Alarm Circuit Using Keypad

Posted on Feb 6, 2014

This is a design circuit for alarm circuit. This circuit is controlled by keypad. For the core of the circuit is using a single transistor with BC547 series. This is the figure of the circuit. For operation, 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 k

Alarm Circuit Using Keypad
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ey 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. The relay energizes, 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. It locks itself high using R2 and it enables gate 2 by taking pin 5 high. The remaining gates operate in the same way, each locking itself on through a resistor and enabling its successor. If the correct code is entered, pin 10 will switch Q4 on and so connect the base of Q5 to ground. This removes the `enable` from gate 1, and the code entry process fails. If `C` or `D` is pressed out of sequence, Q1 or Q2 will also take pin 1 low, with the same result. You can change the code by altering the keypad connections. If you need a more secure code use a bigger keypad with more `wrong` keys wired to `F`.

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