TECHNOLOGY EDUCATION ACTIVITY 5 THE 555 TIMER

Transcription

1 TECHNOLOGY EDUCATION ACTIVITY 5 THE 555 TIMER OBJECTIVE: To determine the circuit configuration and characteristics of the 555 timer when used as a multivibrator. MATERIALS: Protoboard, Vernier MPLI, 555 timer, 100k, 1M (R1), 10k (R2) & 100 (R3) resistors, 3 volt battery,.01 f &.1 f mylar capacitors (C1), speaker. SETUP - PART 1 Configure the circuit shown in the following diagram. Use the 100k resistor for R1 and the.1 f capacitor for C1. Don't connect the battery until the circuit is complete and checked and you are ready to take measurements. The 555 timer chip can only be inserted one way. Make sure that pin #1 (nearest the dot) is oriented as shown. 32

2 METHOD Load the MPLI software. Replace the Plotter window with an Oscilloscope window. Turn off ports B and C. The Oscilloscope should be configured to single sweep, no trigger, 0.5 volts/division and a time-base of 5 milliseconds/division. Click the start button to display a single sweep on the oscilloscope. You should see a graph similar to the next illustration. If you don t recheck the circuit or change the time-base setting. Observe the shape of the graph. How would you describe it? Use the examine function to place the cursor on corresponding points of two adjacent "peaks" and record the time and voltage. Since the graph is a "square wave" use the top left or right corner of two adjacent peaks. It doesn't matter as long as you use the same points on each wave. See data tables, page

3 Replace the oscilloscope window with a Plotter graph window. Change the title and print the graph. Save the data using an appropriate filename. Disconnect the battery pack. Disconnect the MPLI probes from resistor R3. Connect the small speaker to the resistor using "alligator clips". Connect the battery pack. Record a description of what you hear. Restore R3 and probes. Replace the.1 f capacitor (C1) with a.01 f capacitor and repeat the procedure. Make sure to give the graph a different title. You may have to change the time base to 2ms/div or lower to get a usable trace. This will be part 2. When you have finished part 2, replace R1 (currently 100k ) with a 1M resistor. Repeat the procedure. This will be part 3. Replace C1 with the.1 f capacitor. Repeat the procedure. This will be part 4. Use a digital ohmmeter and a capacitance meter to measure the actual values of the components used for R1, R2 and C1. Record in the table below. DATA TABLES Part C1 ( f) Nom. R1 ( ) Nom. Peak 1 T1(s) Peak 2 T2(s) Peak1 Volt (v) Peak2 Volt (v) Period - T* T2 - T1 (s) Freq. 1/T (hz) Exper k k m 4.1 1m *Period is the time for one wave. ie. The time between corresponding point on adjacent waves. 34

4 Part C1 ( f) Nom. R1 ( ) Nom. Description of sound (tone) emitted by the speaker k k m 4.1 1m Part C1 ( f) R1 ( ) R2 ( ) Freq. 1/T (hz) Calculated % Difference Freq. (Exper.) Freq. (Calculated)

5 FOLLOW-UP You will now make a report on this activity. This report will contain the graphs and the measured voltages and times. The analysis of the activity will also include the answers to the following questions. 1. Using the times recorded in part 1, calculate the frequency* of the square wave generated by the 555 timer. Calculate the frequency for parts 2, 3 and 4. Compare all four frequencies. 2. Based on the changes made to the circuit, make a statement as to what determines the output frequency of the 555 timer. 3. The output frequency of the 555 timer is calculated using the equation. Where the resistance (R1 & R2)) is measured in ohms and the capacitance (C1) is measured in farads. Remember a f is 10-6 f. (ie..01 f =.01 x 10-6 f = 1 x 10-8 f) Do a sample calculation for each of the trials and compare it to the measured, experimental frequency. You must use the actual values for each of the components. 4. Compare the voltages measured in all four parts. 5. Do a schematic diagram for this circuit. Use a drawing (Corel Draw, Corel Presentations) or CAD package (Autosketch). Prepare a class presentation on the 555 timer as an example of a simple integrated circuit. Include in your presentation a block diagram with an explanation of how the 555 works internally. Research the various modes that this chip can be configured for. * Remember, frequency is the reciprocal of the period (T) of the wave. f = 1/T 36