[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
Lab #2: Capacitors and the 555 Timer
1
Introduction
The goal of this lab is to introduce capacitors in their role as elements of timing circuits and to convert an analog
oscillation into a digital oscillation with a simple timer IC.
For your convenience, things that should be included and discussed in your report are introduced in a unique
color.
2
Components and Tools Required
• From Your Kit:
Breadboard
Wires
Wire Cutters and Pliers
Various Resistors and Capacitors
A Cat
• From Your TA:
A 555 Chip
• On The Lab Bench:
Variable DC Power Supply
Function Generator
Oscilloscope
3
Meet your New Instruments
3.1
Function Generator
The function generator allows you to create time-varying voltages with different shapes and frequencies. Although
the waveform generator has many capabilities, we will concentrate on a few of the main ones that are common to
most waveform generators. The main parameters for specifying the waveform are:
• Frequency (or its inverse, Period)
• Shape (e.g. square, sinusoidal)
• Amplitude (the magnitude of the signal in volts)
Figure 1: Function Generator
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3.1.1
Introduction to Digital and Analog Electronics
R. McLeod
Changing the frequency
First, we will change the frequency of the to 4 kHz. To change the frequency, push the PARAMETERS
button, then the frequency soft-key. Type in a frequency on the keypad or change it with the knob, then finish
by selecting units with the soft-keys. See Figure 2 for these steps.
(a) First, select parameters.
(b) Selecting frequency.
(c) Adjust the frequency with the keypad or
knob.
(d) Select the units with the soft keys.
Figure 2: Changing the frequency of a function generator.
3.1.2
Changing the waveform shape
The second aspect of the waveform is its shape. Change the waveform shape to a square wave. This
generator can produce a large number of shapes including ones supplied by the user. Press the WAVEFORM
button and select the square wave.
(a) First, press waveform.
(b) Then, select square wave.
Figure 3: Changing the waveform of a function generator.
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Introduction to Digital and Analog Electronics
R. McLeod
Changing the amplitude and offset.
Change the peak-to-peak amplitude to 5 V. This means that the voltage will swing from +2.5V to -2.5V.
Press the UNITS button and use the soft-key to make sure you are in the Amp/Offs (amplitude and offset) mode,
not the High/Low mode. Set the Vpp amplitude to 5 V and the offset to 0 V.
(a) First, press the soft key for ampli- (b) Then, input 5 and select the units of (c) Lastly, set the offset in parameters
tude.
volts.
to be 0 V
Figure 4: Changing the amplitude and offset of a function generator.
Change the output termination. Like voltmeters and ammeters, the resistance of the source matters. This
is a detail you will learn in later classes. For now, press the CHANNEL key to open the channel configuration
screen. Press Output Load, then Set to High Z as shown in Figure 5.
(a) First, press the soft key for channel.
(b) Make sure the output load is Set to HIGH Z.
Figure 5: Changing the output impendance of a function generator.
Now you need some way to see if you have the waveform you want. That brings us to the oscilloscope.
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[ECEN 1400]
3.2
Introduction to Digital and Analog Electronics
R. McLeod
Oscilloscope
An oscilloscope (or scope for short) is used to display a waveform or trace. This particular oscilloscope has the
ability to show 4 different traces simultaneously. The connectors for the four channels are in the lower right-hand
side of the picture. You will use the oscilloscope probe found in your lab kit for connecting your circuit to the
scope.
Figure 6: Oscilloscope
Turn on both the function generator and the oscilloscope. Connect the oscilloscope to the function generator.
At this point, you will probably not see anything displayed.
(a) This is a scope probe. The
pointy bit is positive and the (b) This is a BNC-to-banana plug
clip is negative.
adapter.
Figure 7: Identifying the connectors to be used.
(a) Plug the end of your scope probe
into channel 1 of the oscilloscope.
(b) Plug the BNC end of your BNC- (c) To complete the circuit between the
to-banana plug adapter into
function generator and the oscillochannel 1 of the function generator.
scope, we must connect the ends.
Figure 8: Hooking up the function generator and oscilloscope together.
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[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
To view an output of your function generator on the oscilloscope, you have to turn the output ON.
(a) First, select the soft key channel. (b) Then, toggle the output to ON.
Figure 9: Turning on the output of the function generator.
With the older scopes, you would now start the process of finding the trace, however you are luckier. Press
the AUTOSCALE button. You should see a square wave on the screen with voltage on the vertical axis and
time on the horizontal axis.
Figure 10: Finding the AUTOSCALE button.
Set the trigger level. The trace should be stationary on the screen, which means that the scope has detected
the rising edge of the square wave and placed it in the same place on the screen each cycle. Without this triggering,
the trace will sweep randomly in the time axis. This is controlled by the trigger level, set with the knob on the
right-hand side of the scope. Use the TRIGGER LEVEL knob to change the function voltage which the triggers
the scope to reset the plot on the screen. You should see an indicator on the main screen move. Examine what
happens when the trigger level is outside of the voltage range of the function generator.
Figure 11: Finding the TRIGGER LEVEL button.
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Introduction to Digital and Analog Electronics
R. McLeod
Set the time and voltage scales. The AUTOSCALE function has picked time and voltage increments for
you that are probably reasonable. But you should know how to change them. Find the Horizontal and Vertical
controls. There is a large knob for scale and a small knob for offset on each. Adjust these to understand what
they do. If you lose the trace, just use AUTOSCALE again to get things back. Pick appropriate scales for the
time and voltage increments and record them.
(a) Find the HORIZONTAL knob.
(b) Find the VERTICAL knob
Figure 12: Locating the horizontal and vertical adjustment knobs.
Measure features of your function. Older, analog scopes were capable of displaying traces and not much
more. This digital scope can do much more. One very useful feature is measuring various quantities. Press
the MEASURE button, then either the VOLTAGE or TIME soft-keys. Find the VOLTAGE PEAK-TOPEAK and FREQUENCY), then change them on the function generator to confirm that they track on the
scope. Note that you need at least one period of the waveform on the scope for the measurements to be performed
correctly.
(a) First, press the MEASURE (b) Use the side screen soft keys
softkey.
to select VOLTAGE.
(c) Then, scroll with the selection
knob (shown) and push when
Vpp is highlighted.
(d) Select the TIME measure- (e) Lastly,
select
ments.
QUENCY.
FRE-
Figure 13: Measuring the voltage peak-to-peak and frequency of the signal.
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[ECEN 1400]
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Introduction to Digital and Analog Electronics
R. McLeod
The Capacitor
Now lets combine these two new instruments with a circuit to do something useful.
Build and take measurements from RC circuit. Construct the circuit below using a 2.2 kΩ resistor and a
0.0047 µF (or 4.7 nF) capacitor from parts in your lab kit. Now set the waveform generator to produce a
5 Vpp square-wave, with a 50% duty cycle. Set the frequency to 5 kHz. Record the parameters of the
signal from your function generator. Remember to put (or keep) the waveform generator in High
Z mode. Connect the oscilloscope across the waveform generator just to double check you have the waveform
you expect.
Figure 14: A Simple RC Circuit
Now connect the oscilloscope across the capacitor, so you can measure the voltage as it changes. If you have
a second probe, keep one on the source and use a second for the capacitor voltage. Keep the probe wired to the
waveform generator connected to the input marked 1. Wire the second probe across the capacitor and connect it
to the scope input marked 2.
Save an image of the screen to a USB memory stick. Insert a USB stick into the front-panel and press
the SAVE/RECALL button. In the Storage menu, press Storage. Continue to press this button or use the entry
knob to select a graphics file format (PNG is a good choice). If you want to save the scope parameters, press Para
Save. Press External to access your USB stick and navigate to the desired folder. Finally, create a New File and
press Save.
Check and record the RC time constant. Using either the measurement traces on the scope screen or
your saved image, find the amount of time a decreasing voltage level takes to fall by e-1 = 36.8% or a rising voltage
level grows by 1- e-1 = 63.2%. Measure and Record the actual resistance of your 2.2 K resistor and
compare your measured time to the calculated RC time constant.
Observe the impact of increasing frequency. Now double the frequency of the square-wave to 10k Hz.
Again, save an image of the waveform for you lab report. Does the overall frequency of the voltage
across the capacitor change? What happens to the amplitude of the voltage across the capacitor
as the frequency increases? Try increasing the frequency once again, this time to 15 KHz. What
happens to the shape of the waveform? This is the basis of filtering in which different frequencies in a
waveform can be enhanced or suppressed.
5
The 555 Timer As A Digital Oscillator
Now we will use the rising and falling time response of a charging and discharging capacitor to make a charge/discharge
digital oscillator.
Build and take measurements from timer circuit. Get a 555 timer chip from your TA and assemble the
circuit below. Discuss how you built the circuit and include a picture of what you’ve built. Place a
scope probe between RB and C1 (or pin6 or 2) and a second scope probe on the chip output, pin 3.
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[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
Figure 15: Astable 555 Timer Circuit
Measure and describe your timer performance and include screen capture of its output. When
you have a square wave emerging from your timer, accurately measure the high and low voltages,
the high and low times and the frequency. How accurately would you be able to predict this
frequency from the nominal resistance values? (hint, check the data sheet) Is accurate enough to
be used in your clock?
6
6.1
Extra Credit: 10 Points Max
More Blinking Lights
Select new resistors and a capacitor for your 555 timer to decrease the frequency to several Hz. Drive an LED
with the output if you dont have enough current, you may need to use your transistor driver circuit from the next
lab. Congratulations, youve made a bike light!
6.2
Build A High Pass Filter
The RC circuit of step 4 is called a low-pass filter because it passes low frequencies and blocks high frequencies, as
you found. Now flip the positions of the resistor and capacitor and measure the voltage across the resistor. You
now have a high-pass filter. Examine both the shape of the waveform and the amplitude as a function of frequency
and try to understand it. In particular, look at relatively low frequency square waves and examine what happens
when the square wave transitions from one voltage to another.
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