Skip to content
LearnTronics menu

LearnTronics

Dual-LED Transistor Flasher

Tutorial Index

Recipe #26 — dual-led transistor flasher

Two transistors and two capacitors can form a free-running oscillator. This classic astable multivibrator makes the RC timing action visible by flashing two LEDs alternately.

DUAL-LED TRANSISTOR ASTABLE MULTIVIBRATOR +9 V RC1 1 kΩ LED1 RC2 1 kΩ LED2 RB1 47 kΩ RB2 47 kΩ B C E Q1 B C E Q2 C1 22 µF + Q1 collector → Q2 base C2 22 µF + Q2 collector → Q1 base The two transistor stages are identical. C1 and C2 cross-couple each collector to the opposite base.
How to trace this drawing: Start at either collector dot. C1 carries the change at Q1's collector to Q2's base. C2 does the opposite: it carries the change at Q2's collector to Q1's base. The two transistor symbols are identical NPN transistors drawn in the same orientation.
There is no stable resting state. When Q1 turns on, its collector change is coupled through C1 and pushes Q2 toward off. C2 later lets Q2 recover and the circuit flips. Then the same process happens in the opposite direction.

Why this is called astable

Astable means neither state is permanently stable. The circuit continuously alternates between its two states, so LED1 and LED2 flash back and forth.

Timing is still RC

Each cross-coupling capacitor charges through a base resistor. In a roughly symmetrical circuit, each half-cycle is on the order of 0.69 RBC. Real transistor voltages and capacitor tolerances make this an approximation rather than an exact clock.

Put some numbers on it

For the example shown above, use: 9 V, RB1 = RB2 = 47 kΩ, C1 = C2 = 22 µF, and RC1 = RC2 = 1 kΩ.

Half-cycle ≈ 0.69 × RB × C

≈ 0.69 × 47,000 × 0.000022 ≈ 0.71 second

That is approximately how long one transistor remains in its state before the cross-coupled RC network causes the circuit to flip.

Full cycle ≈ 2 × 0.71 ≈ 1.43 seconds

Frequency ≈ 1 ÷ 1.43 ≈ 0.70 Hz

So each LED lights about once every 1.43 seconds, or roughly 42 flashes per minute. Because the LEDs alternate, there is a visible change from one LED to the other about twice that often.

LED current

For a typical red indicator LED, assume about 2.0 V across the LED and about 0.2 V across a transistor that is fully ON:

I ≈ (9 − 2.0 − 0.2) ÷ 1,000 ≈ 0.0068 A = 6.8 mA

That is a comfortable current for an ordinary indicator LED. The 1 kΩ resistor would dissipate only about 0.046 W, so a common 1/4-watt resistor has plenty of margin.

Changing the flash rate

RB C Approx. half-cycle Approx. full-cycle frequency
47 kΩ 10 µF 0.32 s 1.54 Hz
47 kΩ 22 µF 0.71 s 0.70 Hz
47 kΩ 47 µF 1.52 s 0.33 Hz

Larger resistance or capacitance makes the flashing slower; smaller values make it faster. The actual rate will vary because electrolytic capacitors commonly have fairly wide tolerances and transistor switching voltages are not exact.

Electrolytic capacitor polarity: in this circuit, the positive lead of each timing capacitor goes toward the collector side and the negative lead goes toward the opposite transistor's base.

This is an oscillator made from ordinary transistors

No 555 timer is needed. Positive feedback provides the switching and RC charging provides the delay. The next lessons package related ideas into the very convenient 555 timer IC.

Try these yourself

Why do the LEDs alternate instead of staying steadily on?
Each collector transition is coupled to the opposite transistor base, forcing alternating states.
Which parts mainly set the flashing rate?
The cross-coupling capacitors charge through the base-bias resistors, producing the timing delays.
What does astable mean here?
An astable circuit keeps moving between two states rather than resting in either one.
< Automatic Nightlight with Delay   |   555 Timer One-Shot >