LM555

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SNAS548C – FEBRUARY 2000 – REVISED MARCH 2013

The charge time (output high) is given by:

(1)

And the discharge time (output low) by:

(2)

Thus the total period is:

(3)

The frequency of oscillation is:

(4)

Figure 17 may be used for quick determination of these RC values.

The duty cycle is:

(5)

Figure 17. Free Running Frequency

FREQUENCY DIVIDER

The monostable circuit of Figure 12 can be used as a frequency divider by adjusting the length of the timing

cycle. Figure 18 shows the waveforms generated in a divide by three circuit.

Top Trace: Input 4V/Div.

TIME = 20

μs/DIV.

Middle Trace: Output 2V/Div.

Bottom Trace: Capacitor 2V/Div.

C = 0.01

μF

Figure 18. Frequency Divider

PULSE WIDTH MODULATOR

When the timer is connected in the monostable mode and triggered with a continuous pulse train, the output

pulse width can be modulated by a signal applied to pin 5. Figure 19 shows the circuit, and in Figure 20 are

some waveform examples.

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