This
7-digit combination lock can be easily hard-wired for any combination that you
choose. The circuit uses a 4-bit, divide-by-8 Johnson counter (IC1), ten
pushbutton switches and npn transistor T1.
At power-‘on,’ capacitor C2 connected to pin 15 of IC1 charges to high level through 820-kilo-ohm resistor, holding the counter in the reset state. In this condition, output O0 (pin 2) of counter IC1 is high, while all other outputs are low.
At power-‘on,’ capacitor C2 connected to pin 15 of IC1 charges to high level through 820-kilo-ohm resistor, holding the counter in the reset state. In this condition, output O0 (pin 2) of counter IC1 is high, while all other outputs are low.
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