EECE 206 Lab 7: LM 555 Timer/Oscillator Circuits
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Laboratory Goals
q Familiarize students with the LM 555 IC and its uses
q Design a free-running oscillator to the required specifications
q Design a triggered one-shot circuit to the required specifications
q Compare actual to theoretical values for the circuits
Pre-lab reading
q Student Reference Manual for Electronic Instrumentation Laboratories by Stanley Wolf and Richard Smith, Copyright 1990
q National Semiconductor LM555 Data Sheet, Pages 1 to 8
Equipment needed
q Lab notebook, pen
q Tektronix PS 503A Dual Power Supply
q Tektronix DM 501A Digital Multimeter
q Tektronix 2252 Digital Oscilloscope
q 2 oscilloscope probes (attached to the oscilloscope)
q Tektronix DC 504A Frequency Counter/Timer
q Tektronix FG 501A Function Generator
q 1 test lead, BNC/BNC
q 1 test lead, BNC/EZ Hook
q 2 test leads, red, banana/EZ Hook
q 2 test leads, black, banana/EZ Hook
Parts needed
q Circuit breadboard
q Lab parts kit
q IC, Timer/Oscillator, LM 555
q Capacitor, ceramic disc, 0.01μF
q Capacitor, ceramic disc, (value to be determined)
q Capacitor, electrolytic, (value to be determined)
q Resistors, 1KΩ, 180Ω, ¼ Watt
q Resistors, 3, (values to be determined), ¼ Watt
q Red LED
q Jumper wires
Lab safety concerns
q Make sure all circuit connections are correct, and no shorted wires exist.
q Adjust the power supply to the proper voltage before connecting it to the circuit
q Adjust signal generator to the proper level before connecting it to the circuit
1. Pre-Lab LM 555 Designs
q Design an astable multivibrator (free-running oscillator), referring to Figure 1 below. Choose the oscillation frequency to be the last 4 digits of your student ID number. (Equations will be provided by your instructor during lab recitation)
o Because capacitors in specific values are harder to find than resistors, it is easiest to pick the capacitor and solve for the resistance. For convenience, you should choose a capacitor in the 0.001-0.01 μF range.
o Resistors are probably not available in the calculated values: Use the nearest standard value.
Figure 1 – LM555 Free-running Oscillator Schematic
q Design a monostable multivibrator (“one-shot”), referring to Figure 2 below. The hold time (i.e., the amount of time the circuit will hold its output in a high state) will be 1 second, and the output will be through a Light-Emitting Diode (LED).
o Calculate the current through Rd, which is the same as the current through the LED. Assume the LED to have a 1.5 volt drop, and the output of the 555 to be the same as Vcc.
o Again, it is easier to pick the capacitor and solve for the resistor. This capacitor should be in the 10-100 μF range. For these large values of capacitance, we typically use Electrolytic capacitors. Be careful to note the polarity. The negative terminal should be on the ground side of the capacitor. In future circuits where large amounts of current or voltage are present, the capacitors must be installed correctly or they may explode or leak. The flat side of the LED (cathode) goes towards ground.
Figure 2 – LM555 “One-shot” Schematic
2. Circuit 1 Construction and Signal Measurement
q Before building circuit, measure the values of all passive components (resistors and capacitors) and record the values in your lab notebook
q Build circuit 1 (Figure 3 below), using your values of RA, RB, and C
Figure 3 - LM555 Oscillator Circuit
q Turn on the TM 504 equipment module
q Adjust the PS 503A power supply to 5VDC using the digital multimeter (note: you can turn on/off the output of the power supply by pressing the white OUTPUT button on the power supply)
q Press the white OUTPUT button on the power supply to disable its output (the two orange lights turn off)
q Connect the power supply to the circuit Vcc and ground connections
q Press the white OUTPUT button on the power supply to enable its output
q Connect the CH 1 scope probe and ground clip to the circuit output and circuit ground respectively
q Measure the output signal of the circuit. Record the pulse width, pulse period, frequency, amplitude, rise time, and overshoot in a table in your lab notebook
q Verify the output frequency with the DC 504A Frequency Counter/Timer
q Press the white OUTPUT button on the power supply to disable its output
q Disconnect all test leads from the circuit
3. Circuit 2 Construction and Signal Measurement
q Build circuit 2 (Figure 4 below)
q Connect the power supply to the circuit (still at 5VDC)
q Press the white OUTPUT button on the power supply to enable its output
q Connect the function generator to the circuit input and circuit ground
q Adjust the generator to output a square-wave of 0.3 Hz and a voltage having a negative peak less than 3.33V
q Measure the input signal of the circuit (CH1 scope probe)
q Measure the output signal of the circuit (CH 2 scope probe)
q Record both input and output waveforms (frequency, amplitude, pulse width) – Notice the output pulse width is independent of the input pulse width, as long as the input pulse width is greater than 1 second
q Press the white OUTPUT button on the power supply to disable its output
q Disconnect all test leads from the circuit
Figure 4 – LM555 One-shot Circuit
4. Analysis
Write a summary report for this lab. Be sure to also include the following topics:
Compare theoretical vs. measured values, and calculate the percent error. Why might you see differences?
What applications can you think of to use these circuits?
Why is there a resistor in series with the Light Emitting Diode?
Explain any difficulties you had with this lab. (Please include suggestions to improve the lab, if you have them).
Clean up
Before leaving the lab, take a few minutes to make sure all equipment and test leads are returned to your cabinet, and then lock it. Return the cabinet key to your T.A. Pick up any loose parts on the workstation table, and wipe off any eraser shavings, or other debris with a paper towel. Dispose of the paper towel and debris in the wastebasket.