Now it’s time to turn our attention to the electric door strike – this is the catch which sits in the door frame and is normally closed, so won’t allow the door to open unless 12V is supplied to the strike, in which case it allows itself to be opened, effectively unlocking the door.

If we open up the strike you can see the two very thin wires which were meant to be soldered to contacts on this black plastic top. On this one, the wires are so thin and flimsy that they’d broken off in transit, so to remedy this we’ll remove the black plastic clip holding the contacts and solder some longer pieces of wire on. We’ve got some handy as we just chopped it off the power supply, so let’s take a couple of lengths of wire from that, snip them and then strip and twist the ends.

Now it’s time to get your soldering iron out and tin the exposed ends of the wires with some solder before bringing over the door strike and soldering our new lengths of wire to the two very fragile exposed wires up top.

Now it’s time to test the door strike so let’s wire it directly into our 12V power supply that we created before and watch what happens when we switch on the power.

The door strike now needs to be installed into your doorway, in place of your existing Striker Plate. Here, I’m installing the system into this mock-up of a door frame that I’ve knocked together as a demo, so you’ll have to excuse the slightly shoddy woodwork! On mine, the electronics are going to be located within the stud wall, but make sure you comply with all building regulations for your region before continuing.

The first step is to drill a reasonably-sized hole to allow cables to be poked through the timber to the power supply which will sit below. Run the cables for our dual-voltage supply up the vertical beam and drill a small pilot hole through the centre of the choc block and then screw in place. Then, poke the USB power cable up through the hole to provide power to the Raspberry Pi. Next, we need to decide on the arrangement of our components within the stud wall, and I’m using a spare block of wood from a previous project to create a little platform onto which I can mount the GPIO expansion board. The relay switch will sit just here on the vertical beam, next to the electronic strike.

The Raspberry Pi’s case has mounting points on its base, so measure the distance between them and then transfer that measurement to the horizontal beam, marking two points to insert some small screws, nearly all the way down but not quite. With those in place, the Raspberry Pi can be placed on and slid back, holding firmly in place. The power cable reaches neatly to the socket, and can now be plugged in.

Secure the mounting block in place, and try not to split the wood like I did, oops! Drill a pilot hole if necessary to avoid that. Then, screw down the GPIO Expansion Board and secure the relay switch in place near the electronic strike. Next, wire in one of the cables directly from the door strike to the NORMALLY OPEN terminal on the relay. Either one will do, as it’s not a polarised component. Then connect a length of cable to the GROUND terminal, poke it through the hole we drilled and then connect it to the negative terminal of our dual voltage power supply. Finally, connect the door strike’s other wire to the 12V positive supply.

Next we need to cut the holes in the wall to mount the RFID reader and the LCD touchscreen. I marked the holes out first on a cereal packet to get the placement just right, before using that as a guide to mark out the holes on the wall. Drill a hole in the centre of the area to be cut away, then use a jigsaw to cut out both squares. I used some insulation tape to make the edges look a bit neater on my demo unit, you can of course make yours look much better than mine!

To mount the RFID reader, I’m using some hot glue on the half of the front that doesn’t contain the infographic, and then pressing that up against the wall. I also used some gaffer tape to help secure it in place, as this USB model isn’t one which is specifically intended for wall mounting. I’m mounting my LCD in the same way, which isn’t ideal and I wouldn’t recommend this method in the real-world, but it should work just fine to demonstrate for this build.

Next, connect up the miniUSB cable to the RFID reader and boot up the Pi to test it’s working.

Now it’s just time for the finishing touches, so let’s use some blocks to hold the power supply in place, and then trim down the excess length on all of our cables. Carefully disconnect them one by one, cut to the appropriate length, strip and twist the ends and then reconnect. Then, use tacks to hold them neatly in place.

And we’re done! To summarise our build we have an electronic door strike which is activated by the relay switch here, which in turn is controlled by the output from our Raspberry Pi which denies or grants access using the RFID reader and touchscreen input. It also connects to the internet via WiFi to use a web service to send the one-time passcodes via SMS to the user’s phone, acting as THREE factor authentication. The whole thing is powered by a computer power supply which sits down here inside the wall

So there we are, a ridiculously secure door entry system for that most secret of rooms in your home or office. If you’re going to build one yourself, there are links at the start of this article for all the low-cost components you’ll need to buy, as well as a link to download my Python script to load onto your Raspberry Pi. Make sure you send me a picture or video of your working setup – I’m @paulfp on Twitter.