Kid's Basics: Alarmed Book Safe

Daniel Koch

Issue 24, July 2019

Hide your sugar stash or your prized possessions in plain sight on a bookshelf with our alarmed book safe!


Want to keep your valuables safe from prying eyes, or an annoying sibling? Does someone always eat your chocolate stash? This book safe is easy to make with a little adult help, recycles an old, unwanted book, and ends up very camouflaged on your bookshelf or table. And best of all, it has an alarm that only you can disarm, along with a light to tell you in someone has opened it along the way. And don’t worry parents, you won’t be completely out of the picture, as we have designed this with welfare concerns in mind. As there is no physical lock, you will always be able to access the safe, albeit with a loud noise and an indicator left behind.

Our normal Kids Basics rules apply: No soldering or special tools required, and we’ll take you through step-by-step. Parental help will be required for some of the craft side of this project; a sharp knife and some cutting is required. However, if you don’t feel safe with this, or parental supervision isn’t available, we have a back-up version that will be just as awesome! The alternative version is built in a gift box that does not require the extensive cutting and hollowing of the book.


We’ll construct the electronics first, explain how they work, then give the instructions for the craft. The electronics this month are far simpler than some of our past projects, but we will introduce you to some different types of switches. We use a magnetically operated switch, called a 'reed switch', as well as a lever-operated pushbutton called a 'limit' or 'micro' switch. We also explain a handy little device called a 'Silicon Controlled Rectifier' or SCR, which is a bit like a diode with a control gate to make it work only on command. We explain this in further detail later in the project. Additionally, we’re using four AA batteries instead of a 9V so the LED can run for ages if need be.

Old, Unwanted Book, Big Enough To Make Into A Safe, or A Craft or Gift Box
PVA Glue
Container To Mix The Glue, Such As A Plastic Cup*
Old or Cheap Paintbrush*
Box-Cutter or Snap-Blade Retractable Knife
Double-Sided Outdoor Mounting Tape
5mm × 100mm (or Thereabouts) Balsa Wood
A Heavy Weight That Covers Your Book*
Side Cutters
Small Long-Nose Pliers
Wire Strippers**


* These items will not be needed for the craft/gift box option. ** These are desirable but if you don’t have them, you can strip small wires with your side cutters and a lot of care. *** Which ones you need will depend on exactly which switches you buy.

ELECTRONIC PARTS REQUIRED:JaycarAltronicsCore Electronics
1 × Prototyping BreadboardPB8820P1002CE05102
2 × Breadboard Wire LinksPB8850P1014AP1014ACE05631
3 × Breadboard Jumper Leads, Plug to PlugWC6027P1016P1016PRT-12795
2 × Breadboard Jumper Leads, Plug to SocketWC6027P1017P1017PRT-12794
1 × 4 × AA Battery HolderPH9200S5031 + P0455ADA830
4 × AA BatteriesSB2425S4955BCE04629
1 × Micro/limit SwitchSM1038S3265POLOLU-1403
1 × Reed SwitchLA5070S5153-
1 × Bed Lamp SwitchSP0735S1075A-
1 × Piezo BuzzerAB3462S6109ADA160
1 × 1N4004 Diode*ZR1004Z0109COM-14884
1 × C106D SCRZX7006Z1768-
1 × 5mm LEDZD0152Z0860CE05103
1 × 47Ω 1/4W Resistor*RR0540R7526COM-05092


* Quantity shown, may be sold in packs.

The Build:

We’re assuming very little (or zero) electronics knowledge, and so will try to walk through everything carefully, in order to ensure your success. There is no soldering or programming required - we’ve kept it as simple as possible. If this is your first project, you may also like to read “Breadboard Basics” in Issue 15, to familiarise yourself with the breadboard before getting started.

Step 1:

Peel three different coloured wires from the ribbon of plug to plug jumper wires.

Step 2:

Cut them all in half and strip the ends back about 10mm.

Step 3:

Screw the ends of one of these cut wires into the bed lamp switch as shown.

Step 4:

Screw the ends of another cut jumper wire into the terminals of the reed switch. One end goes to the screw marked COM and the other to the terminal marked N.C. Take care, as different brands may have the terminals in a different order, but all will be marked.

Step 5:

Take the final cut jumper wire and thread the bare ends through the holes in the terminals of the limit or micro switch. One goes to the C terminal, the other to the NC terminal. Again, ours may be a different brand to yours so look for the embossed writing on the plastic body.

Step 6:

Place the breadboard in front of you with the blue (-) rail closest to you.

Step 7:

Insert the wire link between the upper (-) rail (blue) and the spot on the breadboard as shown, in row 1.

Note: The numbers on our board go from right to left. Some brands are left to right and others have no numbers.

Step 8:

Insert the wire link between the upper (+) rail (red) and the spot on the breadboard as shown, row 7.

Step 9:

Insert the 1N4004 Diode as shown, with the end with the silver strip in row 8 and the black end in row 15.

Note: We have trimmed and bent the leads of some of our components, so they lay flat to the board. This helps neatness, fault-finding, and avoiding short circuits.

Step 10:

Insert the SCR with the metal tab facing you and the writing facing away. The left-hand leg as you see it (G) goes in row 8 with the end of the diode. The middle leg (A) goes in row 7 with the wire link, while the right-hand leg (K) goes in row 6.

Step 11:

Insert the 47Ω (yellow-violet-black-gold or yellow-violet-black) resistor from row 6 to row 2.

Step 12:

Peel two wires from the ribbon of plug-to-socket jumper wires, choosing one dark and one light colour.

Step 13:

Insert the LED with the long leg in the socket of the light coloured wire and the short leg in the socket of the dark coloured wire.

Step 14:

Insert the pin end of the wires from step 13 into the breadboard, with the light wire in row 2 and the dark wire in row 1.

Note: Many, but not all, LEDs have a flat spot in the rim at the base of the plastic lens package to identify the negative (Cathode, or K) leg when trimmed. The one we used was completely round.

Step 15:

Insert the pin of one wire from the reed switch into the upper (+) rail (red), and the other 26.

Step 16:

Insert the pin of one wire from the micro/limit switch into row 26, and the other into row 22.

Step 17:

Insert the pin of one wire from the bed lamp switch into row 22 and the other end into row 15.

Step 18:

Insert the buzzer wires so that the positive (+, red) wire will go to row 15, and the negative (-, black) wire will go to row 17. You may have to strip some more insulation to get these to stay in place.

Why is it Important to Test Your Projects?

For some people, ourselves included, there is a temptation, after having assembled a circuit or kit, to simply keep going and install your work in its enclosure, case, or artwork.

However, there are several reasons to test first, and all are designed to save you headaches later.

Making sure everything is connected correctly

All of us are prone to mistakes. When we assume that we haven’t made any, problems can occur. When testing the switches and connections in this project, you are really making sure everything works before anything depends on it.

Changes are easy to make

While your circuit is sitting open on the workbench, it is easy to make any changes you need. Let’s say that you have missed a wire with the screw in the terminal on the bed lamp switch, and instead, the wire is sitting above it, loose. This happened in the prototype, and it was easy to fix because everything was still accessible. It was only during testing that we realised that the switch worked sometimes and not others. This would have been harder with the switch installed in the book, where screwdriver access is hard. The same applies if you mix up the terminals on the limit or reed switch.


When testing, you get familiar with the physical operation of your circuit and its parts. This helps you to understand what’s going on, and in turn, helps you install everything correctly. For example, knowing how the limit switch functions will help you know how close to the edge of the book to install it. In addition, by having to do everything step by step in testing, you get familiar with the process of the circuit, which can help you use it, change it, and expand it. In this case, you get familiar with the fact that if either the limit or reed switch is actuated by the book cover or magnet respectively, the current will not flow.

Step 19:

Insert the battery holder wires into the upper rails, red to the upper (+) rail (red) and black to the upper (-) rail (blue).


Now, it’s time to test the circuit. Carefully insert four AA batteries into the holder, noting the polarity embossed on the plastic. Before you insert the last one, place the magnet for the reed switch next to the switch, then insert the battery.

Hold the limit switch in with your fingers, as shown here, and take away the magnet. If you release the limit switch, the buzzer should sound and the LED light. If it does not, hold the limit switch again, and press the bed lamp switch. Now release the limit switch to try again. If you still have no joy, it is time to remove batteries and check connections and component orientations.

If you did hear a sound and see light, you will notice that the LED stays lit after you press the switch and stop the noise. This feature lets you know if someone has been into your safe while you weren’t around, and the only way to turn it off is to disconnect the battery.

If all went well, you’re done! Read on to find out what’s going on in the circuit and then you can make the craft project.


This circuit has two sections: The switches and the SCR/LED area.

The three switches are connected in series, one after another. First in line is a reed switch, which has two tiny metal ‘reeds’, or thin strips, which are closed together by the presence of a magnet.

Next is a ‘micro’ or ‘limit’ switch, which is a special push button switch in a housing with a metal lever that pushes the button. These are used to sense the limit of travel in machines, hence one of their names. When something touches the lever, like the arm of a machine moving, the switch button is pressed.

Both the reed switch and limit switch have NC and NO states. This stands for Normally Closed and Normally Open. When discussing switches, ‘normal’ refers to the unactuated, or resting state, with nothing acting on it. So, for a reed switch, ‘normal’ is without the magnet acting on it. For the limit switch, this is with nothing touching it. ‘Closed’ means the contacts in the switch are together, and ‘open’ means they are not.

So, for a ‘Normally Closed’ contact on a reed switch, the contacts are held open by the magnet so that no current flows, and close when the magnet is taken away, allowing current to flow. Likewise for the limit switch, but instead of the magnet, something presses the lever.

Looking at the circuit diagram, you can see that the current must first flow through the reed switch. If the magnet is not there, the contacts are closed, and so current can flow to the limit switch. If this is held down by the cover of the book, it is open (because ‘Normally Closed’ means with nothing touching it), and so the path stops here. But if someone opens the cover, the contacts close, and current flows through to the third switch. This is a master on-off switch, and its use is discussed later. If it is ‘on’, current can now flow to the rest of the circuit.

Now current has arrived at the buzzer, which sounds. It also flows through the diode to the SCR. More on that shortly. If someone is quick, they can open your book, take something, and close it quickly. Or they can press your master switch to shut the alarm off (if they figure out what the unlabelled switch is for). But current has flowed through the diode to the gate of the SCR.

A Silicon Controlled Rectifier (SCR) is a very cool component. It is a special construction of a PN junctions that function as a ‘latch’. Diodes normally have two pins, but the SCR has three. One is the Anode (the positive side, marked ‘A’), and one is the Cathode (the negative side, marked ‘K’ for the German word ‘Kathode’). The difference between this and a normal diode is the third connection, the Gate, marked ‘G’.

In a regular diode, current can flow from the anode to the cathode, minus a small voltage drop. This is called ‘forward bias’. If the current is flowing to the cathode, it is blocked. This is called being ‘reverse bias’ and is how diodes are used to control current flow. In an SCR, current is still always blocked if the device is reverse biased.

The difference is that current will not flow with the anode connected to positive either, as a normal diode would. That is, until a current is fed to the gate, which has the effect of ‘switching on’ the SCR. The useful thing is, after this current (which has to be bigger than a value called the ‘holding current’) has disappeared, the SCR will continue to conduct until the current flowing through it stops.

We use this feature so that if anyone opens your safe while you are away, current flows through the switches and buzzer, but also through the 1N4004 diode to trigger the SCR, which has its anode connected to the supply so that it feeds current constantly to the LED until power is removed. In other words, to reset your alarm, you have to disconnect the battery.

One last point before we move on to the craft: Don’t be tempted to use a flashing LED. These have a small circuit in them that flashes the LED, and when they’re in their ‘off’ state, such a tiny current is drawn that the SCR does not notice it. The SCR will turn off as though no load current is drawn, automatically resetting your alarm and defeating the purpose of having the SCR and LED.

Now, on to the craft!!

Construction: Making the Book Safe

For the craft side of your new safe, we have two options for you. The main one is to convert an old book into a safe, but if you don’t want to do this, we have a back-up option: Using a gift or craft box to mount your electronics in instead.

The book project involves lots of carving pages with a sharp knife, as well as down time while you wait for the glue to dry. If you want an instant project or if your adult can't be with you at the time, then choose the box option. Have a read of the instructions for both and see what you think.

Hardcover books work best, and make sure you choose one that no one will read. You will probably upset someone if you cut up a classic, so stick to C-grade titles if you can. Ours came from a local op shop, and our gift box from a dollar shop. The balsa wood is available from many toy, hobby, or hardware stores.

You will notice that we use several different books in our instructions. Because our photo shoot had to happen in one session, we had several books ready for different stages, so our glue drying time wasn’t an issue. Yours will always be the same book, don’t let yourself get confused and think you need to be working on multiple books too!!


If you’re using the craft/gift box option, simply skip steps 1 to 12. Proceed from step 13 onwards, as the process is the same to mount the electronics no matter which safe you choose!

Step 1:

If there is a dust cover on your book, remove it and keep it aside. With an adult, use the knife to carefully slice the book from its cover. Keep the blade facing away from you so that if it slips, it only hits air.

Step 2:

Cover your work area with news or scrap paper.

Step 3:

Pour some PVA glue into a container and mix in a tiny bit of water. About 6 parts water to 1 part glue worked best for us, but different brands of glue will the thicker than others. You need something that ends up about the consistency of thickened cream.

Step 4:

Brush the sides of your book with a thick, but not dripping, coat of PVA.

Step 5:

Sit your glued book on newspaper, put more newspaper on top, and put something heavy on it to stop the pages swelling with the moisture.

Note: Don’t forget to wash your brush as soon as you’ve finished! Unused glue mix can be covered with cling wrap and placed in the fridge. Write on it a with a marker just to be safe.

Step 6:

After the glue is dry, repeat the process for a second coat.

Step 7:

When dry, use the pencil and metal ruler to mark a 2cm margin around the ends and edge of the book. Don’t worry about the spine side.

Step 8:

Here is the tricky part. Have an adult carefully cut along the margin with firm pressure, first along the two ends, then along the edge. Press firmly in the corners or they will not cut properly. Work around ten pages at a time.

Step 9:

Pull out the cut pages as you work them.

Step 10:

Continue cutting through the pages a few at a time, until you get all the way through the book. You may need several sessions to avoid losing your mind.

Step 11:

Add a bead of glue to the bottom of the shell made by the cut-out pages, and along the two remains of the spine.

Step 12:

Position the book on the back cover, close the cover so the spine sticks as well, and close it to allow it to dry.

Step 13:

With the book dry, open it up, and position the breadboard, battery pack, and reed switch as shown.

Step 14:

Using the outdoor double-sided tape (which is much stronger than the normal type), mount the limit switch to the side of the book, with the lever above the top.

Step 15:

Mount the reed switch near the spine, as close as you can, with more double-sided tape.

Step 16:

Mount the bed lamp switch the same way, but a little way out from the board.

Step 17:

Measure the width of the board and battery pack, and cut some balsa wood so that it clears the widest and highest points on the circuit, as shown. Cut two pieces the same size, using the box cutting knife and metal ruler. Note, however, that one end has a cut-out for the reed switch to mount next to the spine. You will need adult help here.

Step 18:

Use PVA glue to attach one piece of Balsa to each end.

Step 19:

Cut a piece of balsa the same height as before, but long enough to span the width of the book as shown. Glue the edges and insert it into its place.

Step 20:

Cut a piece of Balsa to act as a cover for the pieces you have already installed. It just sits on top and does not get glued down. Remember to cut a notch for the limit switch if your book is not deep enough for the electronics to go under it.

Step 21:

Use a screwdriver to make a small hole near one end of the balsa lid, for the LED to come through.

Step 22:

Insert the LED so that it is visible from above.

Step 23:

Remove the lid (just pick it up from the exposed edge with your fingers) and carefully make a hole in the side panel as shown with a screwdriver. Hold your fingers on the other side to take pressure off the glue, but keep them clear if the screwdriver comes through suddenly. If you have access to a drill, use that instead with adult help.

Step 24:

Using small amounts of double sided tape, mount the piezo to line up with the hole you just made.

Step 25:

Place the magnet on the outside of the book spine next to the reed switch.

Step 26:

Connect the batteries and replace the balsa lid. Close the book cover and make sure it is closing the limit switch lever.


Now it is time to test your safe! With the magnet in place, open the cover of the book. You should be able to open it without the alarm going off, and turn off the master switch. If that works, turn it back on, close the cover, and remove the magnet. Now when you open the cover, the alarm should sound and the LED illuminate. If not, double check no connections have come apart since the first test.

To reset the alarm, open the balsa lid, and disconnect the battery. With the magnet in position, reconnect the battery. The LED should not light up. Close the lid, close the cover, and you’re ready to go!!


It is hard to see when the bed lamp switch is on or off. We chose it because it has screw connections and a flat base that can be mounted with double sided tape. You could engineer yourself a way to mount a toggle or rocker switch instead, so you can see easily if the master switch is on or off.

We used some pretty old books with some odd titles. You probably will too. Why not design your own amazing cover? This could be hand-drawn on paper and glued on, but it would be even better to design one digitally if you have those skills, and print it so that it looks like a real book cover. Choose your own title, cover art, blurb on the back, and you can even be the author!

If you used the craft box, you can decorate to your heart’s content. You could paint it, cover it in stickers, print artwork for it, anything you can think of. You might even choose to disguise it, perhaps as a tissue box, or something else that people would not think to look inside of (so not a shoebox!).

If you find the magnet is not strong enough, you may need to investigate super magnets. These are much stronger but carry injury risks. Whichever magnet you use, you could build it into something like an ornament or other disguise to make your own secret key.

You could also make a magnetic closure so that the cover of your book safe stays well closed. Perhaps a magnet recessed into the pages and a strip of thin metal glued under the cover?

Now all you have to do is fill your safe with chocolate, personal stuff, or things you’re hiding from your sibling!