The first two versions of the clock were not really DIY friendly
because of the Altera CPLD used. I decided to design a new version
with a PIC microcontroller and SAA1064 LED drive ICs. When I
started the design some time in 2009 or so, the NXP SAA1064 seemed
to be a good choice: easily available, cheap, uses I2C so just 2
I/O required, supply voltage up to 15V so can drive large displays
directly, drive current programmable via I2C for easy brightness
The project was stalled for years but at 2013 I was bored enough
to finish the design. Even then there was no indication that the
SAA1064 would be discontinued. But now NXP has obsoleted it, and
last time buy will be 30.9.2014. If you are planning to build one
I recommend to build the version 4
which has non-obsolete and cheaper hardware, and more features.
Here's the clock in action on Youtube.
The version 3 uses slightly larger, 58 mm (2.3") displays, which
can be obtained from eBay at cheap price, around 12 EUR for a pack
of ten. The best matching flat top LED for the displays that I've
found is Kingbright WP483SRSGW (or older code Kingbright
L-483SRSGW). This is actually bi-color LED but when mounted as in
assembly drawing, it lights red. I've also used shrink tube around
the LEDs to prevent light leakage from side. The Youtube video
above still has different LED type which is dimmer and different
color than 7-segment displays, without shrink tube. The image on
top of this page has the mentioned Kingbright type.
One major drawback of previous clock versions was lack of battery
back-up, which meant that after power outage, you would need a
chair to climb to the wall to re-adjust the time. Version 3 uses Maxim
DS3231 RTC which provides very accurate timekeeping at very
low power (and a price tag to match...). But very clever chip
altogether. Another improvement is IR remote control, which allows
to change clock to display time, date or temperature (currently
using internal temperature sensor of DS3231 which is not that good
since the electronics warm it up quite a bit). Also the
brightness can be adjusted. A compatible remote control can be
acquired from eBay very cheaply, search for 'keyring universal
remote' and choose the cheapest one you can find. In case the
remote is lost, there is also a joystick-style switch on the clock
which can be used to control the clock.
The schematics are pretty straight forward. The PIC18F14K50
firmware makes most job. It decodes IR messages from the TSOP4838
receiver, reads joystick switch, controls the displays connected
to the three SAA1064 drivers and communicates with the DS3231 RTC.
A 7805 provides +5V to PIC, DS3231 and the IR receiver. The
SAA1064 chips operate directly from input supply to be able to
control the large displays. I recommend using +9 V input supply to
reduce power loss on the display drivers, but +12 V works also OK.
The schematic is available in PDF below:
kello_v31.pdf Schematic version 3.1, changed one joystick line from pin 18 to pin 16 with added pull-up R6 as pin 18 was not working properly as digital input for some reason.
The PCB for the clock is again designed using Cadsoft Eagle. The
design is single-sided with two jumpers. I have designed it to be
possible to etch at home, but after some research ordered from a
Polish company http://en.obwody-drukowane.com/
The price is very competitive and the quality is good. The design
tries to minimize the board area and keep display side as clean as
possible. So mostly SMD at back of the board. The segment order
for SAA1064 chips is chosen to allow simple single-side routing.
This causes some more work in FW but not bad. The assembly drawing
in PDF, Eagle design in .zip and BOM can be downloaded below:
The firmware for the clock is written with CCS PIC-C Compiler. The
source files package including compiled .HEX file are available
for download below. The main source file would benefit from some
clean-up and/or partial rewrite, especially the menu is coded in
awful style. But it works.
The IR receiver code is based on San
Bergmans's NEC reveiver code, but (mostly) converted to C.
The SAA1064.h is based on a version found in the CCS forums. I
can't remember where I've gotten the ds3231.c code. It's quite
probably based on some other Maxim RTC code with my modifications,
as I remember doing the temperature readout part myself.
Kello_v3_110.zip 1.8.2014 Source
files for HW v3.1, FW v1.1. Contains .HEX compiled with CCS
Kello_v3_130.zip 5.9.2014 New features added: Saves brightness level and display mode to DS3231 NVRAM. Adds 'toggle' mode which alternates time, date and temperautre display modes. Possibility to adjust brightness also via 'set' menu, earlier only possible with remote button.
kair.us/ projects/ seven segment wall clocks/ kello version 3
page created 1.8.2014
last updated 14.7.2018 email@example.com