555 timer
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Information
555 is a small timer/oscillator circuit, time or frequency can be adjusted using a resistor and a capacitor. The name 555 originates from three 5 Kohms resistors used in early implementations. Because of the components low cost and good stability it's still wildly in use today. It can be used both astable (oscillator) or monostable (timer), read more about the different operations below.
Oscillator
The output is active for t1 seconds:
t1(secs)=0.693*(Ra+Rb)*C/1000
The output is inactive for t2 seconds:
t2(secs)=0.693*Rb*C/1000
One-time event
Alternative 1
The output is normally inactive, when the button is pushed it goes active for t1 seconds. If the button is held down longer then t1 seconds the output will remain active until the button is released.
t1(secs)=1.1*Ra*C/1000
Alternative 2
The output is normally inactive, when the button is pushed it goes active for t1 seconds. If the button is held down longer then t1 seconds the output will only remain active in t1 seconds.
t1(secs)=1.1*Ra*C/1000
Delay
When the circuit is energized the output will be inactive until t1 seconds has passed, then it will go active and stay active.
t1(secs)=1.1*Ra*C/1000
