Integrated 555 Timer
The 555 timer is very versatile. It can be configured to operate in a running (astable) mode or one-shot (monostable) mode. The ouputs can be normally on or normally off, with time periods ranging from microseconds to hours.
Using a 1000uF capacitor and 9.1 kΩ resitor, the time period is 10.01 seconds. During the timing cycle, any additional trigger pulses are ignored. The timing cycle can be interupted, however, by applying a negative going pulse to the Reset terminal (Pin 4).
In monostable mode, the output switches for a fixed period of time after the timing circuit is triggered. The timing circuit is triggered when a negative going pulse (<1/3 Vcc) is applied to the Trigger terminal (pin 2). The time period is established by the value of Ra and C, using the formula:
The astable mode means the timing circuit is continuously triggered. The Reset terminal (pin 4) is tied to Vcc to prevent the timing ccircuit from being reset. The Trigger terminal (pin 2) is tied to the Threshold terminal (pin6).
The capacitor (C) is charged through Ra and Rb, and discharges through Rb. The duty cycle (% On ratio) can be precisely set by proper selection of Ra and Rb. The times are calculated as:
- Charge Time = 0.693 * ( Ra + Rb ) * C
- Discharge Time = 0.693 * Rb * C
- Cycle Time = Charge Time + Discharge Time
- Frequency = 1 / Cycle Time = 1.44 / [ ( Ra + Rb ) * C ) ]
- Duty Cycle = Rb / ( Ra + 2 * Rb )