Description
The 555 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 (monostable) mode of operation the time is precisely controlled by one external resistor and one capacitor. For stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms and the output structure can source or sink up to 200mA or drive TTL directly. This IC may also be correctly supplied marked as MC1455PI. Supply decoupling must be provided close to the IC to counter the ‘crowbar’ effect of the device’s internal discharge switch, a suitable value is 10 to 100µF as shown in the accompanying diagrams.Monostable ModeTime Delay vs RA and COn time after triggering (i.e. applying a voltage to pin 2 less than 1/3 supply voltage) is equal to 1.1RAC. The load may be connected to VCC for normally-on operation or between pin 3 and ground for normally- off. Connecting reset to ground during on time, drives the output low until a new trigger pulse occurs. Additional trigger pulses during on time have no effect. If reset is not being used, connect it to VCC.Astable Mode (Oscillator)Free Running Frequencyvs RA, RB and CThe frequency is equal to 1.44/[(RA + 2RB)C]. The charge time (output high) is given by t1 = 0.693(RA + RB)C and the discharge time (output low) is given by t2 = 0.693(RB)C.Characteristics (typical)
