Previous Lecture Complete and continue  

  Pins

The 555 timer, as the name suggests, is a timing device. It is an iconic IC and has been used in countless applications over the past. The 555 timer has various modes of operation and delivers voltage pulses of various durations as output. There is much to be said about this component but there is nothing better than getting to know it by applying it on our board.

Let's first look at its pins:

Pin 1 is the ground. Connect it to the (-) side of the circuit to get the chip working.

Pin 2 is the trigger input that triggers/activates the 555 timer. This pin is inverted, which means that any input to this pin is converted to opposite polarity. In other words, a high input to the trigger pin is actually a low input to the 555 timer, and vice versa.

Pin 3 is the output. This output can be either high or low (monostable mode) or switches between high or low (pulse) in astable mode. The duration of the pulse is determined externally and the pulse can be used to time an event or simply blink an LED.

Pin 4 is the reset. When you want to turn the 555 chip back to its initial state, you can "reset" it.

Pin 5 is the control voltage pin. Although we won't be using this pin for now, this pin is able to override the timing control imposed by the threshold pin (Pin 6).

Pin 6 is the threshold pin. Any input voltage that is higher than 2/3 of the timer Vcc switches the state of the timer. If the timer is outputting a high current, a voltage of 2/3 Vcc will trigger the timer to output a low current and vice versa.

Pin 7 is the discharge pin. The timer often requires an external capacitor to direct the timing of the state-switching or pulse output. We will describe this further on but in summary, a capacitor is charged by the voltage source and discharged by this pin (that’s why it’s called the discharge pin!). The charging rate and the voltage of the capacitor determines the time it takes for the 555 timer to switch states.

Pin 8 is the power supply pin. Simply connect it to the (+) of the circuit so the chip can function.