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4025B Triple 3-input NOR gates


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1. Pin connections

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4025B pin connections

The 4025 available from Rapid Online has three separate 3-input NOR gates which you can use independently.

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2. Truth table

The truth table of each individual gate is:

input C input B input A output
0 0 0 1
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 0
3-input NOR gate truth table

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

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3. Basic operation.


You can investigate the behaviour of one of the 3-input NOR gates using this circuit:

3-input NOR gate test circuit

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 µF or 100 µF, across the power supply. (This helps to prevent the transfer of spikes along the power supply rails.)

  •  hot stuff !! Beasties need power supplies! Don't forget to connect pin 14 of the 4025 to +9 V and pin 7 to 0 V.

Here is the circuit on prototype board:


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open in a new window 4025 pins 4025B 3-input NOR gate test circuit

In the prototype circuit, it is not essential to make connections to the unused gate. 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.

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4. Links

reference book..

4.1 Data sheets

The links below allow you to download documents in Adobe Acrobat ©, PDF, format. In the unlikely event that you don't already have Acrobat Reader, you can download the latest version direct from Adobe:

open document 4025B data sheet (Philips Semiconductors, 2001)

open document 4025B data sheet (ST Microelectronics, 2001)

4.2 DOCTRONICS links

4001 4001 2-input NOR

4002 4-input NOR

4078 8-input NOR

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