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HOW THE 555 OSCILLATES


The capacitor charges via the timing resistor
R and when the voltage across it reaches 2/3 of the supply voltage, the output of the 555 goes LOW. The timing resistor is taken to the 0v rail via pin 7 and the capacitor discharges. When the voltage across the capacitor reaches 1/3 of rail voltage the output of the 555 goes HIGH. The timing resistor is taken to the positive rail via the top resistor (pin7 effectively comes out of circuit) and the cycle repeats. Don't worry about pins 4 or 5 at the moment.
The animation below shows how the 555 oscillates:

These are the three points to note:
1. Pin 2 detect the low voltage on the capacitor, and makes pin 7 and the output go HIGH
2. Pin 6 detects the high voltage on the capacitor and makes pin 7 and the output go LOW
3. Pin 7 is "in-phase" with the output. (both are low at the same time)

An improved oscillator is shown in the diagram below. It uses only one resistor to charge and discharge the capacitor and the circuit does not have the wasteful top resistor. The circuit draws less current than the circuit above but the only difference is the frequency of operation will be lower for the same value of components because the voltage delivered by the output line is 1.7v less than the supply rail. The output can deliver up to 200mA but if it is delivering a high current, the output voltage may be reduced and this will affect the frequency of operation. If a reliable frequency is needed, this is not the circuit to choose.