25-06-2013, 12:58 PM
LM555/LM555C Timer
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General Description
The LM555 is a highly stable device for generating accurate time delays or oscillation. Additional terminals are provided for triggering or resetting if desired. In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor. For astable operation as an oscilla- tor, the free running frequency and duty cycle are accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output circuit can source or sink up to 200 mA or drive TTL circuits.
Features
Direct replacement for SE555/NE555
Timing from microseconds through hours
Operates in both astable and monostable modes
Adjustable duty cycle
Output can source or sink 200 mA
Output and supply TTL compatible
Temperature stability better than 0.005% per §C
Normally on and normally off output
Applications
Precision timing
Pulse generation
Sequential timing
Time delay generation
Pulse width modulation
Pulse position modulation
Linear ramp generator
MONOSTABLE OPERATION
In this mode of operation, the timer functions as a one-shot (Figure 1 ). The external capacitor is initially held discharged by a transistor inside the timer. Upon application of a nega- tive trigger pulse of less than 1/3 VCC to pin 2, the flip-flop is set which both releases the short circuit across the capaci- tor and drives the output high.