LM555
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SNAS548C –FEBRUARY 2000–REVISED MARCH 2013
The charge time (output high) is given by:
t1 = 0.693 (RA + RB) C
(1)
(2)
(3)
And the discharge time (output low) by:
t2 = 0.693 (RB) C
Thus the total period is:
T = t1 + t2 = 0.693 (RA +2RB) C
The frequency of oscillation is:
(4)
(5)
Figure 17 may be used for quick determination of these RC values.
The duty cycle is:
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.
VCC = 5V
Top Trace: Input 4V/Div.
TIME = 20μs/DIV. Middle Trace: Output 2V/Div.
RA = 9.1kΩ
C = 0.01μF
Bottom Trace: Capacitor 2V/Div.
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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