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Basically the whole operation of the 555 timer revolves around the three resistors that form a voltage divider across the power supply, each resistor ~5k (for the bipolar version of the IC) so the comparator reference voltages are 1/3 and 2/3 of the supply voltage. The pin 5 identified as ''Control Voltage Input'' can directly affect this relationship, although most of the time this pin is unused. This pin controls the timing of the circuit by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin or connecting via resistor to ground the width of the output signal can be varied independently of the RC timing network. When not used may be connected to ground via a capacitor to eliminate noise and to reduce voltage spikes when the output transistors of the timer change states.
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I love say you :
If you give a voltage to pin 5 and if change the value of voltage you can change frequency of output ! its very interesting and useful !
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