The video has a German voice-over but also English subtitles. So you can watch it or read the instructable, which basically is the same text.

There is one problem with bike locks. No matter how solid and expensive they are, every thief can break them with enough time and the right tools. So I came along with an idea. I want to upgrade my bike lock with an electronic alarm, so that thieves cannot steal my bike unnoticed and hopefully give up.

Step 1: You Will Need

You really do not need much for this modification. Just an alarm like you can buy for windows or doors and a few cables. The alarm will later be installed between the chain links. I also used some 2 part epoxy resin, acrylic sealant, a soldering iron, bare wire and a vice.

Step 2: Disassembling the Alarm

First you have to disassemble the alarm into battery compartment, circuit board and speaker. We do not need the plastic casing any longer, because it is too big. Before you unsolder the cables, mark which connection goes where. This makes the reassembly easier. Now you have to cut the piezo speaker and the battery compartment free, leaving as little plastic as possible on the parts but avoiding any damage on the electric components.

Step 3: Modifying the Circuit-board

We can now unsolder the switch on the circuit board with some stranded copper. By the way, you should never clamp the board as tight as you can see in the video, I just did not realize in the moment. The contacts, where the switch was, should now be connected as if the switch is turned on. I additionally replaced the reed switch with a smaller one to save more space. These reed switches are always turned on when there is a magnet nearby. We can later use this feature to turn off the alarm, when it gets activated. I also bended the black coil inwards to make everything fit nicer.

Step 4: Making the Parts Shockproof

Now we have to wire up the circuit board and the piezo speaker. Leave the pieces of cable a bit longer; you can always trim them afterwards. To shockproof everything a bit more and to avoid short-circuits, I covered the whole thing with another layer of epoxy resin. But keep attention you do not cover the soldering joints of the reed switch yet!

Step 5: Disassembling the Bike Lock

While the resin is hardening, you can already separate the bike lock from its fabric cladding. In order to do that I had to pull out and cut a steel wire first. For the fabric I did not find a better way than to just drag it back and forth until it detached. Repeat this on the other side of the chain. Now you need a piece of a robust cable around four times longer than the bike chain. This cable has to be inserted under the cladding and all the way to the other side. Therefor I used a large iron nail and tied the cable to it. I pushed the nail inch by inch underneath the fabric and then tied the cable around the last chain link. Now the nail should be pulled back again through the cladding, ideally on the other side of the chain, where the first bit of the cable now lays. The result should look like this.

Step 6: Arranging the Parts

Now you can think of ways to arrange the free electronic components onto the chain links, where the ends of the green cable stick out. This cable can now be soldered to the joints of the reed switch. All the components can be soldered together like they were before. All accessible soldering joints should be sealed with epoxy resin or acrylic sealant, which we will need more of later.

Step 7: Mounting the Parts Onto the Chain

After everything dried the parts can be mounted onto the chain. The piezo speaker should be installed with a small gap to the chain link and only glued on the outer ring to allow the speaker to vibrate freely. After that, another layer of sealant can be applied to make it more shockproof.

Step 8: First Test

For testing we put the batteries back into the compartment and just cut the green cable. Remember to have a magnet nearby for deactivating the alarm. When everything works so far, it is almost done. To test the alarm in the future we just solder the old switch to the newly created ends and tape the switched on switch up so that it cannot change its position. Now the fabric just has to be pulled over the alarm and attached to the chain again.