xample Circuits

2. Combination Lock

Most combination lock circuits use NAND gate or NOR gate bistables, but it is also possible to make a bistable with a single AND gate, as shown in the circuit below.

When the power supply is first connected, the output of the AND gate will be LOW. Operating the first push button switch SETS the bistable. The output of the AND gate becomes HIGH making the corresponding input HIGH as well. The other input is held HIGH by the pull-up resistor. The AND gate has two HIGH inputs, so the output is latched HIGH. The second push button switch RESETS the bistable.

Think this through using the AND gate truth table:

input A input B output
0 0 0
0 1 0
1 0 0
1 1 1

The principle can be extended to make a 4-digit combination lock using a single 4081 CMOS integrated circuit:


The four AND gates operate in sequence and should be connected to the push button switches included in the code. The remaining, unselected, switches are connected in parallel and pressing any one of them RESETS all the AND gate bistables.

The 1F, 100 nF and 10 nF capacitors prove to be necessary in practice to eliminate a 'glitch' caused by pressing the next switch in the sequence. Without the capacitors, pressing the next switch can RESET the previous AND gate. With the capacitors, the circuit operates reliably.

the 100 F capacitor is a decoupling capacitor. Its function is to prevent the transfer of signals along the power supply connections.

A chapter on combination locks is planned for Discovering Electronics PROJECTS

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