Hacking a London Underground Jubilee Line Door Button

The London Transport Museum shop is selling decommissioned door buttons from the Jubilee Line (both left and right are available). If you are thinking of carrying out a project which needs a button and an indicator light of some kind, you would be 'hard pressed' to find a more unique button than one of these.

These pieces of London Underground railwayana contain a very useable switch and a set of LED's. One person has made "the worlds fanciest doorbell", I have made a Philips Hue light switch out of mine (source code in Github). The possibilities are endless.

But these door buttons are sold without any information as to how they work. So how do you go about taking them apart, removing the bits you don't need, and then incorporating the switch and lights into your own DC electronics / Arduino / Raspberry Pi project? This guide Instructable will show you how.

Supplies

A London Underground Jubilee Line Train Door Button from the London Transport Museum.
7mm Box Spanner or a Deep 7mm Socket with ratchet - this needs to be as thin as possible.
A 7/32" socket or nut spinner - if you don't have this you can get away with a pair of pliers
Wire strippers
Cutters
Cleaning products: Washing up liquid, clothes washing powder, sponge, tooth brush, wire wool
9v battery
9v battery connector
220 Ohm Resistor

Step 1: Get in to the Casing

Turn the case over and you should see a nut in each corner. Get your 7mm box spanner or socket and remove the nuts. There is not much space around the nut so you will need to get the skinniest one you can find. Mine was a little too big, but with a fair bit of force and determination, the nuts came off.

After this you should be able to pull the back off easily enough.

Step 2: Take a Look Around

Inside you will see 2 PCB's. The bottom one contains a 556 chip, a rectifier, some resistors, capacitors and a diode. A rectifier converts AC input to DC, and this nice Github repo by IgnoredAmbience suggests the 556 is used to send a 500ms signal when the button is pressed.

These are all probably very useful for train doors, but all I want is a light and button we can integrate easily into our DC electronics projects - this is all on the top circuit board.

Step 3: Disconnect the LED's From the Bottom Circuit

Take a look at the top circuit. Each time a letter C appears there is an LED connected on the other side. Taking a closer look shows there are 8 of these LED's, and they are all connected to the purple wires at 4/B and 3/A in the bottom left of the circuit.

These 2 wires connect to bottom circuit near at points 3 and 4, just above ZD1 and R1.

We are now going to separate the LED's from the bottom circuit board - get your cutters and cut the 2 purple wires close to the bottom circuit board at points 3 and 4. With your wire strippers, strip back about 5mm on each wire so we can attach crocodile clips to these in a later step.

Step 4: Disconnect the Button From the Bottom Circuit

Take a look at the centre of the top circuit board. You can see a circuit leading from the centre out to the 2 purple wires on the bottom right. This is how the button is wired up.

Take your cutters and cut the 2 purple wires as close as you can to the bottom circuit board at points 5 and 6, then again use your wire strippers to strip back about 5mm on each.

Step 5: Test the Button

To ensure the button is working, put your multimeter into continuity mode. Connect one test lead to one of the purple wires on the right, and the other test lead to the other purple wire on the right. Your meter should read 0. Now press the button, your meter should now read 1.

Great, we now have the button.

Step 6: Illuminate the LED's

Now this is the really fun bit - lets light up those LEDs.

Take a crocodile lead (preferably black) and connect one end to the black wire of your 9V battery connector and the other to a 220 ohm resistor.

Take another (I've used green) crocodile lead and connect one end to the resistor and the other end to the left most purple wire.

Take another crocodile lead (preferably red) and connect one end to the purple lead 2nd to the left and the other to the red wire on the battery connector.

Now, connect the 9V battery - the LEDs should illuminate.

If they do not light up make sure you have connected the purple wires round the correct way and your battery has sufficient charge.

If you were wondering why I used a 9v battery and 220 ohm resistor without knowing the spec of the LED's a bit of trial and error was involved rather than any formula...

25 to 30 mA is generally the maximum recommended current for an LED. Using the multimeter to check the current, with a multi voltage power supply I worked up from a low voltage and tried different resistors until I got something under that figure with a nice brightness, and also a voltage a common battery could supply. The 9v battery and a 220 ohm resistor gave a current of 19.1mA.

Step 7: Remove the Lower Circuit Board and Wires Going Into the Casing

As you have just found out, if all you need is the switch and light, then you do not need the lower circuit board. Undo the 4 nuts and lift off the lower circuit board. A 7/32" socket or nut spinner would be ideal for this, but you can also get away with using a pair of pliers to loosen them and then unscrew them with by hand.

There is also 2 purple wires going in to the casing. I haven't figured out what these are for, if you do know then please tell me. I cut mine as close to the casing as possible then covered with insulation tape.

Step 8: Disassemble Ready for Cleaning

Remove the 4 nuts on the top circuit board and remove the red washers. The circuit board should then lift off.

Pull the LED's out of their holders - they are held in place with white adhesive, then everything should come apart easily enough.

Step 9: Cleaning

Now this is the hardest part of the Instructable, it required a lot of effort and you may find a better way, but it worked for me:

1. Take all non-electrical parts (so remove the LEDs and circuit boards etc) and soak in a bowl of warm water and washing up liquid. After an hour or so, take out and scrub with the sponge and tooth brush

3. Leave overnight in a bowl of dissolved clothes washing powder

4. Take out and scrub again with the sponge and tooth brush

5. Repeat the previous 2 steps

6. Any dirt that is left, remove it the wire wool (I should have been more delicate on the red plastic ring as I have scratched mine)

7. Rinse off any loose dirt and dry off

Step 10: Reassembly

It is time to get the button back to together. Hopefully you were paying attention to how all those parts came off?

Ok, if not:

Lay the metal plate face down.
First add the translucent orange disc with the protruding side facing downwards.
Next add the 'open' button (make sure it is the right way up)
Fit the metal spring into the back of the button
Add the clear ring
Add the circuit board
Slide the LED's back in to place
Add the 4 red washers
Add the 4 nuts and tighten

Step 11: Incorporating Into a Microcontroller

Now you have a nice clean light and button, what do you do with it next?

The button can be used as per any other switch / button such as in this tutorial for Arduino or Raspberry Pi.

The LED's are a bit more complex as they require 9V power, but the Arduino or Pi usually give 5V at most. I got around this by powering the LEDs with the 9V battery, but used a Mosfet (IRLML6244TRPBF N-Mosfet) so my ESP32 could switch them, see here for a tutorial.

I found the metal casing would short the microcontroller, so placed a small piece of plastic on the lower part of the button.

The code for my Philips Hue button is in Github here.