Panaplex Alarm Clock

I built this Panaplex alarm clock for a friend.  The display is a 12 digit  Burroughs Panaplex BR12400, and the ringer is a Western Electric bell, repurposed from an old discarded phone that I found.  I use a rotary encoder for setting the time, and the giant red button is for the alarm.  My friend prefers 24-hour time, so the photo was taken at 3:04 PM.  I have enough digits to display the temperature also (which my friend likes); I use an LM92 temperature sensor mounted outside the box (on the back).

The bell mechanism is a solenoid which drives the hammer; it takes about 70 volts to drive it.  The old telephone system would drive the hammer at about 20 Hz, creating the familiar telephone ring.  I drive the solenoid using an MPSA42 from the microcontroller (an ATMega48), so I can create any ring tone I want.  I use a cycle consisting of one long ring, two short rings, then a pause.  You can see the bell mechanism a bit more clearly in the photo below.

I had a very difficult time eliminating ghosting from the BR12400.  I finally settled on the following:

• 33 Kohm current limiting resistors at the cathodes (wired in; I use a higher value resistor for the decimal points).
• 175V power supply.
• 1ms per digit, including a .5 ms inter-digit blanking period (both anode and cathodes turned off).
• The supressor pin (which connects to the cage which seperates the digits) biased to 20V using a 500 Kohm potentiometer, through a 220 Kohm resistor.

There is still a small amount of ghosting, but not enough to be unpleasant.  The blank digits have their decimal point lit because otherwise blank digits cause some severe artifacts.

I have scanned some schematics for the panaplex displays, they are here and here.