NE555/SA555/SE555
Figure 13
V
CC
= 5 to 15V
Figure 15 :
Free Running Frequency versus R
1
,
R
2
and C
1
R1
4
Output
3
8
7
C
(µF)
10
1.0
0.1
NE555
Control
Voltage
0.01
µ
F
5
1
2
6
R2
R1
+
R2
1k
Ω
10
k
Ω
10
0
1M k
Ω
=
Ω
C1
555-17.EPS
0.01
0.001
0.1
10
M
Ω
100
1k
10k
f
o
(Hz)
Figure 14 shows actual waveforms generatedin this
mode of operation.
The charge time (output HIGH) is given by :
t
1
= 0.693 (R
1
+ R
2
) C
1
and the discharge time (output LOW) by :
t
2
= 0.693 (R
2
) C
1
Thus the total period T is given by :
T = t
1
+ t
2
= 0.693 (R
1
+ 2R
2
) C
1
The frequency ofoscillation is them :
1.44
1
f
=
=
T
(R
1
+
2R
2
)
C
1
and may be easily found by figure 15.
The duty cycle is given by :
R
2
D
=
R
1
+
2R
2
1
10
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 16 shows the circuit.
Figure 16 :
Pulse Width Modulator.
V
CC
R
A
4
8
7
Figure 14
t = 0.5 ms / div
Trigger
2
NE555
OUTPUT VOLTAGE = 5.0V/div
Output
3
1
6
Modulation
Input
5
C
555-18.EPS
555-19.EPS
CAPACITOR VOLTAGE = 1.0V/div
R1 = R2 = 4.8k
Ω
, C1= 0.1
µ
F, R
L
= 1k
Ω
7/10
555-20.EPS
STMICROELECTRONICS [ STMICROELECTRONICS ]
