The circuit that alternately flashes two LEDs with just two transistors and no IC is the astable multivibrator. It is a classic circuit used since the 1950s, and with few parts and a principle you can see, it makes a good first project for understanding transistors and capacitors. This article covers how it works, the parts list, calculating the blink period, the connections, perfboard layout, and how to check it.
What it does when finished
When you apply 5 V, the two LEDs blink alternately at about 1.5 Hz (one at a time for about 0.33 seconds). You can change the period with the resistor and capacitor values.
Parts list
| Part | Qty | Value | Role |
|---|---|---|---|
| NPN transistor | 2 | 2N3904 | Switch |
| LED | 2 | Red, green, and so on | Indicator |
| Resistors R1, R4 | 2 | 330 Ω | LED current limit |
| Resistors R2, R3 | 2 | 47 kΩ | Base bias |
| Electrolytic capacitors C1, C2 | 2 | 10 µF (16 V or more) | Timing |
| Power | 1 | 5 V (such as a USB power board) |
Use a supply of around 5 V (6 V or below). In this circuit a reverse voltage is applied across the transistor's base–emitter junction, and the allowed reverse voltage of the 2N3904 is about 6 V, so applying 9 V directly slowly damages the transistor.
Connection table
The connections are laid out in a table instead of a schematic. Each line is one net (points connected at the same potential).
| Net | Connected to |
|---|---|
| +5V | One side each of R1, R2, R3, and R4 |
| GND | Emitter (E) of Q1 and emitter (E) of Q2 |
| Q1 collector node | Cathode of LED1, collector (C) of Q1, + of C1 |
| Q2 collector node | Cathode of LED2, collector (C) of Q2, + of C2 |
| Q1 base node | The other side of R3, base (B) of Q1, − of C2 |
| Q2 base node | The other side of R2, base (B) of Q2, − of C1 |
| LED1 | Anode through R1 to +5V |
| LED2 | Anode through R4 to +5V |
Orient the electrolytic capacitors so the + side goes to the collector and the − side to the other transistor's base. Installing them the other way makes the operation unstable (Capacitor Types, Polarity, and Reading Values).
How it works
The two transistors turn on and off in opposition.
- When you apply power, the two transistors turn on slightly differently. Say Q1 turns on a little earlier.
- When Q1 turns on, its collector voltage drops to almost 0 V, and through C1 it pulls Q2's base down, so Q2 turns off. LED1 lights.
- The base of the off Q2 rises as C1 slowly charges through R2. When it reaches about 0.6 to 0.7 V, Q2 turns on.
- When Q2 turns on, in the same way C2 pulls Q1's base down, so Q1 turns off and LED2 lights. The process above repeats.
The time one LED stays on is set by the RC charging time (Understanding RC Time Constants and Capacitor Charging).
Calculating the period
One side's time t ≈ 0.693 × R_B × C Full period T ≈ 1.386 × R_B × C (when R2 = R3 and C1 = C2) Frequency f ≈ 1 / T
With R_B = 47 kΩ and C = 10 µF, T ≈ 1.386 × 47,000 × 0.00001 ≈ 0.65 s and f ≈ 1.5 Hz. Try changing the capacitors to 22 µF to slow it down or 1 µF to speed it up.
If you make the resistors or capacitors on the two sides different, the on-times differ (asymmetric blinking).
How to decide resistor values
- R1, R4 (LED resistors):
(5 V − LED voltage − about 0.2 V) ÷ target current. For a red LED at 8 mA, it is about 330 Ω (LED Current-Limiting Resistors). - R2, R3 (bias): so the transistor saturates when on, make
R_B ≤ hFE × R_C. The 2N3904 has an hFE of 100 or more, so using 330 Ω × 100 = about 33 kΩ as a guide, use 22 k to 47 kΩ. If it is too large, one side does not turn fully on and the LED is dim.
Perfboard layout tips
- Placing the two transistors and two LEDs symmetrically left and right simplifies the wiring.
- Put the power rail on top and the GND rail on the bottom (Power Rails, Grounding, and Decoupling Capacitor Placement).
- The 2N3904 pin order is E-B-C. Check from the left with the flat face toward you (Transistor Switch Circuits).
- The longest run that has to cross is the cross-coupling of the two capacitors. Handling it with a jumper wire is safer (Wiring Methods Compared).
Checking and troubleshooting
| Symptom | Cause | What to check |
|---|---|---|
| Both LEDs stay lit and stop | R_B too small or a capacitor in the wrong direction | Capacitor polarity, resistor values |
| Only one LED stays lit | A bad or reversed capacitor on one side, transistor pin mix-up | Recheck the pinout |
| Neither lights | Power reversed, LED backwards | Check polarity |
| Too fast, looks continuously lit | Capacitor too small | Increase the capacitance |
If you cannot solve it, follow It Is Finished but It Does Not Work: A Step-by-Step Checklist.
Ideas to extend it
- Add one more transistor stage instead of the LEDs to drive a big lamp or buzzer.
- Replace a resistor with a potentiometer to adjust the speed.
- Compare it with the astable circuit made with a 555 timer. It has fewer parts but a far more accurate frequency.
Summary
An astable multivibrator is a circuit in which two transistors and two capacitors pull on each other's bases and turn on and off alternately. Use it at 5 V, orient the capacitors' + toward the collector, and predict the period with 1.386 × R × C.