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 555 timer also has a flip-flop, which is controlled by the two comparators and the reset input.

The trigger and reset inputs are level-sensitive active low inputs. 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 (pin 3) to low. If the reset input is not going to be used, it should be wired to V_CC to prevent false signals.

In its initial state (assuming threshold is low and trigger is high), the 555 timer's internal flip-flop connects the discharge (pin 7) to the ground and sets the output to low. Each time the trigger is pulled down under 1/3 V_CC, the flip-flop will break the discharge pin's connection to ground, and set the output to high. It will hold this state until something (usually a capacitor) forces the threshold pin's voltage equal to 2/3 V_CC, which will reset the flip-flop.

Monostable and Astable mode

The 555 family of timer chips can be used in either monostable mode or astable' mode. In the monostable or "one shot" mode, each time the 555 timer is triggered, the output will go high for a specified amount of time, then return to low and await another trigger signal. In the astable mode, the timer triggers itself periodically and becomes an oscillator, sending out a train of pulses.