Sunday, 6 January 2013

LED Clock Part 4 - The Finished Product

Following on from the previous articles on the LED Clock (a simple replacement and a teardown of the original clock and a more complicated replacement), the parts I ordered from CPC have finally arrived, I've now finished the replacement unit. And here it is.
OK, so it looks like the original. Well yes and no. Yes, it does look like the original, that was the intention. However, it is now a 24 hour clock and the time is derived from a battery backed real time clock. The colons now flash at 1Hz, previously they were on continuously - the TMS3450NL chip on the original clock does have an output to flash the colon, but the designers decided not to use that and wired them permanently on.
Inside it looks different. The original display has been reused, but this now has a row of pin headers and plugs into a new board which contains (from left to right) the CR2032 battery for the RTC, the two MCP23008s to drive the common anodes, the ATMega328P microcontroller and the DS1307RTC. Here it is with the display removed.
On the bottom right are the two common cathode drive transistors and the power supply circuitry. The original transformer wasn't ideal so I've replaced it with one of a similar size (and vintage), with a very simple rectifier, capacitor and 78L05 based power supply. More than enough to supply the 60mA it uses.

The pin header on the left pokes out of the side of the clock, where there used to be a slide switch. This allows the 'Arduino USB2Serial light' to plug in to reprogram the unit, or to set the time.
I could have just gone out and bought a new clock, but that would have been too easy. This has been an interesting project, and I'm happy with the result.