from a dollar shop but we used a spare parcel shipping box, which a lot of homes have around in the age of online shopping. If you’re making yours part of a plane or space ship, maybe you’ll build a control panel instead. See the ‘Where To From Here’ section for more on this.
Step 1:
Take a sheet of thick paper, and roll it into a cone. Pinch the sides for now, and test to see how it fits into your box. The ideal shape is one that will end quite close to the sides of the box. Check the next step before you make a decision.
Step 2:
When you’re happy with the fit, use double-sided tape to secure the paper, then cut the end off the cone so that there is a big enough circle to fit over the speaker. Also, cut the bigger end of the cone down so that it fits into the box below the edge.
Step 3:
Cut a circle out of the lid of the box that’s around the same size as the opening in the cone. Try to get the edges as neat as possible, because this will stay visible. You can look around the house for round objects that are the right size to trace for your circle, but ask your adults first!
Step 4:
Cover the outside of your box in coloured paper of your choice, depending on what look you want. We chose grey because we were going for a Navy look. Carefully cut out the circle for the speaker, and cut along the lines needed to allow the box lid to open.
Step 5:
Lay double-sided tape around the edges of the hole inside the lid, then cut out a square of fabric to cover it. Press the fabric down firmly all the way around, making sure it stays flat. Also, punch a hole in the side of the box with a pencil where your switch will mount.
Step 6:
Place the electronics into the box and secure the batteries, breadboard, and audio module with Blu Tack® or similar. Use more Blu Tack and scraps of cardboard to secure the cone over the speaker, and install the switch. You may need hot melt glue to really secure the switch, but lots of Blu Tack should do it too.
WHERE TO FROM HERE
Now your alarm unit is ready to use. All you need to do is flick the switch for as long as you want the alarm to sound for. You can change the sound by altering the components around IC2: R4, R5, and C3.
You can also change the length of the on and off times by changing R1, R2, and C1 around IC1. Try the following values and see what sound you get:
R1 = 1kΩ; R2 = 56kΩ, C1 = 2.2μ; R4 = 1kΩ; R5 = 1kΩ; C3 = 180nF.
What sound do you end up with? It sounds far more at home in an aircraft cockpit. On that note, you may want a completely different housing for your alarm. Just for fun, we made up a control panel from a fictional spacecraft that uses the faster, higher-pitch alarm with the components above.
Check out the link for images of this online.
This is really up to your creativity, so think about what you might be using your alarm for, and let your imagination do the rest. Also, get an adult to help you browse websites like Pinterest for ideas once you have a scene in mind. A keyword phrase like ‘control panel’ will bring up all sorts of ideas.
You could also fit your alarm box, whether the naval, cockpit, spaceship, or your own version, with a missile switch cover. These are a bigger switch with a hinged cover, that stops the switch being knocked or accidentally pushed.
Some have a spring return and have to be held open, while others spring against either the open or closed position. Those ones usually turn the switch off when they are closed again.