Old Post Office Clock

[Jeffry P]

I purchased a clock display system from a USPS auction about 20 years ago and recently rediscovered it in my storeroom. There are 20 displays and a power supply. The displays were housed in a metal enclosure that has a window cutout for the LEDs.  The window had a piece of red colored film and the LEDs were behind. When I plug the power supply in it will light all 0's and a decimal point (as seen in photos), but will not start keeping time. I'm willing to send one of these boards( at my expense) to someone who can help me get the thing keeping time again.  I've attached a few photos with descriptions of chips that are on the board and voltages measured at the power supply connector.

Thank you, Jeffry

[Dan Hollis]

These are not standalone clock displays. They are driven by an external
controller over serial. If the controller is not integrated into the PSU,
then you are missing the external controller module.

Personally I would just design a completely new clock using the LEDs.

-Dan

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[Bill Stanley]

Correct, these are a differential receiver to receive the serial (BCD) information, 2x8 shift registers to convert to parallel BCD and 4 BCD to seven segment LED drivers.

With a bit of sleuthing, the schematic can be reverse engineered, reverse engineer the serial format and build or use a COTS CPU like Pi to display anything you wish.

The power supply connector also probably has the serial data (look for wires going to the AM26LS32). Somewhere there will be a connection to inject that serial stream.

  -Bill-

[gregebert]

Those are NICE displays. So, you have a total of 20 boards x 4 per board = 80 LED displays ? Someone went to a lot of effort to wire-wrap those instead of making a PCB.

The logic is very simple: MC14511 BDC-to-7 segment decoder, two 74161 shift registers. As Bill said, you should be able to reverse-engineer this fairly easily, and then you would need just a few simple signals to control the shift-registers (clock, data, clear). You could even use some pushbutton switches to test it out; just be sure to de-bounce the clock signal. The data and clear signals dont need de-bouncing.

[Jeffry P]

I can find lots of clock circuits that are for lower voltage LEDs, but these LEDs are 12v.

[Jeffry P]

Yes the displays are very nice. I was thinking a clock with a few more decimal places might be interesting.

[Jeffry P]

Will an Arduino work? Is there alibrary that will produce these  signals?

On Thursday, December 31, 2020 at 4:19:22 PM UTC-6 Bill Stanley wrote:

[Bill Stanley]

From a software point of view the Arduino should be able to generate the signals. It is possible that a SPI interface will do a lot of the serial conversion.

In terms of a library, once the serial format is known and if (big if) a SPI port can be used, there is probably some library code that could be used as a starting point. I'm sure someone can give a lot better information.

There are a couple of things that I am not sure about based on your photos...

• You indicate that the LED displays are 12V. My guess is that the linear regulator with an adjustment resistor for brightness actually drops the voltage (13.6V) to the display The 4.6V is probably the logic Vcc.
  
• One thing I didn't see were current limit resistors. I wonder if they are built into the display?
• I am also guessing that the displays are not multiplexed just based on the fact that if they were multiplexed, the sequencing would probably need to be handled by the missing "host" since I don't see anything on the board to handle that. Also the fact that the 00.00 comes up and is not extremely bright makes me think that current/brightness control is built into the display and the segments/digits are not multiplexed.

I would be willing to take a look at a board and attempt a hardware reverse engineer to generate a schematic for you. I could also take a stab at generating a document on the driving of the unit.

  -Bill-

[Bill Stanley]

Doing a bit or research, I believe the LED display is (or is similar) to an obsolete part, ELS-4005SURWA/S530-A3 ,

datasheet at  https://media.digikey.com/pdf/Data%20Sheets/Everlight%20PDFs/ELS-4005SURWA_S530-A3.pdf

This part matches the size and pinout of your units. Each segment has 5 series LEDs with an If of 25mA and a Vf of 10-12V, thus no resistors required.

  -Bill-

[Dan Hollis]

Resistor is required.

Forward current on the datasheet is maximum rating.

I don't see any current limiting resistor in the datasheet, diodes will
dump current until they self destruct unless you limit them with a
resistor.

The segments are 12V because they are 5x series diodes, forward voltage
drop is typically 2.5v * 5 = ~12V

NOTE! Decimal point is a single LED, therefore you will likely blow it out
if you use 12v.

-Dan

On Sat, 2 Jan 2021, Bill Stanley wrote:

> Doing a bit or research, I believe the LED display is (or is similar) to an
> obsolete part, ELS-4005SURWA/S530-A3 ,
> datasheet at
> https://media.digikey.com/pdf/Data%20Sheets/Everlight%20PDFs/ELS-4005SURWA_S530-A3.pdf
>
> This part matches the size and pinout of your units. Each segment has 5
> series LEDs with an If of 25mA and a Vf of 10-12V, thus no resistors
> required.
>
> -Bill-
>
> On Saturday, January 2, 2021 at 8:26:58 AM UTC-8 Bill Stanley wrote:
>
>> From a software point of view the Arduino should be able to generate the
>> signals. It is possible that a SPI interface will do a lot of the serial
>> conversion.
>> In terms of a library, once the serial format is known and if (big if) a
>> SPI port can be used, there is probably some library code that could be
>> used as a starting point. I'm sure someone can give a lot better
>> information.
>> There are a couple of things that I am not sure about based on your
>> photos...
>>

>> - You indicate that the LED displays are 12V. My guess is that the

>> linear regulator with an adjustment resistor for brightness actually drops
>> the voltage (13.6V) to the display The 4.6V is probably the logic Vcc.

>> - One thing I didn't see were current limit resistors. I wonder if

>> they are built into the display?

>> - I am also guessing that the displays are not multiplexed just based

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[Alex]

I would suggest that these were never actually used as clocks, due to their somewhat homebrew look. I suggest they were weighbridge / scale displays which may of been constructed to give a large readout, driven from the aux port on a scale unit. Would make sense for a postal service to have many of those for vans loading up rather than lots of clocks...

[Robert G. Schaffrath]

I recall that back in the late 1990's, my local post office had a giant USPS logo'ed LED clock on the wall behind the counter that counted down to the millennium. I was very impressed with it and wondered what they would do with it after the count hit zero on January 1, 2000. Well it turns out that after the countdown ended, it became a regular clock. I was hoping it would basically be the end of the program and that they would appear for sale. Never did find out what happened to it as my post office moved about a 1/2 mile away to a larger facility as my community kept growing and the clock was not at the new location.