This is a tutorial for a project kit you can find here.

Introduction. What You’re About to Build

Most modern electronics hide their inner workings behind microchips and software. You press a button, something happens, but you never see the electrical signals at work. This astable multivibrator kit is different. It lets you see, touch, and understand exactly how electrical signals behave in real time.

Figure 1. Astable Multivibrator in action.

This beginner-friendly electronics kit teaches you how circuits work by turning basic components into a mesmerizing light show: two LEDs that blink back and forth, powered by nothing more than resistors, capacitors, and transistors. No programming. No microcontroller. Just pure analog electronics.

 

Why It’s Called an Astable Multivibrator

The name "astable multivibrator" comes from early analog electronics terminology and perfectly describes how this circuit operates:

Breaking Down the Name:

• "Astable" means "not stable." Unlike circuits that settle into one steady state, an astable multivibrator has no resting position. It continuously switches back and forth between two conditions, creating an endless oscillating cycle. Each transistor alternately turns the other off and on.
• "Multivibrator" refers to how the circuit output vibrates (oscillates) between two voltage levels. Back in the 1930s before "oscillator" became the standard term engineers called circuits that generated pulses or oscillations "vibrators." The prefix "multi" indicates the circuit produces multiple alternating states.

In simple terms: An astable multivibrator is a self-switching circuit that continuously flips between two unstable states, producing a square-wave output that makes LEDs blink.

 

Astable Multivibrator, how does it work?

Figure 2. Astable Multivibrator.

This two-transistor oscillator circuit keeps switching between two states automatically that's what makes it "astable" or "not stable."

Step-by-Step Operation

Power-Up: When you first connect the 9V battery, both transistors receive voltage through their base resistors, and both want to turn on simultaneously. However, real-world components aren't perfectly identical one transistor will always react slightly faster. Let's say the left transistor (Q1) turns on first.

• Q1 Turns On: Current flows through Q1's LED and resistor, lighting up the LED. Inside the transistor, the collector voltage drops close to 0V as current conducts directly to ground. This low collector voltage connects through a capacitor to the base of the right transistor (Q2).
• Q2 Turns Off: Capacitors resist sudden voltage changes. When Q1's collector voltage suddenly drops, the capacitor pulls Q2's base voltage down with it sometimes even below ground level. This forces Q2 completely off, regardless of voltage from its base resistor.
• Capacitor Recharge: With Q2 off, the capacitor begins recharging through Q2's base resistor. As it charges, Q2's base voltage gradually rises. When it reaches approximately 0.7V, Q2 begins conducting.
• The Flip: As Q2 turns on, its collector voltage falls, sending a negative pulse through the second capacitor to Q1's base turning Q1 off. Now the right LED lights up and the cycle repeats in reverse.

Controlling the Blink Rate

The two LEDs alternate on and off indefinitely. The blink speed depends entirely on how long each capacitor takes to charge through its resistor:

• Larger resistors or capacitors = slower blinking
• Smaller resistors or capacitors = faster blinking

This RC (resistor-capacitor) time constant gives you complete control over the oscillation frequency without any programming.

Now build your own

Make sure you have or check out our kit if you haven't already.

You will need: 

Components:

• 2x Layer ENIG PCB,55x55mm
• 2x Red Cube LEDs
• 2x NPN Transistors （2N2222A）
• 2x Potentiometers with Knobs (100 kΩ)
• 4x Resistors (1 kΩ)
• 2x Capacitors (10 μF)
• 1x Battery Terminal
• 1x 9V Battery Clip
• 1x Power Slide Switch
• 1x 9V Battery

Tools:

• Soldering Iron
• Soldering Wire
• Clippers

IMPORTANT! Be very cautious about which way you insert the capacitors. The capacitors has two legs. One longer and one shorter, the shorter leg MUST go into the white marked hole. 

Now, insert the capacitors as shown bellow, the red marking should face upwards. 

Flip the Board with the capacitors and solder them.

Now Insert the transistors. The flat face should fce the same direction. On the PCB you will find an outline of the transistor, it should match the flat surface.

Flip the Board with the transistors and solder them.

Take the 4 resistors and bend them as shown in the image bellow.

Now insert the resistors into the PCB as shown.

Flip the Board with the resistors and solder them.

IMPORTANT! Be very cautious about which way you insert the LEDs. The LED has two legs. One longer and one shorter, the shorter leg MUST go into the white marked hole. 

After confirming insert both LEDs.

Flip the Board with the LEDs and solder them.

Now you can insert both blue potentiometers

Flip the Board with the blue potentiometers and solder them.

Find a slide switch and insert it at the top of the board.

Flip the Board with the slide switch and solder it. You can use some surface or tape to hold it in place while you solder it.

Using nail clippers or real ones, snip the unnecessary ends of the components.

IMPORTANT! Be very cautious about which way you insert the connector. The cutout hole should face the inward direction of the PCB.

You can now insert the battery connector on the other side of the PCB as shown bellow.

Flip the Board with the connector and solder it. You can use some surface or tape to hold it in place while you solder it.

Now plug the connector into the soldered port and connect the 9V Battery.

Now flip the switch, and your circuit should come to live! Congratulations!

Your Circuit is now Complete! Go show it to your friends and family!

 

BONUS

If you have an oscilloscope, you can probe the square wave output using the top row.

Connect the oscilloscopes ground probe to the middle pin, and the probe pin to either left or right channels. As you adjust the potentiometers you should see changes in your square wave!