555 Data Sheet from Texas Instruments
The 555 Astable output is a stream of accurately timed pulses. There are zero stable states.
555 Astable Key Facts
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 frequency depends on C1.
The circuit below uses a piezoelectric sounder and a coupling capacitor is not needed.
The output LED indicator is optional.
How it Works - Exam Answer
If the output is high, the discharge transistor is off.
C1 charges towards Vs through Ra and Rb until the threshold rises above 2/3 of the supply voltage.
The output goes low and the discharge transistor turns on.
C1 discharges towards 0 Volts through Rb until the threshold drops below 1/3 of the supply voltage.
The output goes high again and the steps above repeat.
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
Symmetrical Square Wave
To get a fairly symmetrical square wave, Ra should be a lot smaller then Rb.
For adjustable frequencies, Rb can be a variable resistor.
Try using an LDR for Rb. The frequency will depend on the light level.
Things to Try
Use a potentiometer to adjust the frequency.
Replace the Rb timing resistor with a light dependent resistor (LDR).
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