Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
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Article by Ray Wilson

Simplest Oscillator On Earth

U1 is a hex inverting schmidt trigger. There are 6 (hex) inverters in the IC. An inverter means a high input will give you a low output and a low input will give you a high output. With +9V volts on the input you get ground on the output. Conversely, with ground on the input you get +9V on the output.

A "Schmidt Trigger" has built in hysteresis which means that the input voltage has to go higher than 2/3 of the supply voltage before the output goes low and then the input voltage has to go below 1/3 of the supply voltage before the output goes high (keep in mind this is an inverting Schmidt Trigger). The middle 1/3 of the voltage range is called the hysteresis zone.

This oscillator works as follows: When you apply power U1-A's output goes high because the .01uF capacitor is discharged. U1-A's output charges the .01uF cap through both the 1Meg pot and the 1K resistor. When the .01uF capacitor charges to greater than 6 volts U1-A's output goes low (to 0 volts). This causes the .01uF cap to discharge through both the 1Meg pot and the 1K resistor until... the voltage on the .01uF cap gets to less than 3 volts at which time... U1-A's output goes high again and the cycle repeats and repeats and repeats... you get what I mean. Adjusting the 1 Meg pot causes the .01uF cap to charge and discharge faster or slower thus changing the frequency of the oscillatio.