Two-LED Transistor Flasher (Astable Multivibrator)

Introduction

Want to make two LEDs blink alternately without any ICs?
In this project, we’ll build a simple two-transistor oscillator, also called an astable multivibrator, using only NPN transistors, resistors, and capacitors from your parts collection.

This circuit is a fun way to see:

• How capacitors can be used for timing.

• How transistors can act as switches in a feedback loop.

• How changing just one or two components can speed up or slow down the blink.

How It Works

The circuit has two identical halves that “hand off” control.

• When Q1 is on, its LED conducts current while its coupling capacitor pushes Q2’s base negative, keeping Q2 off.

• As the capacitor recharges through a resistor, its voltage rises until Q2 turns on and Q1 switches off.

• The cycle repeats in the opposite direction: Q2’s LED lights while Q1 is held off.

This constant back-and-forth produces the alternating blink.

For symmetric resistor and capacitor values, the oscillation period is approximately:

$$T \approx 1.386 \times R_B \times C$$

And the frequency:

$$f \approx \frac{1}{T} = \frac{1}{1.386 \times R_B \times C}$$

Where:

• $B$ = base timing resistor

• $\mathrm{C =\; cross-coupling\; capacitor}$

Parts Needed 

• 2 × LEDs (any colour)

• 2 × Resistors: 470 Ω – 680 Ω

• 2 × 2N2222 (NPN)

• 2 × Resistors: 100 kΩ

• 2 × Electrolytic capacitors: 47 µF

• Breadboard + jumper wires

• 5 V bench power supply

Circuit Diagram

Wiring Steps

Step 1:

Place the two transistors onto the breadboard and connect the Emitter pins (1) to GND

Step 2:

Connect the two 100 kΩ resistors with one side to the +5V line on the breadboard and the other to the base (2) of each transistor

Step 3:

Connect one side of each of the 470 Ω resistors to the +5v line. Then connect the anode side of the LEDs to the other side of the resistors. The connect the cathode side of the LEDs to the collector (3) of the transistor.

Step 4:

Connect the cathode of each Electrolytic capacitors between the 100k Ω resistor and base (2) of the transistors. Then connect the anode between the LED and the collector (3) of the transistors.

Step 5:

Connect the 5V power supply to the respective 5v and GND rails on the breadboard. 

You probably want to undertake the following checks before powering up:-

• Electrolytic polarity: “+” to collector, “−” to base.
• Pinout: Verify your 2N2222 variant (TO-92 is often E–B–C left to right with flat face toward you, but check).
• Bench supply: 5.0 V, current limit ≈ 100 mA.

Power on and watch the LEDs flash alternatively

Troubleshooting

• Both LEDs stuck on: One cap reversed or cross-coupled to wrong base.

• Only one LED blinks: LED reversed, wrong pinout, or poor joint on the opposite cap.

• No oscillation: Ensure each 100 kΩ goes to +5 V and meets the cap “−” + base node; confirm caps are oriented correctly.

Calculations

1) Blink frequency & period

With $R_B = 100{,}000\ \Omega$ and $C = 47\ \mu F = 47 \times 10^{-6}\ \text{F}$

Step 1 — Multiply $R_B\times C$:

$$\mathrm{100,000}\times 47\times {10}^{-6}=4.700\text{ s}\text{<br>}\text{<br>}\text{<br>}$$

Step 2 — Period:

$$T\approx 1.386\times 4.700\approx 6.51\text{ s}$$

Step 3 — Frequency:

$$f\approx \frac{1}{6.51}\approx 0.154\text{ Hz}$$

Step 4 — On-time per LED:

$$t_{\text{on}}\approx \frac{T}{2}\approx 3.26\text{ s}$$

Result:

• Frequency ≈ 0.154 Hz

• Period ≈ 6.5 s

• Each LED on ≈ 3.3 s

2) LED current with 470 Ω at 5 V

​When the transistor is saturated:

$$I_{LED} \approx \frac{V_{CC} - V_F - V_{CE(sat)}}{R_{LED}}$$

With $V_{CC} = 5.0\ \text{V}, V_F \approx 2.0\ \text{V}, V_{CE(sat)} \approx 0.15\ \text{V}$

$$I \approx \frac{5.0 - 2.0 - 0.15}{470} \approx 6.1\ \text{mA}$$

Resistor power:

$$P = I^2 R \approx (0.0061)^2 \times 470 \approx 0.017\ \text{W}$$

Which is well within a ¼ W resistor rating.

3) Base current with 100 kΩ

• Step 1 — Base current:

  $$I_B \approx \frac{V_{CC} - V_{BE}}{R_B} = \frac{5.0 - 0.7}{100{,}000} \approx 43\ \mu A$$

• Step 2 — Collector current (from LED calc):

  $$I_C \approx 6.1\ \text{mA}$$

• Step 3 — Required transistor gain:

  $${\beta }_{needed}\approx \frac{I_C}{I_B}\approx \frac{6.1\text{ mA}}{43\mu A}\approx 141$$

Most 2N2222s can achieve this at a few milliamps. The capacitors also inject extra transient base current at each transition, which helps switching.

If blinking is weak or sluggish:

• Use red LEDs (lower forward voltage).

• Reduce 100 kΩ to 82 kΩ (more base current).

• Increase LED resistors to 680 Ω (reduces current demand).

Conclusion

The two-transistor flasher is a classic example of a timing circuit made from basic components. Once you’ve built this on 5 V, try swapping capacitor values or resistor values to see how the blink rate changes.