What does it do?
|Turns something off when either input is activated.
How does it operate?
NOR Gate Circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
When two input signals are used with a NOR gate, the NOR gate subsystem provides an output signal that is low when either input signals are high. Otherwise, the output signal is low.
The output from the NOR gate is shown in the truth table below, with 0 meaning “low” and 1 meaning “high”
Truth table for a 2-input NOR gate
American (ANSI) Symbol
European (DIN) Symbol
The simplest NOR gate ICs (such as the 4001B) have two input signals. Other NOR gate ICs are available with more inputs. The 4025B has three input signals, the 4002B has four input signals and the 4078B has eight input signals. In all NOR gates the output signal goes low when any of the input signals is high.
It is possible to combine NOR gates to produce an OR gate. NOR gates can also be made to work like inverters.
- Turning an output device off when either of two sensors is activated
Pins of 4001B
How part of the PCB might look
The PCB shows the basic circuit. Several gates in the IC are not used in this simple design; they can be applied in other subsystems. Any unused input pins should be connected to 0V or Vs, to prevent damage by static electricity.
In the example PCB, the two input signals go to pins 1 and 2, and the output signal comes from pin 3. Any of the other three NOR gates in the IC could be used.
Build and test the units that will provide the input signals before adding the NOR gate IC.
Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that:
- the voltage on pin 7 is low (0V);
- the voltage on pin 14 is high (the supply voltage);
- the voltage on pins 1 and 2 (the blue PCB tracks) goes high and low in response to the units that provide the input signals.
Insert the IC the right way round.
Make sure that the signal going out (on the green PCB track) follows the NOR gate truth table.
If there is a fault, check that:
- The voltage on pin 7 is low (0V)
- The voltage on pin 14 is high (the supply voltage)
If there is a fault, check the tracks and solder joints.
- PICs – more flexible but more expensive.
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