Digital Combination Lock
Notes:
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:
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.
