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Circuits
The circuit above above makes use of the
CMOS 4017 decade counter IC. Each depression of a switch
steps the output through 0 - 9. By coupling the output
via an AND gate to the next IC, a predefined code has to
be input to create the output. Each PBS switch is
debounced by two gates of a CMOS4001 quad 2-input NOR
gate. This ensures a clean pulse to the input of each
CMOS 4017 counter. Only when the correct number of
presses at PBS A will allow PBS B to become active. This
is similar for PBS C and PBS D. At IC4, PBS D must be
pressed 7 times. Then PBS C is again pressed 7 times,
stepping from output 1 to output 8. The AND gate formed
around CMOS4081 then goes high, lighting the LED. The
Reset switch can be pressed at any time. Power on reset
is provided by the 100n capacitor near the reset switch.
Below is a picture of one that I made about 15 years
ago:
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Circuits
Unfortunately, this board was part of a
much larger project containing multiple power supplies.
One day whilst working on another circuit , I slipped
with a wire and splashed 24volts DC onto this board.
There was a small spark, and puff of smoke before all
this chips were cooked! If anyone does consider building
such a circuit, then my advice would be to stop and look
in your local electronic parts catalogue. There are now
dedicated combination lock IC's with combinations many
times greater than this circuit. Incidentally the number
of combinations offered here is 10 x 10 x 10 x 10 x 9 =
90,000. Check out Dean White's Electronic Gadgets, on
the Electronic Sites Alliance web ring, he also has a
combination lock circuit. |
