Binary-Coded Decimal (BCD) Encoder

The circuit monitors the status of ten (10) switches and generates a binary-coded decimal number at the output. The input lines are active-low and are passively pulled high. Only one (1) switch at a time should be closed. The switches should be momentary-contact push-buttons, the SPST Double Break is the only available switch type that closely resembles the momentary-contact push buttons. Four NAND gates were used to generate the four bits needed for BCD encoding. The NAND gate at the bottom generates bit 0, this bit should be high when an odd-numbered switch is active so the input terminals of this gate connect to the odd-numbered switches. The next NAND gate is for bit 1 which should be 1 when switch 2, 3, 6, or 7 is active. The next NAND gate which is for bit 2 should generate logic 1 when switch 4, 5, 6, or 7 is active. Bit 3 will be 1 only for switch numbers which are greater than 7 so the NAND gate at the top connects only to switches 8 and 9.