2-input AND gates

bst_14.gif (721 bytes)

Technology: CMOS Power supply: 3-15 V

14-pin DIL


Pin connections AND gate bistable
Truth table Combination lock
Basic operation
Cascading LINKS . . .

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Pin connections

AND_07.gif (3300 bytes) The 4081 has four separate 2-input AND
gates which you can use independently.


Truth table

The truth table of each individual gate is:

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

where '0' represents a LOW voltage, and '1' represents a HIGH voltage.


Basic operation

You can investigate the behaviour of a single AND gate using this circuit:

testing 4081 AND gates

The inputs of the gate must be connected, either to LOW or to HIGH, and must not be left open circuit. This is the function of the input switches with their pull-down resistors. To avoid loading the output of the gate, a transistor switch indicator circuit should be used. It is good practice with CMOS circuits to insert a decoupling capacitor, 47 F or 100 F, across the power supply. (This helps to prevent the transfer of spikes along the power supply rails.)

Don't forget - - Connect pin 14 of the 4081 to +9 V and pin 7 to 0 V.

This is the test circuit built on prototype board:

4081 AND gate test circuit

In the prototype circuit, it is not essential to make connections to the unused gates. However, in any final circuit, all unused CMOS inputs must be connected either to HIGH or to LOW. Make it an absolute rule that CMOS inputs are never left open circuit.

There is no problem with CMOS outputs. Worry about the inputs and leave any unused outputs unconnected.



It is easy to make 3-input and 4-input AND gates by joining 2-input gates together:

cascading AND gates

3-input and 4-input AND gates are available in integrated circuit form, as listed in the LINKS section.


AND gate bistable

It is a surprise to discover that you can make a bistable circuit using just one AND gate:

AND gate bistable: click for simulation

Click on the diagram and confirm the latching action of this circuit. When you click the SET button, the output goes HIGH and stays HIGH. This is one of the stable states of the bistable. Nothing further happens unless you click the RESET button, making the ouput LOW once more.

The prototype board layout for this circuit is shown below:

4081 AND gate bistable

Build this circuit to confirm that the AND gate bistable works in practice.


Combination lock

Using AND gate bistables, you can make a 4-digit combination lock using a single 4081 CMOS integrated circuit:

AND gate combination lock: click for simulation

Click the diagram to open a Crocodile Technology simulation of the circuit. The four AND gates operate in sequence and are 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.

With a real circuit using 4081 AND gates, the 1F, 100 nF and 10 nF capacitors are found to be necessary 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 47 F capacitor is a decoupling capacitor. Its function is to prevent the transfer of signals along the power supply connections.

Here is the prototype board layout for the circuit:

combination lock using 4081 AND gates

This is an unusual way to make a combination lock but the components needed are cheap and easily available. With no repeat digits, a 10-switch keyboard will give 10x9x8x7=5040 different combinations. The output of the final AND gate can be connected to a solenoid lock system, or other output device.



Cross references in the Beastie Zone:

AND gate

4073 triple 3-input AND gate, 4082 dual 4-input AND gate

74LS08 quad 2-input AND gate, 74LS11 triple 3-input AND gate

74HC08 quad 2-input AND gate

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