An electronic circuit that generates repeated waveforms is an oscillator. The exact waveform generated depends on the type of circuit used to create the oscillator. Some circuits generate sine waves, some generate square waves, and others generate other types of waves. Oscillators are essential ingredients in many different types of electronic devices, including radios and computers.

One of the most commonly used oscillator circuits is made from a pair of transistors that are rigged up to alternately turn on and off. This type of circuit is called a multivibrator.

If the circuit is designed to continuously cycle between the two transistors, it's called an astable multivibrator because the circuit never reaches a point of stability — that is, it never decides which of the two transistors should be on, so it just keeps flipping back and forth between the two. Astable multivibrators are great for producing square waves.

In Project 6-3, you build a circuit that uses an astable multivibrator to alternately flash two LEDs. LED flasher circuits are a favorite of electronic hobbyists because flashing LEDs have all sorts of fun uses. For example, you can add creepy blinking eyes to a jack-o'-lantern for Halloween, or you can add blinking warning lights to your model railroad layout.

The LED flasher circuit is simply an astable multivibrator with LEDs added to the collector circuit of each transistor and filled in the resistor and capacitor values. With the values selected for this project, the lights alternate quickly, a bit faster than once per second.

If you feel like experimenting a bit after you complete this project, here are a couple of suggestions:

  • Try replacing R2 and R3 with larger resistors, such as 220 kΩ and 470 kΩ. What effect does this have on the flasher?

  • Try adding a potentiometer in series with R2 or R3. This allows you to vary the flash rate by turning the potentiometer knob.

  • Try changing the capacitors.

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