The Flip-Flop Transistor Circuit

Nail Your First Flip-Flop!

A flip-flop is a kind of circuit that changes back and forth between two states (on and off) at certain intervals. It flips into one state, then flops to another, and so on. With these two states, a flip-flop serves as the most basic binary storage: 0 and 1, “off” and “on.” As you can imagine, this makes the flip-flop an important circuit to understand! 

 This circuit is pretty simple. All you’ll need is:

  • 4 x 1K Resistors
  • 2 x 100K Trim Potentiometers (A.K.A., Trim Pots)
  • 2 x 10uF 25v Electrolytic Capacitors
  • 2 x N2222A Transistors
  • 2 x Blue LEDs
  • 1 x 9V Battery Connector
  • 1 x BJT Flip Flop PCB

To build this, you’ll want to have on hand:

  • Your Trusty Soldering Iron and Solder
  • A PCB Holder or Helping Hands
  • Wire Cutters 
  • Wire Strippers
  • A 5v-12v Power Source
  • Heat Shrink Tubing
  • Propane Torch
A display of our components for a Flip-Flop

This build is going to be pretty easy, but it will require some soldering in tight spaces. As long as you keep your solder joints separate and orient your components in the correct direction or polarity, you’ll do fine.

Let the Soldering Commence

To begin, place your NPN Transistors through their designated spot on the PCB. If you’re using our custom HackMakeMod board, you can easily tell where they go by the round and flat spot with three through-holes. The shape of the transistor matches the shapes you’ll see on the PCB.

The shape and through-holes for a Transistor

Before you solder, it can be helpful to bend the legs underneath so they don’t move while you’re working.

Transistors Legs being bent to hold the component in place

Now that the transistors are secured and in the right orientation, solder them at their legs.

Transistor Soldering

Once soldered into position, take wire cutters or flush cutters and cut off the legs on the bottom of the PCB. You’ll need the room underneath for the other components and their legs.

Flush cutting the legs from a transistor

Next, you’ll need to insert your capacitors. Take note of two things: A new capacitor always has a shorter leg on the negative side, and there’s typically a white strip with a dash through it, signifying the negative side.

Comparing the positive and negative sides on capacitors

Insert the capacitors into their through-holes with the correct orientation, negative to negative and positive to positive.

Capacitors being placed into a PCB

Because of how flush the capacitors sit on the board, I like to bend the legs underneath and solder them from below.

Soldering two capacitors from below

After soldering, flush cut these legs. Now we can place our resistors! Bend the legs to create shoulders close to the resistor itself and solder them from the top. You can also bend these below, but it’s not entirely necessary. Snip off the legs below to make room for the next component.

Inserting the last 100K Ohm resistor into the Flip Flop Circuit

Next, we’re going to be soldering on our LEDs. Much like the capacitors, you have to solder them in the right orientation. You can tell find the negative side of an LED by its shorter leg, and this longer blade in the LED itself. You can also see a flat spot on the bottom of the LED body. Take a close look!

A close up on the negative-identifying sides of an LED

Place the LEDs in the right orientation, bend the legs outward, and solder them from below.

Inserting an LED into the Flip Flop

Last component! Finally, we have pièce de résistance. At the top of the Flip-Flop, we can add our Trimmer Potentiometers, or Trim Pots for short. These are like any other  potentiometer, but they can be adjusted by screwdriver.

The top and bottom of a trim pot

Match up the legs of the trim pots, bend them outward, and solder them from underneath again. Flush cut those legs off, and we’re ready to add power.

Bending out the bottom of a Trim Pot

We need more power!

Take the exposed ends of your 9 volt battery connector and tin the tips of these wires.

Tinning the 9 volt battery connector's wires

To make life a little easier, you can add a little solder to the GND (Ground) and VCC (Power) through-holes. Then, solder your red wire to the VCC through-hole and your black wire to the GND through-hole.

Soldering in the VCC connector

Then, connect your 9 volt battery, and you should have blinking LEDs!

Connecting a 9 Volt Battery to the Flip Flop Circuit

You can play around with the frequency of the blinking by turning to the left or right on either trim pot. If a light becomes always turned on, its probably just moving faster than the human eye can perceive!

A blinking flip flop circuit

Thanks for joining us on this build! If you had fun and want to share your Flip-Flop, share it on Instagram and tag us with #HackMakeMod!

Tutorial
The Flip-Flop Transistor Circuit
The Flip-Flop Transistor Circuit
January 28, 2022
Skills: Electronics
Difficulty: Beginner