LM555
Applications Information
(Continued)
FREQUENCY DIVIDER
The monostable circuit of
can be used as a fre-
quency divider by adjusting the length of the timing cycle.
shows the waveforms generated in a divide by three
circuit.
shows the waveforms generated in this mode of
operation.
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V
CC
= 5V
TIME = 20µs/DIV.
R
A
= 3.9kΩ
R
B
= 3kΩ
C = 0.01µF
Top Trace: Output 5V/Div.
Bottom Trace: Capacitor Voltage 1V/Div.
V
CC
= 5V
TIME = 20µs/DIV.
R
A
= 9.1kΩ
C = 0.01µF
Top Trace: Input 4V/Div.
Middle Trace: Output 2V/Div.
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Bottom Trace: Capacitor 2V/Div.
FIGURE 5. Astable Waveforms
The charge time (output high) is given by:
t
1
= 0.693 (R
A
+ R
B
) C
And the discharge time (output low) by:
t
2
= 0.693 (R
B
) C
Thus the total period is:
T = t
1
+ t
2
= 0.693 (R
A
+2R
B
) C
The frequency of oscillation is:
FIGURE 7. 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.
shows the circuit, and in
are some waveform
examples.
may be used for quick determination of these RC
values.
The duty cycle is:
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FIGURE 8. Pulse Width Modulator
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FIGURE 6. Free Running Frequency
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