When I came across this old wooden door bell at a boot sale I thought that it would make a great case for a nixie clock. I opened it, and found that the large transformer and solenoids that makes the bell ring, occupy most of the space. My initial idea was to strip it all out and use some other method of alarm. But after a bit of pondering I thought perhaps it's possible.
Challenge accepted !!!
As this clock uses several up-cycled parts and components which you may not be able to find exact pieces, this instructable is a guide to creating something similar.
Step 1: Preparing the Bell and Case
Old bells like this were made by different companies for different uses; Hotels, shops and telephone extensions. This one had a large transformer coil, so I guess it was probably a telephone extension bell used in a large shop or factory.
Disconnect and unscrew the transformer. (Don't be tempted to connect it to the mains power. It will probably catch fire) Once removed you can now begin to test the solenoids which actually make the bell ring.
With only 5 volts this bell began to ring nice and loud. As the contact of the bell is just a piece of metal and an adjustable point, it is very noisy (electrically) causing lots of radio interference. Also, the solenoids produces large amounts of back EMF each time the contact disconnects. This caused the controller to crash. It is very important to isolate the interference and EMF as much as possible. To do this, I began to experiment with different value resistors to reduce the current in the solenoids which, in turn, would reduce the emf. Also it would reduce the force of the bell strike. The sound of the bell is a bit too loud for me anyway. (I want to be woken up gently, not like world war 3 was about to kick off) I found that a 6 ohm resistor worked great. The switching of the bell is done via a transistor. (A relay would work better but I didn't have one available so I used a salvaged BU407. Other transistors will also work ) A flyback diode is used across the solenoid to suppress EMF . (see schematic)
Next the solenoids needs to be shielded. I used some stainless steel plate. For the shape, I made a mock up in paper first and then used that as a template. These methods of isolation work but not completely. The nixie tubes flicker a bit but a least the controller does not crash anymore. I could continue experimenting and isolate further, but I actually like the flicker. It gives the clock a bit of vintage character. (In fact, I have seen some nixie clock kits that have deliberate flicker and fade written into the code as an option)
Put the cover back on and decide how you are going to mount your tubes. Originally I was going to surface mount them; but with lack of space inside for the sockets, I started to play around with different pieces of wood and metal. I had a piece of metal in the shed and thought it was a good angle for the display. I drilled holes for the sockets and then small holes in the wooden bell cover for the wires.
Step 2: Schematic and List of Components
Sorry if the hand drawn schematic is not clear enough. any questions, please contact me. (I don't have any schematic designing software. If anyone can recommend a free and simple one, I'd appreciate it)
Arrange your components so they all fit nicely on the strip board taking into account where the wires for the nixie tubes, button switches and power supply are going to go.
For the clock
GN4 nixie tubes and sockets X 4
Old telephone bell
1307 RTC module
ATmega 328p with Arduino bootloader
74141 bcd or Russian equivalent
817 photo transitors X 4
16 Mhz crystal
7805 voltage regulator
Capacitors: 100uf 16v, 220uf 16v, 22pf X 2,
Resistors: 10kohm X 2, 15kohm X 2, 6ohm, 500ohm,1mohm,1kohm
Transistors MPS42, BU407
Push Button switch X 3
Prototyping strip board
Header pins and sockets for easy connection of tubes, button switches and 12v power.
For HV power supply
100 uh induction coil
UF4004 diode (must be ultra fast !!!)
Resistors : 1k,10k,2k2,220k,
capacitors : 2.2uf 400v, 470uf 16v, 2.2nf
Step 3: Some Images and Video
Step 4: Clock Functions
The clock has 5 modes. The three button switches are mode, set and adjust.
mode 0: clock hours/minutes
mode 1: clock minutes/seconds
mode 2: day / month
mode 3: year
mode 4: alarm
mode 5: scrolling digits.
The mode button selects the modes in this order except mode 5 which is automatic at power up and every five minutes. In date mode the mode will return to clock mode after a few seconds. Pressing the set button in any mode will allow you to change that value using the adjust button. Pressing adjust button in alarm mode will toggle alarm set off/on indicated by the colon. When the alarm is triggered, any button will stop the bell ringing.
Step 5: After Thoughts & Code
Thing's I would do differently.
This is a great clock that looks cool and works well, however, if I were to make it again I would....
1. use DS3231 for greater accuracy; also it can monitor and display temperature.
2. set up an interrupt in the Atmega328 to switch the mosfet. This would lower the component count saving space.
3. use a small relay to trigger the bell.
I wrote the code for this clock quite a while ago. My coding skills have improved greatly since then so I should re-write it. Feel free to change and improve it.
Good luck with your project.