This is circuit that first introduced by Signetics Corporation as the SE555/NE555 about 1971. Pin connections and functions: (See schematic below for basic circuits). Pin 1 (Ground) – The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to circuit common when operated from positive supply voltages. Pin 2 (Trigger) – This pin is the input which causes the output to go high and begin the timing cycle. Triggering occurs when the trigger input moves from a voltage above 2/3 of the supply voltage to a voltage below 1/3 of the supply. This is the figure of the circuit.
For example using a 12 volt supply, the trigger input voltage must start from above 8 volts and move down to a voltage below 4 volts to begin the timing cycle. The action is level sensitive and the trigger voltage may move very slowly. To avoid retriggering, the trigger voltage must return to a voltage above 1/3 of the supply before the end of the timing cycle in the mono stable mode. Trigger input current is about 0.5 micro amps. Pin 3 (Output) – The output pin of the 555 moves to a high level of 1.7 volts less than the supply voltage when the timing cycle begins. The output returns to a low level near 0 at the end of the cycle. Maximum current from the output at either low or high levels is approximately 200 mA. Pin 4 (Reset): – A low logic level on this pin resets the timer and returns the output to a low state. It is normally connected to the + supply line if not used.
Pin 5 (Control) – This pin allows changing the triggering and threshold voltages by applying an external voltage. When the timer is operating in the astable or oscillating mode, this input could be used to alter or frequency modulate the output. If not in use, it is recommended installing a small capacitor from pin 5 to ground to avoid possible false or erratic triggering from noise effects. Pin 6 (Threshold) – Pin 6 is used to reset the latch and cause the output to go low. Reset occurs when the voltage on this pin moves from a voltage below 1/3 of the supply to a voltage above 2/3 of the supply. The action is level sensitive and can move slowly similar to the trigger voltage. Pin 7 (Discharge) – This pin is an open collector output which is in phase with the main output on pin 3 and has similar current sinking capability. Pin 8 (V +) – This is the positive supply voltage terminal of the 555 timer IC. Supply-voltage operating range is +4.5 volts (minimum) to +16 volts (maximum).
