16 March 2016

biNixie Clock schematic diagram

For those interested in reproducing my biNixie Clock, here is the (most likely) schematic diagram:

I have realized I did not take notes while building it, probably because it is so straightforward (and a copy of other designs).

You need to pay attention to the current limiting resistor (one per Nixie tube), which depends on the tube model and your anode voltage ("HV" in the diagram).

Put everything on sockets so it can be easily replaced or reused.

At current prices (3/2016) the whole thing should cost less than 30 Euro if you use common Nixies like B5092/ZM1020.

The firmware for Arduino and compatible boards (ATmega368) is here https://github.com/jh1995/biNixieClock.

Please share pictures of your biNixie clock as well as firmware improvements!

15 March 2016

Four digit display with HP 5082-7300

Arduino Nano counting...
My interest for old display technologies got a bit of satisfaction when I lighted up a bunch of HP 5082-7300 chips. These are 8 pin displays with an integrated BCD decoder (0-9, dash, blank) plus RHS decimal point that emulate a dot matrix configuration. The required power supply is 5 V and today they can be easily driven with the microcontroller of your choice. The integrated logic includes a latch, so you can connect more displays in parallel and multiplex data through the [Latch Enable] control.

One remark not clear from the datasheet: the decimal point is latched as well as ABCD inputs.

My first application is a simple 0000 to 9999 endless counter. It is a good way to test all dots work, measure current consumption and do other optimizations.

As expected, these little displays produce a good amount of heat. According to the datasheet, at 5 V the current drawn is well above 100 mA, so that's more than 0.5 W of heat to dissipate. To minimize the risk of failure due to heat I mounted my 5082-7300 on a "socket", so I can add a small fan on the side if I want to.

Then I tried to reduce the voltage supply, both to the Arduino and to the display string. The good news is that there is no change in dot luminosity down to 4.1 V and the average current goes down to 60-70 mA per display: that's about 0.25-0.3 W dissipation per display. Cool, isn't it?

Click to zoom and see the logic through the red plastic. Last digit was changing from 6 to 7.
Voltage supply can be further decreased but then pixel luminosity is not uniform anymore.

So, in the final application I can run the whole setup Arduino+HP 5082-7300 at 4.2 V or so. Given the amount of power needed by the display rack itself it makes no sense to optimize the firmware for a lower consumption or use a stand-alone microcontroller in place of an Arduino.

Now it is time to fill the perfboard with a Nano, an RTC and some form of alarm function. And write some code, of course.

I have some HP 5082-7300 spares in case you land on this post looking for this specific part.

04 March 2016

Saba Compact Clock A - display sorted out

With the help of a DVM and the circuit diagram I sorted out few things on the Saba Compact Clock A.

One trace is hidden under the display support frame, and probably it even contains a diode. I cannot unplug the custom IC to look what is underneath it, so I must trust my DVM readings.

The IC does work, since it outputs voltages compatible with a blinking display (typical of "time not set" state), but there's something wrong on the path of the common cathode I couldn't sort out. On the positive side I could light up segments on the display and confirm they are plain red LEDs with a common cathode, so I can re-wire my own cock circuitry if I want to.

I also spotted a curious optimization: segments a-d-g of tens of hour digit are connected together. That makes sense since they always light up together in numbers 2 and 3, and don't display anything higher than 23 (hours) or 31 (day of the month).

Now I might try to replace a couple electrolytics badly corroded (externally) and then give up on fixing this device, for now.