A 4-Digit Keypad Controlled Switch

This is a universal version of the Four-Digit Alarm Keypad . I have modified the design to free up the relay contacts. This allows the circuit to operate as a general-purpose switch. It also means that it can be used to control all of my Alarm Circuits. I’ve used a SPCO/SPDT relay – but you can use a multi-pole relay if you wish.

Do not use the “on-board” relay to switch mains voltage. The board’s layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage – mount the relay somewhere safe – [u:5cbdb54b67]Away From The Board[/u:5cbdb54b67].

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 turns Q6 on – and energizes the relay. The two transistors – Q5 & Q6 – form a “Complementary Latch”. So – when you release the key – 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 turns Q4 on through R8. Q4 connects the base of Q6 to ground. This unlatches the complementary pair – and the relay drops out.

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 sequence 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 a 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 – use a bigger keypad with more “Wrong” keys wired to “F”. A 16-key pad gives over 40 000 different codes.

The Support Material ([url]http://uk.geocities.com/ronj_1217/u4s.html[/url])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 u4dl.png

Look at [url]http://uk.geocities.com/ronj_1217/al1/fourd.html[/url] for a detailed description.
Ron J’s Circuit Page: [url]http://uk.geocities.com/ronj_1217/circ.html[/url] – updated regularly.
Write To Ron: [url]http://uk.geocities.com/ronj_1217/mail.html[/url]

Copyright Ron J

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