Overview

The 555 timer is one of the most versatile integrated circuits in the world and can be used in everything from sound circuits to radio modulation. It's impact on the modern world is arguably unmatched by any other IC, and has revolutionized the way we use passive components such as resistors & capacitors. This lesson will explain (in detail) the three main use cases of the 555 timer, and the incredibly clever math that makes it all work.

Video Coming Soon

Pinout & Explanation

Beginning on the left are the power pins. Pin 8 is the positive supply voltage (\(V_{CC}\)) and pin 1 is the negative supply voltage (\(V_{SS}\)). The voltage range of the 555 timer is 4.5 V (minimum) to 16 V (maximum), the reason for this will be explained later. Internally, pins 1 & 8 are connected through three 5 kΩ totaling 15 kΩ. These resistors set up a simple voltage divider circuit which will be used as reference voltages later. We can determine the voltages using Kirchoff's Voltage Law (KVL) & simple algebra:

\(V_{R_1}+V_{R_2}+V_{R_3}=V_{Total}\) *Recall all resistors are the same (5 kΩ)

\(3V_R=V_{Total}\)

\(V_R=\frac{V_{Total}}{3}\)

Therefore each of the three resistors account for a voltage drop of \(\frac{1}{3}\) (\(33.\overline{3}\%\)) the supply voltage, essential for performing the functions of the IC.

Pin 2 (known as the trigger pin) is connected to the input of a comparator. Recall that a comparator's output is based on the comparison of its two input voltages: If the non-inverting input (labeled as +) is greater than the inverting input (labeled as -), the output will saturate towards the positive supply voltage. If the opposite is true, the output will saturate towards \(V_{SS}\). The voltage divider is connected to the non-inverting input of the comparator in such a way that it compares the voltage of the trigger pin (pin 2)