When there was no microcontroller and integrated circuits at that time Keypad matrix was controlled by diodes and momentary push on switch. Due to lack of space, we have shown 2 Bit keypad matrix by 4 numbers 0 1 2 & 3. Two SET RESET Flip-flop circuit used for Bit 1 and Bit 2 position. When we press 3 (1 1) both Bit 1 and Bit 2 SET Relay energized. When pressed 1(0 1) Bit 2 Relay RESET to de-energized and Bit 1 SET Relay energized. This way Keypad is working.
