Astable multivibrator (two-transistor oscillator, LED flasher)

Objective

Two cross-coupled transistors with capacitors form a free-running oscillator that flashes two LEDs.

Procedure

1. Run and compare the two collector waveforms on the oscilloscope.
2. Change a capacitor with the Controls slider (Live on): the flash rate and duty change.

Theory

Each capacitor charges through a 47 kΩ base resistor until the other transistor turns on; the half period is t ≈ 0.69·R·C = 0.69 · 47 kΩ · 10 µF = 0.32 s, so the period is of the order of 1.4·R·C ≈ 0.65 s (the simulation gives about 0.5 s because the LEDs and collector resistors lower the swing) with the LEDs alternating.

Questions to answer

1. How would you get an unequal on/off ratio?

Components used

Battery / DC source, 2 × Electrolytic Capacitor 100uF 25V, LED 5mm Green, LED 5mm Red, Oscilloscope (2-channel), 4 × Resistor 220 ohm 1/4W, 2 × Transistor BC547 (NPN)

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.

More Logic Gates from Components experiments

• OR gate from diodes
• AND gate from diodes
• NOT gate (inverter) from an NPN transistor
• NOR gate (RTL: resistor-transistor logic)
• NAND gate (transistors in series)