LM555

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SNAS548D – FEBRUARY 2000 – REVISED JANUARY 2015

Feature Description (continued)

capacitor equals 2/3 of the supply voltage. The output pulse width can be extended or shortened depending

on the application by adjusting the R and C values.

•

Astable (free-running) mode: The LM555 timer can operate as an oscillator and puts out a continuous stream

of rectangular pulses having a specified frequency. The frequency of the pulse stream depends on the values

7.4 Device Functional Modes

7.4.1 Monostable Operation

In this mode of operation, the timer functions as a one-shot (Figure 11). The external capacitor is initially held

pin 2, the flip-flop is set which both releases the short circuit across the capacitor and drives the output high.

Figure 11. Monostable

and drives the output to its low state. Figure 12 shows the waveforms generated in this mode of operation. Since

the charge and the threshold level of the comparator are both directly proportional to supply voltage, the timing

interval is independent of supply.

Top Trace: Input 5V/Div.

TIME = 0.1 ms/DIV.

Middle Trace: Output 5V/Div.

Bottom Trace: Capacitor Voltage 2V/Div.

C = 0.01

μF

Figure 12. Monostable Waveforms

During the timing cycle when the output is high, the further application of a trigger pulse will not effect the circuit

so long as the trigger input is returned high at least 10

μs before the end of the timing interval. However the

circuit can be reset during this time by the application of a negative pulse to the reset terminal (pin 4). The output

will then remain in the low state until a trigger pulse is again applied.

false triggering.

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