WEEK

This week I decide to jump ahead and begin working with the 555 timer IC. It will take us several weeks to become familiar with the circuit and we can also use the time to become more competent with the oscilloscope.

GOALS:

       Build an oscillator with a 555 timer IC.

       Understand the operation of the 555 timer.

       Understand what we mean by an oscillation. (frequency, period)

       Be able to display an oscillation on the oscilloscope.

       Be able to describe a capacitor in terms of our water model.

 

Below we see a picture of a sinusoidal oscillation. This is a signal that repeats a pattern over and over again. Using our frequency generators we were able to examine several different types of repeating patterns (sawtooth, square wave and others). The picture below follows the pattern of a function called the sine. This is a very common oscillation.

A square wave oscillation appears as shown in the following picture.

This is the type of oscillation that we will make using a 555 timer.

 

555 timer

The above picture shows the how the 555 timer IC works by showing the internal functionality in terms of components or parts. We will look at the parts separately and then try to understand how they work together. We will try to make an oscillator based on these internal elements.

We have already built an RS flip flop. You should see the RS flip flop as a box with two inputs and two outputs in the above circuit. You can review its operation in your notes. This particular flip flop will be set by a true (5V) on the set (S) line and reset by true (5V) on the reset line (R).

The resistor chain serves to divide the voltage. After the first resistor the voltage will be 3.33V after the second resistor the voltage will be 1.66V. These voltages remain constant and fix one of the inputs on each of the voltage comparators. The diagram that represents a voltage comparator is shown below.

 

When the voltage on the + input is greater than the voltage on the - input the output is true (5V).

When the voltage on the + input is less than the voltage on the - input the output is false (0V).

 

Finally there is a transistor in the circuit. The transistor is a complicated component but for this application it simply works as a switch that can be opened or closed.

When the base of the transistor is 5V the switch is closed and a connection is made from pin 7 to 0V (internal). If a component is attached it will hold that point in the external circuit at 0V. When the base of the transistor is 0V the switch is open and pin 7 will have any influence on any external components attached. Pin 7 id called discharge because when the switch is closed our capacitor will be discharged through this pin.

A more complete discussion of the workings of the 555 timer will be given in class.

 

We also need to introduce another component, the capacitor. The symbol for a capacitor is.

We will imagine that it is a bucket. As you fill the bucket with water through a tube (resistor) the height of the water in the bucket increases. As you charge up a capacitor through a resistor you fill the capacitor with charge and the voltage on the capacitor increases. We will discuss the model for capacitors more during class. Our goal will be to fill and empty the bucket using a fixed filling and emptying tube. The time to compete a cycle will therefore remain fixed and we can use this to generate an oscillating output.

 

QUIZ *******

 

QUIZ *******

In order to receive a grade of A you must successfully complete the lab portion of the course, read prelab material, and pass the lab quiz. In order to pass you must get 75% of the questions correct.

In order to receive a grade of B you must successfully complete the lab portion of the course. The quiz is optional. For fun feel free to try the quiz.

You are not allowed to consult with other students while taking the quiz. You will be given a limited time period to complete the quiz. You may refer to any written material.

You may take the quiz at any time during the week but only once.

The quiz will be available starting on the day you cover the material and ending one week later.

 If you have troubles please discuss the difficulties with DRG.

You are not allowed to consult with other students while taking the quiz. The quiz will be available at least from lab until following lab. You will be given 30 minutes to complete the quiz and you may refer to any written material.

USE LON CAPA