Try to make this 0-9 Keyboard for your project work, it's not so hard. First make a Clock by 555 IC, easy. Send the clock pulse to a Counter and get 2 Bit output. Now get 0 1 2 3 four lines output by 2 NOT Gate and 4 AND Gate. Take 3 output for your Keyboard. Your Keyboard is ready with Clock pulse.
Now try to understand the logic of the circuit. We used 10 AND Gates to identify the 10 keys easily. AND Gate one port connected with clock pulse and other port with Key. When both ports of the AND Gate become high the output become high and send the high output to the counter via OR Gate. Why via OR Gate?
4 Bit Counter counts from 0 to 9 so AND Gate high output where to send Bit 0 or Bit 1 or Bit 2 or Bit 3? That's why Bit wise high pulse send through OR Gates. 5 input OR Gate used for Bit 0 position. AND Gate 1 3 5 7 and 9 output connected with Bit 0 OR Gate. 2 3 6 and 7 connected with 4 input OR Gate for Bit 1. 4 5 6 and 7 connected with 4 input OR Gate for Bit 2 position. 8 and 9 connected with 2 input OR Gate for Bit 3. And key 0 connected with a 5 input OR Gate with all OR Gates output to control the counter Load.
Hope the article will help to understand the logic of Keyboard Matrix.
