How to build a 4-digit siren control keypad

The keyboard must be with a common terminal class - and for each key a separate connection. A 12-key keyboard - 13. A matrix type with 7 or 8 terminals will not work. On the support page you will find details on how to make your own keyboard.

The relay energizes by pressing a key. Select the key you want to use - and connect it to terminal "E". Select the four keys you want to use for your security code - and connect them to the "ABC&D". Wire the common lead to R1 and all remaining keys to "F".

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

When you press the keys in the correct sequence - the relay de-energizes - and the 12 volt output returns to the "off" terminal. The green indicator light also turns off - to indicate the alarm is off.

The remaining keys -- those not wired to "ABCD" -- are wired to "F." Whenever one of these "wrong" keys is pressed -- a failed code entry attempt -- the code entry sequence is reset.

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

A 12-key keyboard - over 100,000 different codes available. If you need a more secure code - you can simply use a larger keyboard with more "mistakes" wired to the "F" keys. A 16-key keyboard offers over 400,000 different codes. If you make a mistake while entering a code - simply start over.

Support materials for this circuit include a step-by-step guide to board construction - a parts list - a detailed circuit description - and more.