The 555 is a monolithic timing circuit that can produce accurate & highly stable time delays or oscillation. The timer basically operates in one of two modes: either
(i) Monostable (one - shot) multivibrator or
(ii) Astable (free running) multivibrator
The important features of the 555 timer are these:
(i) It operates on +5v to +18 v supply voltages
(ii) It has an adjustable duty cycle
(iii) Timing is from microseconds to hours
(iv) It has a current o/p
PIN CONFIGURATION OF 555 TIMER:
Pin description: Pin 1: Ground:
All voltages are measured with respect to this terminal.
Pin 2: Trigger:
The o/p of the timer depends on the amplitude of the external trigger pulse applied to this pin.
Pin 3: Output:
There are 2 ways a load can be connected to the o/p terminal either between pin3 & ground or between pin 3 & supply voltage
i. When the input is low:
The load current flows through the load connected between Pin 3 & +Vcc in to the output terminal & is called the sink current.
(ii) When the output is high:
The current through the load connected between Pin 3 & +Vcc (i.e. ON load) is zero. However the output terminal supplies current to the normally OFF load. This current is called the source current.
Pin 4: Reset:
The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to +Vcc to avoid any false triggering.
Pin 5: Control voltage:
An external voltage applied to this terminal changes the threshold as well as trigger voltage. In other words by connecting a potentiometer between this pin & GND, the pulse width of the output waveform can be varied. When not used, the control pin should be bypassed to ground with 0.01 capacitor to prevent any noise problems.
Pin 6: Threshold:
This is the non inverting input terminal of upper comparator which monitors the voltage across the external capacitor.
Pin 7: Discharge:
This pin is connected internally to the collector of transistor Q1.
When the output is high Q1 is OFF.
When the output is low Q is (saturated) ON.
Pin 8: +Vcc: The supply voltage of +5V to +18V is applied to this pin with respect to ground.
