Home > Designing basic digital circuits/circuit wizard > 4000 series (logic gates)

4000 series (logic gates)

Now I have had a play with one IC in the 555 timer I decided to have a look at some different integrated circuits.  When doing my research the 4000 series integrated circuits kept getting mentioned and seemed to be popular.  We had many different ones at University but I bought the 4011 as this is comprised of NAND gates and you can use NAND gates to make any other type of gate.  This is the schematic for the circuit I constructed.


4011 data sheet

I have also included the breadboard with this project (as I did pay for the chip, unfortunately the 555s had to go back!).  Notice that in the print screen both inputs are high 1 and the led is off this is because as I know from the NAND truth table featured in logic gates

input A    input B   Output

0                    0                 1

0                    1                  1

1                     0                 1

0                    0                 1

Another interesting thing about this little circuit is the inclusion of the two 10 k resistors.  These are actually known as pull down resistors.  They are included in the circuit because CMOS gates are sensitive to static electricity and can actually be damaged by high voltages.  If they are left to ‘float’ they can assume any logic level.  For example if I had a switch connected to a CMOS chip through to ground, if I pushed the switch, ground is connected to the input pin.  When the switch is open however the signal to the pin is open to static and interference and is known to be in a ‘floating’ state.  This can lead to damage to the chip and cause the circuit to malfunction.  To combat this I have to include a pull down (or pull up) resistor.  The pull up resistor surprisingly pulls the signal up. It will keep the pin in a high state and prevent it from floating and will remove any unwanted inaccurate signals.  The pull down resistor does the same but pulls it down.  The other thing to note about this circuit is that I had to get my fault finding hat on.  At first the circuit didn’t work and I couldn’t understand why, everything seemed to be in the right place and it was confusing me a little.  It wasn’t until I took a closer look at the transistor that I realised the problem.  I needed a BC547 transistor but the young gentleman at maplins had given me a BC557 one.  Sometimes the components can be swapped as they will function in a similar manner, but when I checked the data sheets for the two resistors the 557 is actually a PNP and the 547 is an NPN  hence the reason it wouldn’t work!  On a final note about CMOS circuits, I was informed that it was good practice to include the 47uf de-coupling capacitor across the power supply because it helps to prevent the transfer of spikes along the power supply rails.

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