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This switch will suit the Modular
Burglar Alarm circuit. However, it also has other
applications. 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. 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,
almost 100 000 different codes are available. The Alarm
is set using the first four of your five chosen keys.
When '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 energizes,
and then holds itself on by providing base current for
Q6 through R12. The 12-volt output switches 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 & E 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. Pressing 'A' takes pin
1 high for a period of time set by C1 and R2. This
'enables' gate 1, so that when 'B' is pressed, the
output at pin 3 will go high. This output does two jobs.
It locks itself high using R3 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
within the time allowed, pin 10 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 removes the 'enable' from gate 1,
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. 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. 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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