4 Digit Alarm Keypad

  
The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13 terminals. The matrix type with 7 terminals will NOT do. The Alarm is set by pressing a single key. Choose the key you want to use and wire it to `E`. Choose the four keys you want to use to switch the alarm off, and connect them to `A B C & D`. Your code can include the non-numeric symbols. With a 12-key pad, over 10 000 different codes are available. Wire the common to R1 and all the remaining keys to `F`. When `E` is pressed, current through D2 and R9 switches Q5 on.
4 Digit Alarm Keypad - schematic

The relay energises, and then holds itself on by providing base current for Q5 through R10. The 12-volt output is switched from the "off " to the "set " terminal, and the LED lights. To switch the Alarm off again it is necessary to pre



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