Instructables
Picture of Numitron clock & thermometer
SDC12105.JPG
SDC12110.JPG
SDC12111.JPG
I really like nixie and numitron clocks, but I never worked with them before. So I decided to give it a go. I choose numitrons because of 2 reasons: first of all nixies need a higher voltage than numitrons to work. Nixies need around 170V DC and numitrons only 4,5V so they are safer to work with and don't need a special powersupply.

The timekeeping is done by a ds1307 I2C realtime clock and the temperature is measured with a DS18B20 1wire temperaturesensor.



An Atmega48 is used to proces the data and to drive the segments in the numitrons. I used Bascom to write the code and a MyAvr MK2 Programmer to stuff it into the microcontroller. You can find a free Bascom demo here. The only limitation of the demo version is that you can only compile 4Kbytes of code (but the atmega48 has only 4Kbytes so that will work fine.)

I added this instructable to the microcontroller contest, so give it a vote if you like it.
 
Remove these adsRemove these ads by Signing Up

Step 1: The DS1307 realtime clock

Picture of The DS1307 realtime clock
ds1307.png
The timekeeping will be done by a DS1307 IC. This is a handy little IC, because it not only keeps track of time but also of the date and the day of the week.

For this little project we'll only use it to keep track of the time. Therefore it needs a 32.768kHz quartz crystal connected between pins 1 and 2. We can also add a battery with + to pin 3 and - to pin 4. This enables the IC to keep working when the mainpower is switched off. If you don't want to use this feature, you can just connect pin 3 to pin 4 and everything will work fine.

Pin 5 and pin 6 will be used to transfer the data to our microprocessor. They should be connected to the SCL and SDA pins on your microprocessor. These lines need to be pulled high by a 4K7 pullup resistor.


Bascom makes working with I2C devices easy. You only need to know 4 commands:
  1. I2cstart: This commant will startup I2c communications
  2. I2cstop: This command will stop I2c communications
  3. I2crbyte var: This command reads a byte from the device and stores it in 'var' 
  4. I2cwbyte var: This command writes the variable 'var' to the device
Using the write or read command is not enough, we will also have to tell the device whether we want to write to it or read from it. We do this by using the right address. These addresses can be found in the datasheet. The write-address for the DS1307 is D0H and the read-address D1H (the H behind it tell us that these are hexadecimal figures).

The DS1307 sends and wants to receive data in BCD format. This is a variation on binary for diplays where every digit is represented by four bits. More about that here. Luckily converting from BCD to decimal and visa versa is very easy in Bascom.
  • var = Makebcd(var) will convert decimal, hex and binary into BCD
  • var = Makedec(var) will convert hex, binary and BCD into decimal


The data is stored on the IC in register. You can imagine them as those oldfashioned filingcabinets. Each drawer has its number and contains some info:

00H Seconds
01H Minutes
02H Hours
03H Day
04H Date                                               The H tell us that these are hexadecimal figures.
05H Month
06H Year
07H Control
08H to 3FH Ram

If we want to read or store some data we'll first have to tell the device in which drawer we want to be. We can do this by writing the hex code for that drawer to the device. The device then will grant us acces to that drawer. After you write or read something from or to this register the device will automatically jump to the next one. So there is no need to send the location every time

Now lets put this in code:

For this code you will need to dim hours as byte, minutes as byte and seconds as byte.

First, we will set the clock:


Seconds = Makebcd(Seconds)                             We convert our variables into BCD format
Minutes = Makebcd(Minutes)
Hours = Makebcd(Hours)

reset hours.6                                                            We reset bit 6 of the hoursbyte to make sure that
                                                                                    our Clock runs in 24h modus. If bit 6 is 1 then the
                                                                                    clock runs in 12h modus and bit 5 will then 
                                                                                    contain the AM/PM data.


I2cstart
I2cwbyte &HD0                                                         We tell the device that we want to write a byte
2cwbyte &H00                                                           We start at the register for seconds hex 00  
I2cwbyte Seconds                                                    Adding seconds
I2cwbyte Minutes                                                      Adding minutes
I2cwbyte Hours                                                         Adding hours
I2cstop

 Now our clock is set! Lets read from it now.

I2cstart
I2cwbyte &HD0                                                        We tell the device that we want to write a byte.
I2cwbyte &H00                                                         We ask the device to go to the seconds register.
I2cstop
I2cstart                                                                       
I2cwbyte &HD1                                                        We tell the device that we want to read bytes.
I2crbyte Seconds , Ack                                           We read the data and acknowledge that we want 
                                                                                     to read the next byte too.                  

I2crbyte Minutes , Ack
I2crbyte Hours , Nack                                             We don't ackowledge here so the device knows
                                                                                     that we are done reading.

I2cstop

Hours = Hours And &B00111111                        We remove bits 6 and 7 as they contain other
                                                                                    data. If you are in 12h modus, then you need to
                                                                                    remove bit 5 too

Hours = Makedec(Hours)                                      We convert back to decimal format.
Minutes = Makedec(minutes)
Seconds = Makedec(seconds)

Now we know what time it is.

In the next step we will take a closer look at the DS18B20.

gmg5 months ago

Nice case.

But Numitron segments are cathodes. Aren't the diodes backwards in the schematic?

janw (author)  gmg5 months ago

Numitron segments are filaments that light up in either way. The position of the diodes is correct as it allows the filaments to light up when a positive voltage is applied to them but it block them when connected to GND. This is needed When multiplexing otherwise you might get filaments that glow softly when they should be of because of power flowing 'backwards' to the filaments connected to GND.

gmg janw5 months ago

Now I see you're driving them as common cathode. Should have noticed that. Thanks.

andrea biffi11 months ago
I love it, I will try to build mine! :-)
mazzmn12 months ago
Neat design...thanks for info about numitron...I've wanted to work with Nixie tubes before but didn't like the voltage requirements, this looks like the answer!
PICme2 years ago
Hi Janw,
I have the control board built and sourced the IV-9 Numitrons. There is just one point i hope you can make clear. The Numitrons are numbered IC4 to IC1 in your schematic. How does that equate to the the hours and minutes. For example if the time is 09.24 does ic4=0, ic3=9, ic2=2, ic1=4. This is my first Numitron clock and any help would be greatly appreciated.
janw (author)  PICme2 years ago
The order is correct as you mentioned it.
PICme2 years ago
Hi Janw,
Thank you for your prompt reply i understand the pin connections now.
PICme2 years ago
Hi janw,
I would like to build your excellent clock. Could you confirm the numitron pinots. I understand it is as follows:
(8 pin Socket) pin 1=com
2="b"
3="c"
4="a"
5="f"
6="g"
7="d"
8="e"
Also did you include any option to blank the screen at night. Thanks again for a great project.
janw (author)  PICme2 years ago

Pin Connection:

1 common
2 right hand deciamal point
3 Segment (b)
4 Segment (c)
5 Segment (a)
6 Segment (f)
7 Segment (g)
8 Segment (d)

I did cut pin 2 because I did't use the decimal point, So i ended up with 8 pins instead of 9.

You don't need a blank out option. You can put a powerswitch on the powercord of the device. The battery of the DS1307 will take over the power for the timekeeping and all other functions are switched off. The battery can power a DS1307 for a few years so no worries there.
sgleason12 years ago
Can you add two more numitrons so it can display a temp with 1 decimal plagce, display the time in seconds and display the date?
janw (author)  sgleason12 years ago
Yes you can but bear in mind that you will need to speed up the multiplexing.
masterbalby2 years ago
I have great difficulty to draw a proper pcb.
Can you send me yours?
thank you in advance
masterbalby2 years ago
hello,
why have connected the 13 pin connector JP1 to +5 V while the pin 13 of the second connector is connected to anything.
thank you
janw (author)  masterbalby2 years ago
When I designed the pcb, I had the intention to add blue LEDs under the tubes for more colour. So a 5V supply was connected to JP1. The LED's never made it to the final design but I forgot to remove the 5V. You can leave it out.
thank you very much
astroboy9073 years ago
Hey- what do you think is an ok price for a numitron? Also, how large are your numitrons? Both the total physical size, and an estimated digit hight/width wold be nice- but its up to you. Great clock! I think im gonna build one for my geek lair :)
janw (author)  astroboy9073 years ago
I paid about 25euro for 12 numitrons. So that's around 2euro for one and I think that this is a reasonable price.

The dimentions are:
11 x 8 mm (0.44" x 0.32") for the digit
32 x 10 mm (1.28" x 0.4") total size.


did u enter it in the clock contest

u could win!!!!!!!!!!!
janw (author)  harry potter rules3 years ago
I can't as this is an old instructable and I an not a US citizen
are you from england
janw (author)  harry potter rules3 years ago
No I am a Belgian who lives in The Netherlands.
k thanks :)
TerryKing3 years ago
How-To Information on the DS18B20 Temperature sensor here:
http://arduino-info.wikispaces.com/Brick-Temperature-DS18B20
CThoma0313 years ago
i love the look and operation of it! If you sold these I would be the first customer.
astroboy9073 years ago
Sweet! Mainly I just like the vintage look of the Numitrons :) Found some on ebay for cheap (2$ each!) so i might buy a few and use them in a project (or replace them for my seven segment displays in my 'ible!)

They would make an awesome desk clock!
F-173 years ago
Thats Pretty neat, 5*
n-2-stuff3 years ago
Awsome. Very nice case....
jeff-o3 years ago
I love the case! Very nice indeed.