The 555 Timer - An Introduction

The 555 timer analog IC (Integrated Circuit) is very economical and convenient for use in mutivibrator circuits since few external components are required. It's packaged as a black box.

The black box has eight connections called ‘pins’ and there is little white dot and dimple denote pin 1. The 555 uses a 3 – 18V DC power supply.

• The 555 timer IC has two comparator, one SR flip-flop and one switching BJT transistor.
• Power supply voltage Vcc is applied to a series of three resistors so that almost 1/3Vcc and 2/3Vcc is established.
• The reset, threshold and trigger control the state of the flip-flop.
• If reset is low, Q output is low regardless of the input applied, and transistor is saturated in this case. So, reset has the highest priority in setting Q.
• When reset is high (connected to Vcc) it does not affect the output Q.
• If the trigger input is lower than 1/3Vcc, then the comparator output is high, setting Q to high and transistor is off.
• If  the threshold input becomes higher than 2/3Vcc, then the output of the comparator is high, resetting Q to low and transistor is in saturation.