555Timer_datasheet

　　These devices are precision timing circuits capable of producing accurate time delays or oscillation. In the time-delay or monostable mode of operation， the timed interval is controlled by a single external resistor and capacitor network. In the astable mode of operation， the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor. The threshold and trigger levels normally are two-thirds and one-third， respectively， of VCC. These levels can be altered by use of the control-voltage terminal. When the trigger input falls below the trigger level， the flip-flop is set， and the output goes high. If the trigger input is above the trigger level and the threshold input is above the threshold level， the flip-flop is reset and the output is low. The reset （RESET） input can override all other inputs and can be used to initiate a new timing cycle. When RESET goes low， the flip-flop is reset， and the output goes low. When the output is low， a low-impedance path is provided between discharge （DISCH） and ground. The output circuit is capable of sinking or sourcing current up to 200 mA. Operation is specified for supplies of 5 V to 15 V. With a 5-V supply， output levels are compatible with TTL inputs.