LM555/NE555/SA555
5
Figure 1 illustrates a monostable circuit. In this mode, the timer generates a fixed pulse whenever the trigger voltage falls
below Vcc/3.
When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low, the timer's internal
flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor C1and
setting the flip-flop output at the same time.
The voltage across the external capacitor C1, VC1 increases exponentially with the time constant t=RA*C and reaches 2Vcc/3
at td=1.1RA*C. Hence, capacitor C1 is charged through resistor RA. The greater the time constant RAC, the longer it takes
for the VC1 to reach 2Vcc/3. In other words, the time constant RAC controls the output pulse width.
When the applied voltage to the capacitor C1 reaches 2Vcc/3, the comparator on the trigger terminal resets the flip-flop,
turning the discharging Tr. on. At this time, C1 begins to discharge and the timer output converts to low.
In this way, the timer operating in monostable repeats the above process. Figure 2 shows the time constant relationship based
on RA and C. Figure 3 shows the general waveforms during monostable operation.
It must be noted that, for normal operation, the trigger pulse voltage needs to maintain a minimum of Vcc/3 before the timer
output turns low. That is, although the output remains unaf fected even if a dif f erent trigger pulse is applied while the o utput is
high, it may be affected and the waveform not operate properly if the trigger pulse voltage at the end of the output pulse
remains at below Vcc/3. Figure 4 shows such timer output abnormality.
2. Astable Oper ation
Figure 4. Waveforms of Monostable Operation (abnormal)
84
7
1
2
3
5
6
CONT
GND
Vcc
DISCH
THRES
RESET
TRIG
OUT
+Vcc
RA
C1
C2
RL
RB
100m 1 10 100 1k 10k 100k
1E-3
0.01
0.1
1
10
100
10MΩ
ΩΩ
Ω
1MΩ
ΩΩ
Ω
100kΩ
ΩΩ
Ω
10kΩ
ΩΩ
Ω
1kΩ
ΩΩ
Ω
(RA+2RB)
Capacitance(uF)
Frequency(Hz)
Figure 5. Astable Circuit Figure 6. Capacitance and Resistance vs. Frequency