This is a clock powered by mains via 12vac transformer, which isolates as well as provides power and the 60Hz frequency for time keeping. It is in military time format and has momentary switches for setting the current time, resetting the seconds count and a SPST for activating SET mode, indicated by a red LED. It uses 8 bit counter IC’s, binary to 7 segment converters and 7 segment LED displays. Counter resets are done with AND logic IC’s. LED’s are used here as blocking diodes for adjustment signals in simulation because I can set the Vf to 100mV, a low Vf Schottky diode should be used as blocking diodes.
Rectified ac is sent to a comparator which outputs a high signal when the AC crosses a 2.5V reference, supplying a 60Hz square wave. This is sent to a counter, which counts to 6 and resets. This reset signal is also sent to another counter IC’s input, which counts 10 sets of 6 and resets. Thus a single pulse is supplied every second. This is sent to a counter which displays the seconds counted, forwarding and resetting on the 10th. The next counter counts and displays tens of seconds, forwarding and resetting on the 6th. The following counter displays minutes, counting out 9 minute pulses and displaying them on a 7 segment, forwarding and clearing on the 10th. The following counter counts 5 tens of minutes and displays them, resetting and forwarding on the 6th, to the hour counter. The hour counter displays the hour pulses, forwarding and clearing on the 10th hour count. The final display counts out tens of hours, clearing both the ten and single hours on the 24th hour. I’ve included RC circuits on the AND gate outputs, this adds a small delay between the signal to the next counter and the reset of the current counter which is helpful with overcoming switching delay in the IC’s. Works on a breadboard although simulation is a bit grumpy. The display order is not in 00:00:00 format, I’ve arranged them so the schematic is easier to read.
