IC 555 Timer
Pin-1, GROUND: It is the GROUND PIN of the IC. The negative terminal of DC power supply or battery is connected to this pin. Here note that IC555 works always on single rail power supply and NEVER on dual power supply, unlike operational amplifiers. Also note that this pin should be connected directly to ground and NOT through any resistor or capacitor. If done so, the IC will not function properly and may heat up and get damaged. This happens because all the semiconductor blocks inside the IC will be raised by certain amount of stray voltage and will damage the IC.
Pin-2, TRIGGER It is known as TRIGGER PIN. As the name suggests in triggers i.e. starts the timing cycle of the IC. It is connected to the inverting input terminal of trigger comparator inside the IC. As this pin is connected to inverting input terminal, it accepts negative voltage pulse to trigger the timing cycle. So it triggers when the voltage at this pin LESS THAN 1/3 of the supply voltage (Vcc). In number of applications, the IC must be triggered by a pulse. The amplitude and minimum pulse width required for triggering depend on operating temperature and supply voltage of the IC. Generally the current required for triggering is about 0.5uA for a period of 0.1uS. The triggering voltage may be in a range from minimum 1.67V when Vcc = 5V to maximum 5V when Vcc = 15V. The triggering circuit inside the IC is very sensitive and may be accidently activated due to surrounding noise. To avoid this, the pin is always connected to a pull-up resistor (10k-ohm), if this pin is used separately.
Pin-3, OUTPUT This is the OUTPUT PIN of the IC. It can SINK or SOURCE a maximum current of 200mA. Sinking the current means, when the output of the IC is at logic-0 state i.e. LOW and so it can absorb current into its output. Similarly sourcing the current means, when the output of the IC is at logic-1 i.e. HIGH and so it can give out current from its output. Due to this property of the IC, we can use it in number of typical digital applications also. Also note that the output voltage of the IC is slightly greater than zero, when it is in logic-0 state. Similarly it is slightly less than supply voltage (Vcc), when output of the IC is in logic-1 state.
Pin-4, RESET It is the RESET PIN of the IC. When it is connected to positive terminal of battery, the IC works normally. However, when it is grounded (either directly or through a maximum of 100k-ohm resistor), the IC stops its working completely and its timing cycle stops i.e. the charging or discharging of the external capacitor stops, so output of the IC is locked in logic-0 state. It is interesting to note that the reset voltage required by this pin is typically 0.7V at a reset current of 0.1mA. However in general applications, this pin is always connected to positive terminal so that the IC works normally.
Pin-5, C. VOLTAGE This is known as the CONTROL VOLTAGE pin. The 2/3 of supply voltage point on the terminal voltage divider is brought out to pin-5, known as the control terminal of the IC. The timing cycle can be modified by applying external DC control voltage to this pin. This allows manual or electronic remote controlling of the time interval of the IC. The control terminal is frequently used when the timer is operated in MMV mode. But if you are NOT using this pin for any such purpose, then this pin MUST BE GROUNDED THROUGH A CAPACITOR OF 0.01uF. This prevents the time interval from being affected by picking up of stray AC or RF noise from the surrounding. Also note that, when the IC is used as an oscillator, in AMV mode, we can modulate the output waveform of the IC by applying a variable DC control voltage to this pin, as shown below.
Pin-6, THRESHOLD This is known as the THRESHOLD PIN. It finalizes the timing cycle of the IC, when its voltage is equal to or greater than 2/3Vcc, the output is at logic-0 state.
Since this pin is connected to non-inverting terminal of threshold comparator inside the IC, it accepts positive going pulse to end up the timing cycle, also. Note that the typical value of threshold current is 0.1mA, just like the RESET PIN. The time width of this pulse should be greater than or equal to 0.1uS.
Pin-7, DISCHARGE It is known as DISCHARGE PIN. It discharges the external capacitor into itself, but when fully charged…! It is connected to the collector of an NPN transistor inside the IC. Due to this, the discharging current going into this pin MUST NOT EXCEED 50mA, otherwise the internal transistor may get damaged. It is interesting to note that this pin can also be used as output pin with open collector output. I am working on one such practical circuit and will publish the circuit very soon.
Pin-8, +Vcc It is known as the +ve supply terminal of the IC. The battery voltage connected across this pin and ground pin SHOULD NOT EXCEED 18V. Generally the range of operating voltage of the IC is 3V–18V.