4-Digit Alarm Control Keypad

Schematic Diagram


The Keypad must be the kind with common terminal  & a separate connection for each key. On a 12 key pad  look for 13 terminals. The matrix type with 7 or 8 terminals will not do. On the Support Page you will find details of how to Make Your Own Keypad.

The relay is energized by pressing a single key. Select the key you need to make use of and connect it to terminal E. Select the keys you need to make use of for your security code and connect them to A B C & D. Wire the common lead to R1- and all the remaining keys to F.

When you press E the relay energizes & the 12-volt output moves from the off to the set terminal. The green LED also lights. It provides  visual indication that the alarm is set.

When you press keys "A B C and D" in the right order - the relay de-energizes - & the 12-volt output returns to the "off" terminal. The green LED is also extinguished - to indicate that the alarm is switched off.

The remaining keys - those not wired to "A B C D and E" - are connected to "F". Whenever of these "Wrong" keys is pressed - the tried code entry fails - and the code entry sequence is reset.

With a 12-key pad - over ten 000 different codes are available. In the event you need a more secure code - you could basically use a bigger keypad with more "Wrong" keys wired to "F". A 16-key pad gives over 40 000 different codes. In the event you make a mistake while entering the code - basically start again.

The Support Material for this circuit includes a step-by-step guide to the construction of the circuit board - a parts list - a detailed circuit description - and more.

The same thing happens if "C" or "D" is pressed out of sequence. If "C" is pressed before "B" - or "D" is pressed before "C" - the tried code entry will fail. And the code entry sequence will reset.

Veroboard Layout








source by zen22142.zen.co.uk


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