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.
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 ([url]http://uk.geocities.com/ronj_1217/4_digi.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 k4l.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]
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