There is both a
Four-Digit and a Five-Digit version of this switch. Although they will have other applications, both are designed to control the
Modular Burglar Alarm circuit. Simply use the relay's changeover contacts in place of SW1. The poles and the points marked 'set' and 'off' represent the corresponding connections on both the keypad relay and the alarm switch.
Click Here for a Diagram.
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.
Choose the five keys you want as your code - and connect them to ''A, B, C, D & E''. Wire the common to R1 - and all the remaining keys to 'F'. Because your choice can include the non-numeric symbols - there are almost 100 000 different codes available.
The Alarm is set using the first four of your five chosen keys. The sequence begins when you press ''A''. This charges C1. If ''A, B, C & D'' are pressed in the right order - and within the time set by C1 and R2 (about 10 seconds) - current through R11 switches Q6 on. The relay energises - and then latches itself on by providing base current for Q6 through R12. The 12-volt output moves from the "off " to the "set " terminal - and the LED lights.
To switch the Alarm off - it's necessary to press ''A, B, C, D & E'' in the right order. The IC has four 2-input AND gate - a Cmos 4081. These gates only produce a high output when both inputs are high. Again, the sequence is started by pressing ''A''. This takes pin 1 high for a period of time set by C1 and R2. Taking pin 1 high ''enables'' gate 1 - so that when 'B' is pressed - the output at pin 3 will go high. This output does two jobs. It latches itself high by using R3 to hold its own input pin high - and it "enables" gate 2 by taking pin 5 high. In other words - pin 3 is doing for gate 2 what C1 is doing for gate 1.
The remaining gates operate in the same way. When the correct key is pressed - the output of the gate goes high. It then latches itself on by holding one of its own input high - and it "enables" the next gate in the chain by taking one of its inputs high.
If the correct code is entered within the time allowed - pin 10 will go high. It will switch Q5 on - and so connect the base of Q6 to ground. This causes Q6 to switch off and the relay to drop out.
Any keys not wired to ''A, B, C, D or 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 - removes the 'enable' from gate 1 - and causes the code entry process to fail.
If C, D or E is pressed out of sequence - Q1, Q2 or Q3 will also take pin 1 low - with the same result. For example - if C is pressed before B then pin 3 will still be low. That is - pin 3 will be connected internally to the negative line. So Q1 will be able to discharge C1 into pin 3. D1 prevents the unwanted recharging of C1 through the base-collector junctions of Q1, Q2 and Q3.
When the code is entered - and the relay drops out - the collector of Q6 goes high. At this point C7 conducts briefly - and Q4 switches on for a moment. This empties C1 and resets the circuit immediately. Without C7 - it would be necessary to wait for C1 to finish discharging through R2 - before the alarm could be set again.
You can change the code by altering the keypad connections. If you make a mistake entering the code - just start again. If you need a more secure code - you can use a bigger keypad with more ''wrong'' keys wired to 'F'. A 16-key pad gives over half a million different codes.
R1 limits the base current through Q1, Q2 and Q3. The 100n capacitors are there to slow down the inputs and make them less sensitive to stray electrical interference picked up by the wires going to the keypad.
There is nothing special about the transistors. Any small npn transistors with a gain (hfe) greater than 100 and capable of coping with a current of at least 100mA should do.
The relay coil should have a resistance of at least 270 ohms. This gives a maximum current of about 45 mA - and Q6 will easily be able to handle it. Relay coils produce large reverse-voltage spikes that will destroy Cmos ICs. D3 short-circuits these spikes at source - before they can do any damage.
Parts List
Construction
Click here if you're new to constructing stripboard projects.
The terminals are a good set of reference points. To fit them, you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 28 places where the tracks are to be cut. Before you cut the tracks, use the "actual size" drawing to
Check That The Pattern is Correctly Marked .
When you're satisfied that the pattern is right - cut the tracks. Make sure that the copper is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass. It only takes the smallest strand of copper to cause a problem. If you don't have the proper track-cutting tool, then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.
Actual Size Of Pattern
Next make and fit the
Thirteen Wire Links. I used bare copper wire on the component side of the board. Telephone cable is suitable; the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it; and also allow the insulation to slip off.
Next fit the transistors, diodes and relay. Again, the diodes are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright.
Finally, add the 9 solder bridges to the underside of the board. Then examine the underside of the board carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. Finish off by inserting the Cmos 4081 into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 14 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.
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