The keyboard entry circuit of Figure 9-16 is to be used as part of an electronic digital lock that operates as follows: when activated, an UNLOCK output goes HIGH. This HIGH is used to energize a solenoid that retracts a bolt and allows a door to be opened. To activate UNLOCK, the operator must press the CLEAR key and then enter the correct three-key sequence.

(a) Show how 74HC85 comparators and any other needed logic can be added to the keyboard entry circuit to produce the digital lock operation described above for a key sequence of CLEAR-3-5-8.

(b) Modify the circuit to activate an ALARM output if the operator enters something other than the correct three-key sequence.

Figure 9-16

Transcribed Image Text:

Digit kays +5V 1 kn For switch bounce >T OS Q Q = 20 ms Pull-up resistor for each input 74LS147 decimal- to-BCD encoder J >CLK CLEAR 1 k ā MSB 0₂ CLA +5 V ō. LSB X IX J CLK CLA Y To CLR inputs of Qo-Q11 J PRE CLK D D D D D D D D To decoder/drivers and displays CLK CLK CLK 0₂ 0₁ CLK CLK CLK Q₁ CLK CLK CLK d D Q CLK BCD code for MSD Second MSD LSD