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.
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 ([url]http://uk.geocities.com/ronj_1217/5_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 k5l.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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