Alarm Control Electronic Keypad Project

  
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The IC is a quad 2 input `AND` gate, a CMOS 4081. These gates only produce a HIGH output, when BOTH the inputs are HIGH. When the key wired to `E` is pressed, current through R1 and D1 switches Q5 on. The relay energizes; and Q5 is `latched on` by R8. Thus, the Alarm is set by pressing a single key, say one of the two non-numeric symbols. The alar
Alarm Control Electronic Keypad Project - schematic

m will switch off when the 4 keys connected to "A, B, C, D" are pushed in the right order. The circuit works because each gate `Stands` upon its predecessor. If any key other than the correct key is pushed, then gate 1 is knocked out of the stack, and the code entry fails. Pin 1 is held high by R4. This `Enables` gate 1; and when button `A` is pressed, the output at pin 3 will go high. This output does two jobs. It locks itself `ON` through R2 and it `Enables` gate 2, by taking pin 5, high. Now, if `B` is pressed, the output of gate 2, at pin 4 will go high. This output does two jobs. It locks itself `ON` through R3 and it `Enables` gate 3 by taking pin 12 high. Now, if `C` is pressed, the output of gate 3 will lock itself `ON` through R5 and, by taking pin 8 high, `Enable` gate 4. Pressing `D` causes gate 4 to do the same thing; only this time its output, at pin 10, turns Q4 `ON`. This takes the base of Q5 to ground, switching it off and letting the relay drop out. This switches the alarm off. Any keys not connected to `A B C D E` are wired to the base of Q1. Whenever `E` or one of these other keys is pressed, pin 1 is taken low and the circuit is reset. In addition, if `C` or `D` is pressed out of sequence, then Q2 or Q3 will take pin 1 low and the circuit will reset. Thus nothing happens until `A` is pressed. Then if any key other than `B` is pressed, the circuit will reset. The Keypad needs to be the kind with a common...



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