There are occasions when a circuit needs to be fed with a square wave signal. To provide for such a scenario, we will build a square wave signal generator that uses a **555**. A **555 **can be configured to run in 3 different modes, the one that we are concerned with here is called the **astable mode**. We will use the 555 in astable mode to build an **astable multivibrator**. **Astable **means that it runs without settling into a stable state, it constantly oscillates between a **LOW **and **HIGH **state. We can adjust the amount of time that it is in a **LOW **state, as well as how long it is in the **HIGH **state. This is known as the **duty cycle**. A **duty cycle** of **50%** means that the circuit is in a low state for the same period as it is in the high state. Or we can express this as a ratio of **1:1**. A **multivibrator **is a device that operates in a two-state mode.

The output signal frequency of our astable multivibrator will be determined by the 2 resistors (**R1 **and **R2**) and the 1 capacitor (**C1**), governed by the formula **f = 1 / time period**. The time period can be calculated by **1.44 / ((R1 + R2 x 2) x C1)**.

A fixed frequency signal generator wouldn’t be very useful, so we will substitute **R2 **with a potentiometer, so we can vary the **signal frequency **that is generated. Varying the value of our capacitor, **C1 **would also alter the frequency. However, we will keep **C1 **at a fixed value as it is easier to alter a resistance. We will also use a potentiometer for **R2**, which will allow us to vary the **duty cycle** between **50%** and **100%**.

R1, R2 and C1 form a **resistor-capacitor** network. The capacitor charges and discharges at a rate that is proportional to the resistance. When the voltage on C1 reaches 2/3 of the supply voltage, the 555 triggers its flip-flop (pin 2). The capacitor starts to discharge and when it drops to 1/3 of the supply voltage the flip-flop flips again and starts the capacitor charging again. When C1 is charging, the RC is dependent on both R1 and R2. When it’s discharging, only R2 comes into play, because of the connection to pin 7 between the 2 resistors. The time taken to perform these charge-discharge cycles is what determines the frequency of the square wave generated. The frequency is the reciprocal of the time taken for the circuit to go high plus the time taken to go low.

There are 2 parameters that we are interested in altering, the frequency and the duty cycle.

Just what I needed for my project, it worked first time.