The circuit described here is a hex to binary encoder using four, 8-input NAND gate 7430 ICs. The input keyboard switches SW1 to SW15 are connected in such a manner that pressing of any switch produces the binary ABCD outputs of corresponding decoded number. When all inputs is a NAND gate are high, it produces a low output, But when any of its input goes low, its output becomes high. Normally, all switches being open, the binary output is 0000. Pressing of a switch gives a ‘low input’ to the corresponding NAND gate, and hence the output code becomes a correct binary equivalent of the input hex number.
IC1 – IC4 = 7430
SW1 – SW15 = Input Key Board