7400 Series Guide: 74HC78/74LS78 (Dual J-K flip-flop)

The 74×78 (ex 74LS78) is a chip that has two negative-edge-triggered J-K flip-flops inside.

Learn how it works and how to use it with this beginner-friendly guide and start using JK flip-flops in your projects.

What does the 74HC78 / 74LS78 do?

The 74×78 gives you two J-K flip-flops with a common clock and common reset pins. A JK flip-flop is a flip-flop that can be set, reset, and toggled using its J and K pins. It can be used for making counters, event detectors, frequency dividers, and much more.

In the table below, you can see how to set, reset, and toggle the flip flop output:

Clk	J	K	Q	Description
0	X	X	Q	Memory (no change)
1	0	0	Q	Memory (no change)
1	1	0	1	Set
1	0	1	0	Reset
1	1	1	Q̅	Toggle

How To Use This Chip

The 74HC78 comes in a 14-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC78 supports 2V to 6V, while the 74LS78 only supports 5V.

The maximum current you can pull out of one output pin of the 74HC78 is 4 mA. For the 74LS78 you can pull out only 0.4 mA when the pin is high (sourcing), but 8 mA when the pin is low (sinking). This can differ between models, so check the datasheet of your model to verify.

Once you’ve connected it to power, you can use any of the two negative-edge-triggered J-K flip-flops inside.

74×78 Pinout

The 74×78 has 14 pins and contains two negative-edge-triggered J-K flip-flops with common clock and common reset as shown in the pinout diagram below:

Pin Name	Pin #	Type	Description
CLK	1	Input	Input to the unknown flip-flop (inverted).
1SET	2	Input	Input to the first flip-flop (inverted).
1J	3	Input	Input to the flip-flop.
VCC	4	Power	Positive power supply (VCC). Connect to +5V power.
RST	5	Input	Control signal that controls the flip-flop (inverted).
2SET	6	Input	Input to the second flip-flop (inverted).
2K	7	Input	Input to the flip-flop.
2Q	8	Output	Output from the second flip-flop.
2Q	9	Output	Output from the second flip-flop (inverted).
2J	10	Input	Input to the flip-flop.
GND	11	Power	Connect to ground (GND).
1Q	12	Output	Output from the first flip-flop (inverted).
1Q	13	Output	Output from the first flip-flop.
1K	14	Input	Input to the flip-flop.

Alternatives and Equivalents for 74HC78 / 74LS78

There are many versions of the 74×78 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

• 74HC78 (High-speed CMOS)
• 74HCT78 (High-speed CMOS, TTL compatible)
• 74LS78 (High-speed TTL)
• 74LVC78 (Low Voltage TTL)
• 74AC78 (Advanced CMOS)
• 74ALS78 (Advanced Low-Power Schottky TTL)
• 74F78 (Very High Speed)
• 74C78 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC78 and SN74LS78 by Texas Instruments.

Can’t find the 74×78 anywhere? Then try one of the following IC alternatives:

• 74×67 – AND-gated master-slave J-K flip-flop
• 74×70 – AND-gated positive-edge triggered J-K flip-flop
• 74×72 – AND-gated master-slave J-K flip-flop
• 74×73 – Dual J-K flip-flop (positive-edge-triggered).
• 74×76 – Dual J-K flip-flop.
• CD4027 – Dual J-K flip-flops (positive-edge-triggered).
• CD4095 – Gated J-K flip-flop.
• CD4096 – Gated J-K flip-flop.

If you can’t find the 74×78 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS78 and 74HC78 chips

Download the PDF datasheet for your version of the 74×78 here:

• 74LS78
• 74LS78 (National Semiconductor)