05 June 2016

A clock with Arduino and four HP 5082-7300 displays - 2

Part two of how I designed and built my own bedside clock with a 1970's touch.

The finished clock, in the operating position.
This time my adventure started from the enclosure: a wooden box with the top cover almost completely transparent I had bought to use with a VFD clock but turned out to be too tight for it. Inside there is enough room to fit the whole circuit and ensure air flow for heat transfer (remember an HP 5082-7300 dissipates 500 mW on average!).

I have read online that these displays fail mainly because of overheating or stress from prolonged heat. I do have few spares but I really care to do my homework properly!

Therefore I cut a piece of perfboard to the desired size and started planning the layout: displays would be on the upper-right corner and the rest underneath. I decided that everything should be visible through the glass window (except for wiring) so that it would be easier to describe the clock to friends visiting my place (in that case the box will be moved to a more visible spot, like the kitchen furniture). Nevertheless I tried to keep a clean wiring scheme, pleasant to be seen.

Since these displays have a Latch Enable line, they keep displaying the digit once it has been loaded into their internal buffer. In order to drive these displays I need:
  • 4 lines for the 8421 BCD code
  • 1 line for the decimal point
  • 1 line for each display (4)
Lid open.
So it is 9 lines going from the Arduino-like to displays. Instead of running them through the board I used a set of pin headers just in case one day I will decide to separate logic from display areas. This holds true for +V and GND as well. The effect is a bunch of colored wires connecting two areas of the single board: they add a touch of color :-)

The RTC board is a cheap battery-backed DS1302 I2C module that appears to be pretty stable. I added an LDR to do brightness control (ADC in microcontroller and PWM control on displays) and made provision for two pushbuttons that help setting the time.

As microcontroller board I chose a clone of Arduino Nano v3, because it is small and still has a strong USB socket for firmware updates.

Given the power requirement of HP 5082-7300 displays I need an external PSU, but since I want to run the whole thing at 4.2 V, I had to add a further switching step-down module inside (on the back of the perfboard).

Isn't display brightness affected with the lower supply voltage, you may ask? It is indeed, they look dimmer (and dissipate much less power!), but I will look at this clock at night in a dark room, so that's what I am looking for afterwards! The voltage can be even lower, but then brighness is not even anymore.

And after running it for 24h I came up with few firmware adjustments, covered in the next article of this series.

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