Diode-transistor logic in a discrete transistor clock. The diodes at A decode 1 of 12 lines to light the seven segment displays to show the hours 1 to 12. The transistors at B drive the decode diodes and displays. The diodes at C trigger one of the 12 pairs of transistors based on the state of the transistor flip-flops at D. Additional diodes in the chain of flip flops allow this to count hours.