LC Tank Oscillator

Build an oscillator using an inductor and capacitor—watch electrical energy bounce back and forth between magnetic and electric fields!

Overview

When you connect an inductor and capacitor in parallel, energy sloshes back and forth between them—from the capacitor's electric field to the inductor's magnetic field and back again. This creates oscillation at a specific frequency. Add a transistor to sustain the oscillation, and you've built a radio-frequency oscillator! This is how radios generate and receive signals.

Components Needed

  • 1x Inductor 1mH (or wind your own)
  • 1x Capacitor 10nF or 100nF ceramic
  • 1x NPN Transistor (2N2222 or 2N3904)
  • 2x Resistor 10kΩ
  • 1x Resistor 1kΩ
  • 1x Capacitor 10µF electrolytic
  • 1x LED (optional, for low-frequency version)
  • 1x Piezo buzzer or speaker (for audio)
  • 1x 9V Battery
  • 1x 9V Battery clip
  • 1x Breadboard
  • severalx Jumper wires
  • 1x Oscilloscope (optional but helpful)

Instructions

  1. Build the LC Tank

    Connect the inductor and capacitor in parallel—this is your "tank circuit." For 1mH and 10nF, the resonant frequency is about 50kHz (ultrasonic). For audible oscillation, use a larger capacitor (100nF → 15kHz) or larger inductor.

  2. Test the Tank (Optional)

    If you have an oscilloscope: charge the capacitor briefly with a battery, then disconnect. You'll see decaying oscillation (ringing) as energy bounces between L and C. Measure the frequency—it should match your calculation!

  3. Build the Colpitts Oscillator

    A Colpitts oscillator uses capacitive voltage divider for feedback. Connect: collector to one side of LC tank, emitter through a capacitor to a tap in the LC tank, base biased with 10kΩ resistors. The transistor amplifies and feeds back to sustain oscillation.

  4. Alternative: Build a Hartley Oscillator

    A Hartley oscillator uses inductive tap instead. If your inductor has a center tap (or you use two inductors), you can build this variation. The principle is the same—feedback sustains oscillation.

  5. Add Output

    Connect a piezo buzzer or speaker (through a capacitor to block DC) to hear the oscillation if in audio range. For higher frequencies, use an oscilloscope or AM radio tuned to the frequency to detect the signal.

  6. Tune Your Oscillator

    Try different capacitor values. Smaller C = higher frequency. If you built a variable capacitor project, use it here! You've just built a Variable Frequency Oscillator (VFO) for a radio transmitter!

Challenges

  • Calculate the resonant frequency for several L and C combinations, then verify with oscilloscope or frequency counter
  • Build a crystal oscillator using a quartz crystal instead of LC tank for precise frequency
  • Create an AM transmitter by modulating the oscillator with audio (very low power, short range)
  • Build a variable-frequency oscillator using a potentiometer and varactor diode
  • Measure the Q factor by observing how quickly free oscillation dies out
  • Build a regenerative radio receiver using LC tuning and transistor amplification