A two-part series to explore some environmental sensors in a real-world setting, with some fun along the way - by Daniel Koch

In the first instalment of this series, we are going to build a lighthouse which starts flashing when the day gets dark enough. We are going to learn about comparators and use a light-dependent resistor to achieve this.

The lighthouse will flash steadily but later on in the article, we will discuss how you can use Arduino to give the lighthouse an identity through a flash pattern, and how real lighthouses do the same. In the next instalment, we are going to make an automatic fog horn to go with our lighthouse, and build some sensors that can trigger it in the right conditions.

Real lighthouses use very complex lenses to create their flash pattern and project a light beam far out to sea. However, that would be far too complex for Kids' Basics or even most serious models. Instead, we are going to use a timer IC to drive an LED, with a flash every few seconds or so. We will have a look at some other options if you really want to go crazy, but they are not for the faint-hearted.

Way back in Issue 28, we built a lighthouse night light based on a Passive InfraRed (PIR) motion sensor, an Arduino, and a USB light strip. This used the PIR to trigger the Arduino into turning the light on for a duration of the user's choosing, but we set ours for half an hour. The idea was for small kids, who might stir during the night, as the light would come on automatically as they woke up, reducing the freak-out many small kids experience when waking up in the dark.

This build is different. This one is more of a model for fun than a decorative utility. It certainly won't light up the room! It will turn itself on when the light level falls far enough, and beyond the fun of the model itself, the circuit that drives it is the main point of the project. However, to save ourselves a lot of time and effort, we are going to re-use the original printable model from that PIR project.

There is a bit of modifying required, and of course, you could freelance if you wanted and build your own lighthouse model from scratch. If you are using the supplied printable model, however, you'll need to download it and print it before you start the craft build. The electronics build stands alone, and you can build that first if you are making your own model.

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.

THE ELECTRONIC BUILD

Parts Required:	ID	Jaycar
1 x Solderless Breadboard	-	PB8820
1 x Packet Breadboard Wire Links	-	PB8850
2 x Plug-to-plug Jumper Wires	-	WC6024
1 x 150Ω Resistor	R5	RR0552
1 x 1kΩ Resistor	R4	RR0572
1 x 10k Resistor	R1	RR0596
1 x 15kΩ Resistor	R7	RR0600
1 x 150kΩ Resistor	R6	RR0624
1 x 1MΩ 25-turn Trimpot	R3	RT4658
1 x Light Dependent Resistor	R2	RD3485
1 x 100nF Capacitor	C1	RM7125
1 x 47µF Electrolytic Capacitor	C2	RE6100
1 x BC327 PNP Transistor	Q1	ZT2110
1 x LM311 Comparator IC	IC1	ZL3311
1 x NE555 Timer IC	IC2	ZL3555
1 x High-brightness White LED	LED1	ZD0290
1 x 4xAA Battery Pack	-	PH9200
4 x AA Batteries.	-	SB2425

Step 1

Take two plug-to-plug jumper wires, cut them in half, and bare the ends. Also cut two lengths of twin-core hookup wire, and bare both ends of both cores. The wire length will vary depending on your lighthouse design, as one is for the light sensor and the other for the LED. Ours were 50cm for the LED and 20cm for the light sensor.

Step 2

Twist the two halves of one plug-to-plug jumper wire onto the two cores of one end of the longer wire. Tape the joins, but we left them exposed for you to see. On the other end, twist the wires around the LED legs and then bend them with small pliers so the wires do not slip off. Tape these joins too. More twin-core wires have a trace down one core, so use this to remember which is the long (+) leg of the LED and which is the short (-) leg.

Step 3

Twist the two halves of one plug-to-plug jumper wire onto the two cores of one end of the shorter wire and tape the joins. On the other end, wrap the wires around the legs of the Light Dependent Resistor (LDR) and bend the legs like the LED. Tape these joins too. The LDR is not polarised so you do not have to remember which side is which later.

Step 4

Place the breadboard in front of you, with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Install two wire links, one joining the two red (+) rails and one joining the two blue (-) rails.

Step 5

Insert the LM311 into the breadboard and use wire links to join pin 8 to the upper red (+) rail, and pins 1 and 4 to the lower blue (-) rail.

Step 6

Install a 10kΩ resistor (BROWN BLACK BLACK RED SPACE BROWN) between pin 3 of the IC, and a spot to the left. Add a 1MΩ 25-turn trimpot (Marked 105) with one leg in the same row as pin 4 of the IC, and the others off to the right.

Step 7

Place a 1kΩ (BROWN BLACK BLACK BROWN SPACE BROWN) resistor from pin 7 of the LM311 off to the right. Add a wire link to the end of it, and then three more below the IC: One from the end of the 10kΩ resistor to the lower red (+) rail, one from the middle leg (wiper) of the trimpot to pin 2 of the IC, and one from the right-hand leg of the trimpot to the lower red (+) rail.

Step 8

Insert a BC327 PNP transistor into the breadboard with its flat side facing away from you, its middle (base) leg in the same row as the wire link from the 1kΩ resistor. Add a wire link between the left-hand (emitter) leg and the upper red (+) rail, and another from the left-hand (collector) leg off to the right.

Step 9

Install an NE555 timer IC into the breadboard, with pin 8 at the other end of the wire link from the transistor. Add a 150kΩ (BROWN GREEN BLACK ORANGE SPACE BROWN) resistor between pin 7 and the right-hand (collector) leg of the transistor, and three wire links to join pin 6 around to pin 2.

Step 10

Place a 15kΩ (BROWN GREEN BLACK RED SPACE BROWN) resistor between pin 7 of the IC and the wire links that join pin 6 to pin 2. Add a 100nF (100n or 104) capacitor between pin 5 and the upper blue (-) rail. We used an MKT capacitor but any type of capacitor with a value between around 100nF down to 10nF will work fine.

Step 11

Install a wire link between pin 4 of the IC and the lower red (+) rail, and another between pin 1 and the lower blue (-) rail. Insert a 47µF electrolytic capacitor with its negative lead (marked with a stripe) to the lower blue (-) rail and its positive lead (unmarked) to pin 2 of the IC. Also add a 150Ω resistor (BROWN GREEN BLACK BLACK SPACE BROWN) between pin 3 and a spot off to the left.

Step 12:

Add the black wire of a 4xAA battery pack to the lower blue (-) rail of the breadboard and the red wire to the upper red (+) rail. Plug the wire from the long Anode (positive, +) leg og the LED into the same row as the wire link, resistor, and collector (left leg) of the transistor. The short leg wire (cathode, -), goes to the end of the 150Ω resistor. One of the LDR wires goes to pin 3 of the LM311, and the other goes to the lower blue (-) rail.

SET UP AND TESTING

First of all, double-check the connections you have made against the pictures above before applying power.

Setup is fairly simple but you will need a small flat-blade screwdriver, the batteries, and some patience. You will need access to the circuit but will also need to position the light sensor in the way it will be positioned when used. For example, if you position the sensor in the flat roof of the lighthouse base building, facing up at the ceiling, do not point it on its side at a sunny window when adjusting the darkness cutoff.

'Trimpot' is the name for small potentiometers that are usually set with a tool and forgotten about. It is an abbreviation of 'trimmer potentiometer'. The 25-turn trimpots we use here are more expensive than the basic 270° ones, but far more accurate. Position your light sensor or partially cover it until the amount of light falling on it is similar to the darkness level where you want your lighthouse to come on. Power up the circuit and adjust the trimpot until the light starts flashing. Work slowly, because the long off time means you might go past the right point just at the start of the off time and not notice. Remember, the NE555 circuit is running all the time, it is only the LED that is turned on and off by the daylight switch.

The 25-turn trimpots should sound a very soft 'click' once per rotation when they reach the end of their travel but they will not stop turning, so listen carefully. If your light does not turn on (or off if it was already on) when you hear clicking, then start turning the other way.

By the way, there is nothing wrong with using the much cheaper flat 270° type of trimpot, it's just harder to find the perfect point because the entire travel is covered in three quarters of one rotation, not spread out across twenty five full rotations.

If you want to, swap out the 150kΩ resistor R6 for a much smaller value, down to around 10kΩ or so. This will speed up the cycle and make testing easier.

If you get no light, the first thing to do is check the LED - on long wires like this, it is easy to mix up the order of the legs. If you still get nothing, move the 150Ω resistor so that it goes from the LED to the lower blue (-) rail. This eliminates the NE555 section and allows you to find out if the problem is there, or in the LM311 side.

HOW IT WORKS

The circuit has two sections, so we will deal with them separately. The first half is the light sensor, based on a Light Dependent Resistor (LDR) and an LM311 comparator. The light dependent resistor is a form of variable resistor, as its resistance changes depending on the amount of light falling on it.

They are made with special chemical combinations to make this happen, and the most common is Cadmium Sulphide (shortened in chemistry to CdS), so they are sometimes called CdS Cells. However, there are other specialised ones. There are a great variety of LDRs around, but the two main ways they differ are the resistance, and how they behave. Some have a resistance that reduces with more light, while others have a resistance that increases with light. Most reduce with light.

The other main way they differ is the dark and light resistance. This is really important later on. Of the two we had on hand, one had a dark resistance of over 10MΩ and a light resistance of between 50kΩ to 150kΩ (depending on light strength and component tolerance). The other type we had, however, has a dark resistance of around 500kΩ and a light resistance between 2.5kΩ and 10kΩ. This is the one we went with.

The LDR and 10kΩ resistor R1 form a voltage divider. For an in-depth look at voltage dividers, including maths with M&Ms, check out Classroom issue 51. In simple terms, the 6V supply voltage is split across R1 and R2, according to the amount of the total resistance that each makes up. Let's say that the LDR is at 10kΩ right now. R1 and R2 add to 20kΩ, and each makes up half of that. So, half the supply voltage drops across R1 and half across R2, meaning the voltage at the junction is half the supply, or 3V.

Now, imagine that R2 was 90kΩ, and R1 is still fixed at 10kΩ. Now, the total is 100kΩ, and R1 is one tenth of that, while

R2 is nine tenths of that. Therefore, one tenth of the voltage drops across R1 and nine tenths across R2. That's less than 1V on R1 and the rest on R2, so the junction of R1 and R2 sits at over 5V. That becomes really important shortly.

IC1 is an LM311 comparator. A comparator is a device with two inputs, an inverting (-) and a non-inverting (+). The comparator compares the voltages at its two inputs by subtracting the inverting input from the non-inverting input. If the difference between them is positive, the output is high. If the difference is negative, the output is low. We use a separate voltage divider, a variable one this time made by connecting the two ends of a 1MΩ trimpot R3 to the supply rail and the wiper to the inverting input. The wiper of a potentiometer moves across the length of the resistor inside it, changing the amount that is before and after the wiper. So, it also changes how much of the supply voltage is dropped across each section.

So, we have two variable voltage dividers connected to the comparator. In strong light, the resistance of the LDR is low, as low as 2.5kΩ depending on component tolerance. Therefore, around 4V drops across the 10kΩ resistor and the rest over the LDR, giving around 1V at the junction connected to the non-inverting input of the comparator. The voltage at the inverting input is adjusted with the trimpot. Remember, the inverting input is subtracted from the non-inverting input. Let's say the non-inverting input is 1V, and the inverting input is 2.5V. 1 - 2.5 = -1.5. That means the output is low. As the light falls, however, the LDR's resistance increases, and therefore so does the voltage at the junction of it and R1, connected to the inverting input. When the voltage rises above 2.5V, then the difference is positive and the output goes high.

The exact point at which this happens can be adjusted by turning R3 one way or the other. In this way, you can adjust how dark it must be before the output of the comparator goes high. The output of the LM311 is not directly connected to the pins of the IC. Instead, it is connected to the base of an internal NPN transistor. When current is provided to the base of an NPN transistor, it flows out the emitter to ground and allows a much larger current to flow from the collector to the emitter. In the case of the LM311, both collector and emitter are broken out to pins, so you can control how the transistor is connected. However, we use it as a switch for a separate transistor.

A PNP transistor works differently. Current flows from the emitter, out the base and to ground. The larger current can then flow from the emitter to the collector. The NPN transistor should generally be used as a low-side switch, meaning that it is between the load and ground.

The PNP transistor, on the other hand, is generally used as a high-side switch, which means it goes between the power supply and the load. We say generally, because there are exceptions to the rule and reasons for both cases that determine whether to follow or break the rule. Most of the time you would follow that rule but be aware that if you see it broken, there may be a good reason, or not!

So, when the output of the comparator is high, its internal transistor is on. That means current can pass through the emitter of Q1, and out its base, through resistor 1kΩ R4 which limits current to a level safe for both the base of Q1 and the internals of the LM311, and the through the LM311 itself to ground. That in turn allows a larger current to flow from the emitter of Q1 and out its collector. This switches current on or off to the second half of the circuit.

The second half of the circuit is based around IC2, an NE555 timer IC. The IC works by charging and discharging a capacitor, and measuring the voltage across it. Inside the IC are two comparators, and a circuit called a flip flop. One side of each comparator is connected to an internal voltage divider while the other side of one is connected to pin 2, the trigger pin and the other to pin 6, the threshold pin. The flip flop is a circuit that changes state based on its two inputs, whether high or low, to give a high or low output.

Pins 2 and 6 are tied together to work in 'astable' mode, where the circuit repeats a cycle. The 47µF capacitor C2 is connected to both. Pin 7 is the discharge pin. It is connected internally to the flip flop via an NPN transistor. At first, the internal flip flop is high, and so is pin 3, the output pin.

However, there is an internal inverter so that while the output pin is high, the discharge transistor is actually off. Current flows through the 150kΩ resistor R6 and 15kΩ resistor R7 to charge C2. However, because the output at pin 3 is high, there is no voltage difference between there and the supply rail, so current will not flow through LED1 and R5. More on that in a moment.

When the voltage across C2 rises to two thirds of the supply voltage, the comparator inside the IC changes state to low. Now, the discharge transistor turns on, allowing the capacitor to discharge through 15kΩ R7 only. The charge goes through the resistor and the internal transistor to ground. However, the output at pin 3 is also low. When it is in a low state it can sink current, meaning carry it to ground. On some ICs, the output is simply off when low, not necessarily a current path to ground, so the NE555 is useful here in that it can both source current (supply when high) and sink current (drain to ground when low). That means current can now flow through LED1, and R1, and to ground via pin 3.

As the voltage on the capacitor falls to one third of the supply voltage, pin 2 of the NE555, the trigger pin, carries this to the comparator and the flip flop changes state again. The output goes high and the discharge transistor turns off, allowing C2 to charge again. Because the output is high, the current through the LED cannot flow. There is no voltage difference between pin 3 and the supply voltage, and current only flows from a higher voltage to a lower voltage or ground (which is 0V, as low as you can go in simple terms).

Why is the LED not just connected to output pin 3, and ground? We said the NE555 can source and sink current. The high time for an NE555 in astable mode cannot be longer than the low time.

We want a short flash followed by a long pause. So, we connect the LED so that its anode (long, positive (+) leg is connected to the supply rail and its negative cathode (-) leg is to pin 3. As above, current can only flow through the LED when the output pin is low, sinking current, and we have made this low period quite short so that we have our short flash and long pause.

We pointed out that the on and off times for an NE555 in astable mode are determined by the capacitor value, combined with R6 and R7 for charge time (output high) and R7 only (output low). There is a mathematical formula and you can find it by browsing some previous projects in Kids' Basics where we used the NE555, or in Classroom issue 58, but for most people, it's just easier to use the online calculator that we have linked to in the 'Reading and Resources section.

THE CRAFT BUILD

Craft Materials:

Paper Printout, see text.

Fast-drying All-purpose Adhesive

Hot Melt Glue and Glue Gun

Rubbr Bands

Clothes Pegs

Scissors

Match Sticks

Small Cardboard Box, See text

Additional craft materials of choice for decoration, see text.

The craft build recycles a previous file we made for issue 28, but used a little differently. Also, while we stick the lighthouse to the top of a box to make a service building, we haven't dressed it up in the instructions. We have decorated in a couple of example photos. Every lighthouse is different. Some have a service building at the base, while others are just a tower. Some have separate keepers' cottages, some have living quarters integrated to the tower. Generally, an onshore lighthouse has more buildings, which offshore lighthouses built on rocks are tower only. Island lighthouses vary depending on the size of the island.

Instead of following rigid instructions for this, we suggest an internet search for lighthouses, to find some that appeal to you. Our builds were inspired by the lighthouses in our local area but the design is really up to you. You will need somewhere to mount the circuitry, but this can be a small box that you build into a 'rock' or 'ground' base if you want a tower-only light. Decorating your lighthouse can be as simple as some colour, or as extensive as a full diorama base. Materials can be simple or purpose-made scenery materials if you have access to those (and money for them). Paper mache or hardware-store patching plaster work just as well though, particularly for making rock.

Step 1

Start by choosing a box to mount your lighthouse circuitry, as big as you like or as small as can still fit the circuit and batteries. We used a postal packaging box but shoe boxes and gift boxes work well too. Make sure the circuity fits.

Step 2

After deciding where the lighthouse will go, make a hole in the lid for the LED and its wires to pass though. Also make a hole for the LDR wires, wherever you choose to mount the sensor. Pass the wires and components through, and put the lid on the box. Glue the LDR down at this point, because we weill not do anything else with it.

Step 3

Download and print the lighthouse cutout on firm paper. We used 300gsm paper but anything 160gsm or above should work ok. Make sure your printer settings are set to 'print actual size' for A4 paper. Do not allow any scaling or 'print to fit' type options.

Step 4

Take the fist sheet with the red stripes, and cut around the blue tabs. Cut off and keep the red strip that sts to the side of the main body.

Step 5

For the second sheet, cut around the gallery, including careful cutting around the yellow spokes. The grey sections are waste. Don't cut the centre opening, because this was from the previous lighthouse. Just make a hole here big enough for the LED.

Step 6

To make the base retainer ring, first cut around the outer circle. Then, fold it gently in half but do not press down to crease it. Pinch the middle with scissors to nick it, then unfold it. Use the nick to start the cut, and cut toward the inner circle, then carefully around it.

Step 7

Cut around the outside of the gallery support, then go through the same process as the last step to cut out the grey centre section. Cut along the black lines to cut the tabs out and then fold them upwards.

Step 8

Cut out the roof section, being careful not to cut off the blue tab. Also, cut out the railing from the edge of the page.

Step 9

Apply glue to the tab at the long side of the lighthouse tower. Carefully wrap the paper around until the tab is fully inside the cylinder created by this. Make sure the end edges are straight, lining up squarely, before clamping with clothes pegs and rubber bands. The best kind of glue is a fast-drying alcohol solvent glue used in crafting. One common brand is Uhu, but there are others.

Step 10

Glue the blue tab at the end of the railing to the inside of the opposite end, making a ring. Peg it to dry. While it does, lay the gallery piece coloured side up on the table, and glue the gallery support ring onto it.

Step 11

Turn the gallery over and bend the yellow tabs upward. These become the railing posts. Use the marks on the railing as position guides to glue the railing onto the posts. Clamp with paper clips or pegs until dry.

Step 12

Slide the base retainer ring over the top of the tower. Bend the blue tabs so they stick out, and apply glue to the undersides. Push it down firmly with pressure on the tabs with the tower where you want it positioned. Then, spread glue on the top of the tabs and the exposed sections of the box lid, before pressing the retainer ring over the top. Remember to pull the LED wire through before you do this.

Step 13

Roll the roof piece into a cone, and apply glue to the blue tab before fixing it in place with a peg while the glue dries.

Step 14

Pull the LED though the hole in the middle of the gallery piece. Open the tabs out on the gallery and slide them over the tower. Glue them down one at a time, as quickly as you neatly can, then use a rubber band to hold them down while the glue dries.

Step 15

Glue the red strip saved from earlier over the gallery tabs, with a bit of extra glue on top of the tabs as well as at the bottom edge of the strip. This hides the tabs and adds a bit of strength.

Step 16

Slide the LED down with the excess wire inside the tower, until it is in a good position visually. Glue it in place. You can leave it as-is, or use crafting materials to make a base like you would find in the lamp of a real lighthouse.

Step 17

Take some match sticks and glue them into the inside edge of the roof section. Work in pairs, with two sticks on opposite sides first.Then, turn it a quarter of a turn and add two more so that you have four, equally spaced to make a square. Then, add four more, in the middle of those already there, so you have eight equally spaced around the cone. You may need hot melt glue instead of the craft glue for this step.

Step 18

Glue the other ends of the match sticks to the gallery piece, over the LED. This forms your lantern room. You can cut a scrap of paper, too, and wrap it around the base so that the lantern itself starts higher from the floor, as many but not all real lighthouses do. That's the build complete. Have a look at the 'Other Build Options' and 'Where To Next' sections for ideas on how to dress up your lighthouse.

A SHORT HISTORY OF LIGHTHOUSE TECHNOLOGY

Real lighthouses are as important now as when they were first invented. In ancient times, navigating the oceans or even bigger inland waterways was a challenging task. At first, simple fires on a hill marked the entrances to harbours, or at least a headland nearby. These evolved into stone fire towers, where a fire was kept burning in an open grate. In some parts of Europe, some of these still stand, albeit with no fires kept burning now.

Eventually, tallow candles replaced fires. Many, many candles were used in these lighthouses and they needed to be maintained all night long, just like a fire. Wicks need trimming, candles need replacing. However, candles allowed the light room, called the 'lantern' of a lighthouse, to be enclosed in glass to protect it from wind and rain.

Around this time, some offshore lighthouses were being built as well. Most of these were on small islands, but some of the first rock lighthouses were built then, too. Also in the mix was the first use of mirrors to reflect the candle light. At first, these were polished metal, because the mirror as we know it today had not been invented yet (or reinvented, as more recent research suggests). This would be placed behind the candles to reflect light seaward.

The next big step in lighthouse technology came with the invention of the oil lamp. At first, these were simple wicks in a container, being little brighter than a candle. However, development of fuel allowed these to become brighter, and it was found that whale oil gave the best light. That is, until kerosene was developed. Also called paraffin oil, this was the brightest of all fuels. It also led to the development of the mantle lantern, where the fuel is vaporised and burned as a gas inside a fine mesh enclosure. This is brighter than any flame.

Electric lights began replacing lighthouse lamps from around the 1920s, but at first, only inshore and harbour lights. It would take the development of much brighter electric lights before they could replace the kerosene mantle or even gas mantle lamps used in the brighter lighthouses. Because of the difficulty of transmitting electricity, kerosene lamps continued in use, even while automated, into the 1980s in very remote locations.

The greatest development came with the glass prism lens, called a Fresnel lens. These made the modern lighthouse possible. They are far more efficient reflectors than mirrors, and lighter for the same power. They also create a better beam, capable of being transmitted further because the light is parallel.

It is also this lens type that allows lighthouses to have a distinctive flash pattern. Earlier, lighthouses all looked the same, and that made navigation hard. Giving each lighthouse its own identity would allow navigators to know where they were. Early attempts at this involved screens and shutters that rotated around the light. Rotating groups of mirrors were also tried. However, the Fresnel lens made the idea really work. Different panels of prisms reflect the light at different angles, and so as the prism rotates, a pattern of flashes and periods of darkness is seen from afar.

Early lighthouses had keepers, to maintain first the fire, then the candles, and later the lamps. Each had to be maintained and refuelled. Onshore lights were the first to be automated and keepers made redundant, as electric lights made automation possible. It was not until much later, the 1950s at the earliest, that systems were developed that allowed kerosene lamps to be automatically controlled and maintained. Many remote lighthouses kept their keepers until the 1970s or 80s. Now, most are solar powered and very few staffed lighthouses exist.

OTHER BUILD OPTIONS

You could build your lighthouse from scratch using reference photos and crafting materials. Cardboard tubes make great towers but many lighthouses are tapered, so you can use cardboard to make a slight cone. The build process would be similar, consisting of cutting out parts of various materials and glueing them together. You could also 3D print a lighthouse, or parts for one. There are great online tutorials to show you how to make a truncated cone, which is the name for a tapered cylinder.

If you really want a challenge, you could make a motorised lighthouse. Lego released a model of a lighthouse not too long ago, which used a motor in the base to drive a long shaft through the middle of the lighthouse. At the top, this turned a gear which was mounted around the central light. It held a mirror and lens, a tribute to the Fresnel lens, and the light stayed fixed. The lens and mirror rotate. You could build something like this into your lighthouse, but you would likely need 3D printed parts. You could use lenses from toys or small science kits, or even just mirrors. This would enable you to give your lighthouse a flash pattern.

FLASH PATTERNS

The flash pattern is what gives modern lighthouses their utility. Being able to tell one from another helps a navigator know exactly where they are along a coast. Before this was possible, shipwrecks still occurred when mariners thought they were looking at one lighthouse but were actually looking at another. Even in modern times of GPS navigation, lighthouses are still an important visual reference, which is why governments around the world still maintain them, and even build new ones.

Modern lighthouses are made with modern, compact optics and bright light sources and so are much smaller than their historic cousins. Smaller ones simply flash the light on and off to give a flash pattern but bigger ones, and all historic lighthouses with a pattern, rely on rotating lenses. Some of these are simple, like Norah Head lighthouse, not too far from DIYODE HQ on the New South Wales Central Coast. It is a huge Fresnel lens with two sides, and makes one full rotation every thirty seconds. So, it flashes every fifteen seconds. Barrenjoey Head light, at the border between the Central Coast and Sydney, NSW, was automated with acetylene gas in 1932 and since then has had a flash pattern with a fixed (non-rotating) lens. Its pattern is four flashes with two-second intervals between them, with a burst starting every thirty seconds.

It is patterns like this that allow lighthouses to be identified at night from a distance. There are some very creative ways that lighthouses were given flash patterns and other identifiers in the pre-electric era, or even with early light bulbs, so if it interests you, several very good resources on lighthouses are around online or in print. However, we could only come up with a simple flash pattern that could be driven by the NE555, so we went with that.

WHERE TO NEXT

You can dress up your lighthouse in a variety of ways using basic crafting skills and some imagination. You can build windows into the tower or the base building, make an island, rock, or land diorama or small base display section, dress up the base building with windows and a roof, or anything like that. If you add a roof, be careful of the light sensor. We glued ours flat down to the box lid because Norah head lighthouse, the one we used as inspiration, has a flat roof with a parapet wall around it.

The easiest way to take your build further is to mount it on some sort of display base, as we did with the original lighthouse night light project. This photo shows some foam core board slapped on the sides. You could also substitute the 5mm LED for a brighter type, like a 3W power LED. The transistor would need to be bigger to suit, and the voltage carefully considered. Further, side-emitting LEDs are a better option than the 5mm one we used. However, they are a little harder to get. Some online sellers have them, and you will always find something at LEDsales.com.au. The other option is an LED with a wide viewing angle, like some superflux-style LEDs which have up to a 120° viewing angle.

The other option you have is to add lighting detail with more LEDs. You could cut windows into your tower and put LEDs inside. Many offshore lighthouses in particular had the living quarters built into the tower, so there were several floors within this. You could detail them, so someone can get up close and look through the windows; or, you could just use tracing paper so that light shines through but you cannot see in. If you are building an onshore or island lighthouse, you could add a keepers' cottage too, and light that up.

Electronically, you could abandon the discrete circuit we build and use an Arduino. A simple sketch would use an analog input to read the light sensor. Still set up as a voltage divider as it is here, and switch accordingly. Using a microcontroller allows you to program in a flash pattern that could give your lighthouse an identification like a real lighthouse. Alternatively, there are analog ways to do this but we think they might be beyond Kids' Basics.