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7400 Series Guide: 74HC109/74LS109 (Dual J-K flip-flop)

The 74×109 (ex 74HC109) is a chip that contains two J-K flip-flops where the K input is inverted.

In this guide, you’ll learn the things you need to know about this chip in order to use J-K flip-flops with added functionality in your own projects.

74HC109 / 74LS109 Package And Pinout

What does the 74HC109 / 74LS109 do?

The 74×109 is a dual J-K flip-flop with clear and preset capabilities.

This means it contains two independent J-K flip-flops in one chip, each having inputs for J, K, clock (CP), and outputs (Q and Q̅). Additionally, it has clear (CLR) and preset (PRE) inputs for each flip-flop.

Note that the K input is inverted.

Here’s a simple explanation of what each input does:

  • J and K Inputs: These control the state of the flip-flop. K is inverted, so it means that when J is HIGH and K is LOW, the flip-flop toggles its current state on the clock’s rising edge.
  • Clock (CP): The flip-flop checks the J and K inputs and changes state based on them when the clock edge occurs.
  • Clear (CLR): This input sets the Q output to LOW (0) immediately, regardless of other inputs.
  • Preset (PRE): This input sets the Q output to HIGH (1) immediately, regardless of other inputs.

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

ClkJKCLRPREQDescription
 0XX11QMemory
(no change)
10111QMemory
(no change)
111111Set
 100110Reset
 11011Toggle
XXX010Async Reset
XXX101Async Set
JK Flip-Flop Truth Table

The 74×109 is useful for counting, storing data, and toggle switch applications in digital circuits. The clear and preset functions makes it particularly handy in situations where you need to be able to immediately set or reset the flip-flop state.

How To Use This Chip

The 74HC109 comes in a 16-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 74HC109 supports 2V to 6V, while the 74LS109 only supports 5V.

74HC chips can normally supply a maximum of 4 mA from an output pin. If you’re using the 74LS version, the maximum current you can pull out of one output pin is 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking).

But these values can differ between models, so check the datasheet of your model to verify.

Once you’ve connected it to power, you can use the JK flip-flops inside.

7400 series IC connected to 5V on a breadboard
The 74HC109/74LS109 IC connected to power

74×109 Pinout

The 74×109 has 16 pins and contains two J-K flip-flops with set and reset laid out as shown in the pinout diagram below:

74HC109 Pinout
Pin NamePin #TypeDescription
1RST1InputReset input for the first JK flip-flop (active low).
1J2InputJ input for the first JK flip-flop.
1K3InputK input (inverted) for the first JK flip-flop.
1CLK4InputClock input for the first JK flip-flop.
1SET5InputSet input for the first JK flip-flop (active low).
1Q6OutputDirect output from the first JK flip-flop.
1Q7OutputComplementary output from the first JK flip-flop.
GND8PowerConnect to ground (GND).
2Q9OutputComplementary output from the second JK flip-flop.
2Q10OutputDirect output from the second JK flip-flop.
2SET11InputSet input for the second JK flip-flop (active low).
2CLK12InputClock input for the second JK flip-flop.
2K13InputK input (inverted) for the second JK flip-flop.
2J14InputJ input for the second JK flip-flop.
2RST15InputReset input for the second JK flip-flop (active low).
VCC16PowerPositive power supply. Connect to +5V power.
Pin overview for the 74×109 IC

Alternatives and Equivalents for 74HC109 / 74LS109

There are many versions of the 74×109 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:

  • 74HC109 (High-speed CMOS)
  • 74HCT109 (High-speed CMOS, TTL compatible)
  • 74LS109 (High-speed TTL)
  • 74LVC109 (Low Voltage TTL)
  • 74AC109 (Advanced CMOS)
  • 74ALS109 (Advanced Low-Power Schottky TTL)
  • 74F109 (Very High Speed)
  • 74C109 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74HC109 and SN74LS109 by Texas Instruments.

Can’t find the 74×109 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 positive-edge triggered J-K flip-flop.
  • 74×76 – Dual J-K flip-flop.
  • 74×78 – Dual negative-edge triggered J-K flip-flop.
  • 74×107 – Dual negative-edge triggered J-K flip-flop.
  • 74×112 – Dual negative-edge-triggered J-K flip-flop.
  • 74×113 – Dual negative-edge-triggered J-K flip-flop.
  • 74×114 – Dual J-K flip-flop.
  • CD4027 – Dual J-K master-slave flip-flop.
  • CD4095 – Gated J-K flip-flop.
  • CD4096 – Gated J-K flip-flop.

If you can’t find the 74×109 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 74LS109 and 74HC109 chips

Download the PDF datasheet for your version of the 74×109 here:

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