A relaxation oscillator is an oscillator in which a capacitor is charged gradually and then discharged rapidly. It's usually implemented with a resistor, a capacitor, and some sort of "threshold" device such as a neon lamp, diac, or unijunction transistor.
The capacitor is charged through the resistor, causing the voltage across the capacitor to rise on an exponential curve. In parallel with the capacitor is the threshold device. Such devices don't conduct at all until the voltage across them reaches some threshold (trigger) voltage. They then conduct heavily, quickly discharging the capacitor. When the voltage across the capacitor drops to some lower threshold voltage, the device stops conducting and the capacitor can begin charging again, repeating the cycle.
The electrical output of a relaxation oscillator is usually a sawtooth wave. If the threshold element is a neon lamp, the circuit also provides a flash of light with each discharge of the capacitor. If a truly linear sawtooth is required, then the charging resistor should be replaced by some sort of constant current source.
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