555 Timer

Connection digram for the 555 IC timer (click for larger image).

Datasheet for the LM555

Inside the 555 timer is a voltage divider the divides +V_CC into thirds. A voltage comparator compares the trigger input (pin 2) with 1/3 V_CC, and another comparator compares the threshold (pin 6) with 2/3 V_CC. The point at 2/3 V_CC on the voltage divider is connected to the control voltage (pin 5). This pin can be used to modify the values of 1/3 V_CC and 2/3 V_CC without having to change V_CC. However, if this input is not going to be used, it should be connected to a bypass capacitor (0.01uF) to protect it from noise.

The trigger and reset inputs are level-sensitie active low. To activate the trigger, the voltage on the pin must be pulled down to under 1/3 V_CC. To reset the timer, the voltage on the pin must be pulled under 0.4V. The reset input will override other inputs and set the output to "low".

Monostable and Astable mode

The 555 family of timer chips can be used in either monostable mode or astable' mode.