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Timing 555 Astable
555 Data Sheet from Texas Instruments
The 555 Astable output is a stream of accurately timed pulses. There are zero stable states.
This is an oscillator circuit providing a square wave output.
The astable has zero stable states. Whichever state it is in (high or low), it will soon switch to the other state.
The 555 chip CAN NOT drive a normal 8 Ohm loudspeaker. Piezoelectric sounders can be plugged straight in.
The astable will oscillate at this frequency ...
F = 1.44 / ((Ra + 2Rb) C1)
The square wave output is low for the following period ...
tL = 0.7 RbC1
The square wave output is high for the following period ...
tH = 0.7 (Ra + Rb) C1
Use a potentiometer to adjust the frequency.
Replace the Rb timing resistor with a light dependent resistor (LDR).
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