Little notes of what I’ve learned and re-learned so far.
by Jae686 on Oct.24, 2011, under Electronics, Nixie Clock
At this time, the nixie clock has passed trough 2 revisions.
The 1st version
The 1st one was a clock with a purely transistor based nixie driver.
The I2C bus was able to handle the multiplexing fast enough for the clock to be usable, but there were random Micro controller crashes. Those crashes were caused by noise induced on the SDA and SCL lines on the I2C bus (these lines were passing under the wiring of the nixes driver, including the HV PSU. This PSU is nothing more that a boost converter, so electrical noise should be expected).
Although the prototype reached the working stage, it never got past the test type (PCB). The clock failed promptly at its first power up test (aka smoke test).
The root cause was on the PCB design.
Due to the fact that EMI interference from the boost converter could bring the micro controller to a crash, I though it would be a good ideia to leave as much grounded copper on the PCB as possible.
In theory its a good ideia, but it did not take into account that the method used to build PCB can’t achieve the needed precision.
PCB issues, lacquer between tracks.
It was observed that lacquer residue was present between tracks, and that this residue was not always visible.
I believed that this could be fixed, by slightly over-etching the board, but this proved to be ineffective.
And, during soldering, the solder would easily stick to the grounded copper that surrounds the tracks and pads, shorting them. Therefore, this added another point of failure to the test type unit.
Re-work proved quite tedious.
And to make thing even worse, the Micro controller source code was lost, due to the fact that the Arduino IDE saved the sketch the a folder that was not backed up.
Therefore, I’ve chosen to make a simpler nixie driver design.
The 2nd revision of the nixe clock.
PCB, 2nd version
The 2nd version of the nixie clock used anode drivers similar to the 1st version, and used a K155ID1 (equivalent tho the 74141) IC as the cathode driver. The interesting thing about this IC is that it driver 10 outputs from a 4 pin BCD input.
Other diferences include :
- The Micro controler drives the nixies trough its own IO’s instead of the I2C bus
- The I2C in only used to get the RTC data, user IO and this bus is exposed trough a expansion header, allowing the addition of expansion modules.
- The nixies themselves are on the PCB instead of a separate socket.
- Although the 2nd version of the clock uses the same PCB area that the 1st version used, the PCB is separated into 3 boards, the nixie board, that holds the nixies and the nixie drivers, a board the has the micro controller and the board the handles the user IO.
- Nixie clock with PSU attached
Nixie Clock with the PSU detached.
Nixie driver board
