Pressing a single key on the keypad - will energize the relay. Entering a four-digit code of your choice - will de-energize it. The circuit was designed to control the Modular Burglar Alarm System - but it will have other applications. If you require added security - A Five-Digit Version - of the circuit is also available.

Notes
The relay is energized by pressing a single key. Choose the key you want to use - and connect it to terminal "E". Choose the four keys you want to use to de-energize the relay - and connect them to "A B C & D". Wire the common to R1 and all the remaining keys to "F".

The circuit is easy to use. When you press "E" - current through D2 & R9 switches Q5 on - and the transistor energizes the relay. The 12-volt output moves from the "off" to the "set" terminal - and the green LED lights. At the same time - R10 takes over the job of supplying base current to Q5. This means that - when you release "E" - the relay will remain energized.

To de-energize the relay - you need to press keys "A B C & D" in the right order. When you do so - pin 10 of the IC goes high - and it switches Q4 on through R8. Q4 connects the base of Q5 to ground. This switches Q5 off - and the relay drops out. The 12-volt output moves from the "set" to the "off" terminal - and the green LED is extinguished.

Any keys not wired to "A B C D & E" are connected to the base of Q3 by R7. Whenever one of these "Wrong" keys is pressed - Q3 takes pin 1 low - 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. If you make a mistake while entering the code - simply start again.

The Keypad must be the kind with one common terminal - and 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. With a 12-key pad - over 10 000 different codes are available. If you need a more secure code - you could simply use a bigger keypad with more "Wrong" keys wired to "F". A 16-key pad gives over 40 000 different codes.

The Support Material (http://uk.geocities.com/ronj_1217/4_digi.html)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.

For PCB layout see k4l.png

Links
Look at http://uk.geocities.com/ronj_1217/al1/fourd.html for a detailed description.
Ron J's Circuit Page: http://uk.geocities.com/ronj_1217/circ.html - updated regularly.
Write To Ron: http://uk.geocities.com/ronj_1217/mail.html

Copyright Ron J