The relay is energized by entering the first four digits of your chosen five-digit security code. Entering the full five-digit security code - will de-energize the relay. When "A, B, C & D" are pressed in the right order - and within the time set by C1 and R2 - current through R11 switches Q6 on. This energizes the relay. When "A, B, C, D, & E" are pressed in the right order - and within the time set by C1 and R2 - current through R7 switches Q5 on. This de-energizes the relay. The circuit was designed to control the Modular Burglar Alarm System - but it will have other applications. A simpler - A Four-Digit Version - of the circuit is also available.

Choose the five-digit security code you want to use - and connect the keys to "A B C D & E". Wire the common to R1 and all the remaining keys to "F".

The circuit is easy to use. When you enter the first four digits of your security code - the relay energizes. The 12-volt output moves from the "off" to the "set" terminal - and the green LED lights. At the same time - R12 takes over the job of supplying base current to Q6. In effect - the relay latches itself on.

To de-energize the relay - you need to press keys "A B C D & E" in the right order. When you do so - pin 10 of the IC goes high - and it switches Q5 on through R7. Q5 connects the base of Q6 to ground. This switches Q6 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 Q4 by R9. Whenever one of these "Wrong" keys is pressed - Q4 takes pin 1 low. This discharges C1 - and the code entry process fails. If "C", "D" or "E" is pressed out of sequence - Q1, Q2 or Q3 will also take pin 1 low - with the same result. If you make a mistake while entering the code - simply start again.

Notes
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 - almost 100 000 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 half a million different codes.

The Support Material (http://uk.geocities.com/ronj_1217/5_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 k5l.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