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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 terminals will NOT do. The Alarm is set by
pressing a single key. Choose the key you want to use
and wire it to 'E'. Choose the four keys you want to use
to switch the alarm off, and connect them to 'A B C &
D'. Your code can include the non-numeric symbols. With
a 12-key pad, over 10 000 different codes are available.
Wire the common to R1 and all the remaining keys to 'F'.
When 'E' is pressed, current through D2 and R9 switches
Q5 on. The relay energises, and then holds itself on by
providing base current for Q5 through R10. The 12-volt
output is switched from the "off " to the "set "
terminal, and the LED lights. To switch the Alarm off
again it is necessary to press A, B, C & D in the right
order. The IC is a quad 2-input AND gate, a Cmos 4081.
These gates only produce a high output when both inputs
are high. Pin 1 is held high by R5. This 'enables' gate
1, so that when 'A' is pressed, the output at pin 3 will
go high. This output does two jobs. It locks itself high
using R2 and it enables gate 2 by taking pin 5 high. The
remaining gates operate in the same way, each locking
itself on through a resistor and enabling its successor.
If the correct code is entered, pin 10 will switch Q4 on
and so connect the base of Q5 to ground. This causes Q5
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 Q3
by R7. Whenever one of these 'wrong' keys is pressed, Q3
takes pin 1 low. This removes the 'enable' from gate 1,
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. You can change the code by
altering the keypad connections. 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. All components are shown lying flat on the board;
but some are actually mounted upright. The links are
bare copper wires on the component side. Two of the
links must be fitted before the IC.
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