THE 555 TIMER IC PIN DESCRIPTION
THE GROUND PIN (PIN 1) The ground pin of the 555 timer ic which is the first counting pin of the timer ic is called the ground pin, these pin connects the 555 timer to the negative(0V) supply rail.
THE TRIGGER PIN (PIN 2) This is the second counting pin of the timer ic which is the trigger pin and the negative input to the comparator 1. A negative pulse on this pin set the internal Flip-flop when the voltage drops below 1/3 vcc causing the output to switch from a LOW to HIGH state.
THE OUTPUT PIN (PIN 3) This is the third counting pin of the timer ic, which serves as the output pin for the ic. The output pin can drive any TTL CIRCUIT and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately vcc -1.5V, so small speakers, leds or motors can be connected directly to the output.
THE RESET PIN (PIN 4) This is the fourth pin of the timer ic called the reset pin, this pin is used to reset the internal Flip-flop controlling the state of the output pin 3. This is an active low input and is generally connected to a logic "1" level when not use in other to prevent any unwanted resetting of the output.
THE CONTROL VOLTAGE (PIN 5) This is the fifth counting pin of the timer ic, this pin controls the timing of the 555 by over riding the 2/3 vcc level of the voltage divider network by applying a voltage to this pin the width of the output signal can be varied independentely of RC timing network. when not used it is connected to the ground via a 10nF(103 fixed capacitor) to eliminate any noise.
THE THRESHOLD PIN (PIN 6) The threshold pin is the sixth counting pin of the 555 timer ic , the positive input to the comparator number 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3 vcc causing the output to switch from HIGH to LOW state. This pin connects directly to the timing network(RC) network.
THE DISCHARGE PIN (PIN 7) This is the seventh counting pin of the 555 timer ic called the discharge pin, the discharge pin is connected directly to the collector of the internal NPN transistor which is used to discharge the timing capacitor to ground when the output at pin 3 switches LOW.
THE SUPPLY PIN (PIN 8 ) This is the last counting pin of the 555 timer ic which is the supply pin, these pin then supply +VCC and for general purpose TTL 555 timer is between 4.5V and 15V.
