Mengenlehreuhr

The Mengenlehreuhr (German for "Set Theory Clock") or Berlin-Uhr ("Berlin Clock") is the first public clock in the world that tells the time by means of illuminated, coloured fields, for which it entered the Guinness Book of Records upon its installation on 17 June 1975. Commissioned by the Senate of Berlin and designed by Dieter Binninger, the original full-sized Mengenlehreuhr was originally located at the Kurfürstendamm on the corner with Uhlandstraße in what was then West Berlin. After the Senate decommissioned it in 1995, the clock was relocated to a site in Budapester Straße in front of Europa-Center, where it stands today.

The clock at its original location in May 1979, displaying 17:54 (5:54pm).

Time encoding

Clock displaying time from 16:50 to 17:05 (4:50 pm to 5:05 pm) in time lapse.

The Mengenlehreuhr clock face utilizes 24 light switches (1+4+4+11+4=24) to display time in 0–23 hour, 0–59 minute and even/odd second.

Second: The top big circular light is the second mark. Since 0 is represented as an OFF in the most logical circuit, It’s an even second when the light is OFF. When the second light is ON, it’s on an odd second.

 //Second encoder in Source Code
 //Assuming neon[3] holds 1-bit for the 1 second light
 if(tsec&1){ neon[3]=1; } else{ neon[3]=0; }

Hour: The next two rows of 8 light switches are the hour mark, using a similar BCD system, where the upper row is a multiplier of 5, and the lower row is a multiplier of 1. Therefore the maximum hour number is 4*5+4=24 and the minimal hour number is 0 where everything is OFF. Since a 24-hour clock only uses a number from 0 to 23. The 24th (25th counting from 0) position is unused on this clock.

 //Hour encoder in Source Code
 //Assuming neon[0] holds 8-bit for the top 8 lights
 //Hour-top
 for( uint8_t i=0; i<(thr/5);i++){
   neon[0]|=(1<<i);
 }
 //Hour-bottom
 for( uint8_t i=4; i<((thr%5)+4);i++){
   neon[0]|=(1<<i);
 }

Minute: The last two rows consist of an upper 11 light switches and a lower row of 4 light switches. It’s also encoded similarly to hour mark, with a maximum possible number of 11*5+4=59 minute mark and 0 minute mark when everything is OFF. The upper row also have 3 unique colored light, representing the minute mark at 3*5-minute intervals of 15, 30, 45.

 //Minute encoder in Source Code
 //Assuming neon[1] and neon[2] hold 15-bit for the bottom 15 lights
 //Minute-top
 for( uint8_t i=8; i<(tmin/5)+8;i++){
   neon[i/8]|=(1<<i%8);
 }
 //Minute-bottom
 for( uint8_t i=19; i<(tmin%5)+19;i++){
   neon[i/8]|=(1<<i%8);
 }

Time decoding - How to Read the Clock

Mengenlehreuhr clock recreation using high voltage neon lamps with clock in 12:32:x1

Both hour and minute's upper rows are a multiplier of 5, while the lower rows are a multiplier of 1.

Hour (upper 2 and lower 2): 2*5+2=12. In a way, the clock can be read visually at a glance. Half neon lamps ON, implies it's half day or 24/2=12. Each of these 4 upper lights represents a 5 increment hour: 5, 10, 15, 20.

Minute (upper 6 and lower 2): 6*5+2=32. Similarly each of these 11 upper lights represents a 5 increment minute: 5,10,15,20,25,30,35,40,45,50,55; 15,30 and 45 are marked, assisting minute read back. 60 minutes can also be read visually as well.

Second ( top 1 ): It's odd second or second&1 logically. The second is mostly not read. Its main function is to indicate that the clock is running.

Kryptos

This clock may be the key to the unsolved section of Kryptos, a sculpture at the Central Intelligence Agency (CIA) headquarters in Langley, Virginia, United States. After revealing that part of the deciphered text of the sculpture, in positions 64–69, reads "BERLIN", the sculptor, Jim Sanborn, gave The New York Times another clue in November 2014, that letters 70–74 in part 4 of the sculpture's code, which read "MZFPK", will become "CLOCK" when decoded,^[1] a direct reference to the Berlin Clock. Sanborn further stated that in order to solve section 4, "You'd better delve into that particular clock".^[2] However, Sanborn also said that, "There are several really interesting clocks in Berlin."