Flip Flops and Latches

This is a guide for making Flip Flops and Latches using NAND gates.

When I was studying about Flip Flops and Latches in my class, all I wondered was how in the world we can store a bit by interconnecting 4 NAND gates. So I decided to make Flip Flops on my own and verify.

Material Required

• IC - 7400 (Quad 2-input NAND Gates)
• 4-5 LEDs
• Breadboard
• 5V power supply

These are all the things needed to make FFs.

IC-7400 is used in each circuit for NAND gates.

SR Latch

This is a Active LOW SR Latch, output is HIGH (SET) when S is LOW while output is LOW (reset) when R is LOW. Another type is Active HIGH SR Latch build using NOR Gate.

Here Active Low SR Latch is demonstated.

• Using IC-7400 connect the above shown circuit on a breadboard and supply 5V VCC to IC.
• Initially set S and R to 1, i.e. HIGH.

This is how my circuit was looking. I was using Arduino for 5V power supply. Don't forget to add resistor before an LED to prevent from over voltage.

• Now verify the truth table for the Latch.
• Active Low Latch changes output, i.e. SET or RESET only when S or R is LOW respectively.

S	R	Q	Image	Comment
HIGH	HIGH	Don't Care		Initially output is default set to LOW
LOW	HIGH	HIGH		Now ouput (Q) becomes HIGH (SET) as S is LOW
HIGH	HIGH	No Change		S is set to HIGH but the previous ouput i.e. 1 is stored
HIGH	LOW	LOW		Now output (Q) becomes LOW (RESET) as R is LOW
HIGH	HIGH	No Change		R is HIGH but previous output i.e. 0 is stored
LOW	LOW	Invalid State		The output is indeterminate

So it is clear that a Latch stores a bit after it is SET or RESET even after the the input is changed. 🤘

Raw Video of Latch demo :

SR FlipFlop

CLK or CK is CLOCK

This is a SR Flip Flop, output is HIGH (SET) when S and CLK is HIGH while output is LOW (reset) when R and CLK is HIGH. SR FF is clock synchronized SR Latch and has several advantage over SR Latch.

Due to unavailability of function generator and my laziness of making a square wave generator, I made a SR Gated Latch, which is nothing but a SR FF with ENABLE instead of CLK. You can make a square wave generator and use it as a clock.

EN or E is ENABLE

Here Gated SR Latch is demonstated.

• Using IC-7400 connect the above shown circuit on a breadboard and supply 5V VCC to IC.
• Initially set S, R and EN to 0, i.e. LOW.

This is how my circuit was looking. I was using Arduino for 5V power supply. Don't forget to add resistor before an LED to prevent from over voltage.

• Now verify the truth table for the Flip Flop.
• Flip Flop changed ouput when EN is HIGH and S or R is HIGH.

E	S	R	Q	Image	Comment
LOW	LOW	LOW	Don't Care		Initially output is default set to LOW
HIGH	HIGH	LOW	HIGH		Now ouput (Q) becomes HIGH (SET) as S is HIGH
HIGH	LOW	LOW	No Change		S is set to LOW but the previous ouput i.e. 1 is stored
LOW	LOW	HIGH	No Change		R is HIGH but Q is HIGH as EN is LOW i.e.1 is stored
HIGH	LOW	HIGH	LOW		Now output (Q) becomes LOW (RESET) as R is HIGH
HIGH	LOW	LOW	No Change		R is LOW but previous output i.e. 0 is stored
HIGH	HIGH	HIGH	Invalid State		The output is indeterminate

So it is clear that a Flip Flop stores a bit after it is SET or RESET even after the the input is changed. And a Flip FLop changes its value only when EN or CLK is HIGH. 👍

Raw Video of Flip Flop demo :

Further Development

You can similarly make JK, D and T Flip Flops. Further the same concept can be applied to make counters and registers.