Minelco Clock

I finally got around to using the Minelco electromechanical displays shown at the bottom of this page.  Minelco is actually a mining and minerals company.  The displays are "magnetic latch indicators".  If you search on "minelco magnetic latch indicator" you can find some explanation of their origins and uses.  They seem to be intended to indicate the presence of transient shorts to ground in electrical equipment - once triggered the indicator stays latched until reset.  I can see how this display is useful for industrial and mining applications, but Minelco doesn't seem to be an electronics manufacturing company.

The indicator is a tube with a little white peg in it.  The peg is a magnet, so by energizing an electromagnet in the device with a positive or negative voltage (+24 vs. -24), one can set or reset the display.  I had an old LaCie external IDE hard drive enclosure.  Since IDE is obsolete, but the case looks cool, I decided to make an industrial looking clock.  I decided to use two columns of 12 pegs, the left column for hours, the right for minutes in 5 minute increments.  I extracted the front cover, machined off various protuberances, and drilled 24 holes into the tin/aluminum alloy.  I confidently asumed that I could multiplex the displays, so I wired it up to be drive by 12 relays: 4 X 6, plus one SPDT relay to switch between +24/-24 volts.

I built the circuit, wrote the code ... and could not get the thing to work reliably.  The problem is that these indicators are not designed to work well with multiplexing, as they have a relatively low resistance and tend to store energy internally  - they are a bunch of electromagnets.  I ripped up and replaced the drivers several times until I struck upon the following:

• I need to diode-protect the Minelco displays to prevent stray currents from unintentionally tripping displays/
• The displays have two electromagnet circuits, presumably one to set and one to reset.  I could choose between which circuit to energize using +24V and a SPDT relay, and diode-protect but combine the output to ground.
• Since the output is always +V to ground, I could use a semiconductor power latches for switching - in particular three TIPC6259's.
• To remove some flakiness, I added a lot of dead time between energizing the indicators - .9 seconds.
• Some of the indicators were still flakey so I diode-protected their +V sides.

After a long effort (and nearly pitching the thing into the dump) I made the clock display work reliably.  But the wiring became really ugly.  I'm not showin' that picture.

Here is a picture of the finished  clock, showing 10:03

The lamp on the bottom is a 3-color LED, the red level increases as the minute modulo 5 increases from zero (not lit) to 4 (fully lit).  I use green and blue to indicate that the clock is in the mode to set minutes / hours.  Here is 10:05 when in minute-selection mode