555 Timer.

The 555 IC  is available as an 8-pin metal can, an 8-pin mini DIP(dual-in-package) or a 14-pin DIP, This IC consists of 23 transistors , 2 diodes and 16 resistors. The explanation of terminals coming out of the timer IC is as follows. The pin number is used in the following discussion refer to the 8-pin DIP and 8-pin metal can packages.

Pin1: grounded Terminal-

All the voltages are measured w.r.t this terminal.

Pin2: Trigger Terminal-

This pin is an inverting input to a comparator that is responsible for transition of flip-flop from set or reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.

Pin3: Output Terminal-

Output of the timer is available at this pin. There are two ways in which a load can be connected to the output terminal either between pin3 and ground pin or between pin3 and supply pin .The load connects between pin3 and supply in is called the normally  on load and that connected between pin 3 and ground pin is called the normally off load.

Pin4: Reset Terminal-

To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose. It should be connected to +vcc to avoid any possibility of false triggering.

Pin5: Control Voltage Terminal-

The function of this terminal is to control the threshold and trigger levels. Thus either the external voltage or a pot connected to this pin determines the pulse width of the output waveform. The external voltage applied to this pin can be used to modulate the output waveform. When this pin is not in used , it should be connected to ground through a 0.01uf to avoid any noise problem.

Pin6: Threshold Terminal-

This is the non-inverting input terminal of comparator 1, which compares the voltage applied to this terminal with s reference voltage of +2/3 vcc . The amplitude of voltage applied to this terminal is responsible for the state of flip-flop.

Pin7: Discharge Terminal-

This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor saturates ,capacitor discharges through the transistor. When transistor is cutoff, the capacitor charges at a rate determined by external resistor and capacitor.

Pin8: Supply Terminal-

 A supply voltage of +5v to   +18v is applied to this terminal with respect to ground .

The timer combines a relaxation oscillator, two comparators, an R-S flip-flop and a discharge capacitor.R-S flip -flop A pair  of cross-coupled transistors each collector drives the opposite base through resistance.On the other hand if transistor Q1 is cut off, its collector voltage , which is approximately equal to _vcc . drives the transistor Q2 into saturation. The low collector voltage of this transistor then keeps the transistor Q1 in cutoff.

Depending on which transistor is saturated, the Q output is either low or high. By adding more components to the circuit , an R-S flip-flop is obtained , R-S flip-flop is a circuit that can set the Q output to high or reset it low. The circuit latches in either two states. A high S input sets Q to high a high R input  resets Q To low. Output Q remain in given state until it is triggered into the opposite state.