ECEN 2250 - Introduction to Circuits & Electronics

Peter Mathys, Fall 2011, 11/07/11

myDAQ Experiment 1: Capacitors and the 555 Timer

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The goals of this experiment are:


This assignment has the following tasks:

  1. Check out a NI myDAQ device and familiarize yourself with the hardware, the software, and the basic functionality of the Digital Multimeter, the Function Generator, and the Oscilloscope. For more details check the Introduction to NI myDAQ
  2. Lookup and use the datasheet of a 555 timer IC to determine the maximum operating voltage, the typical power supply current, and to design an astable multivibrator with frequency 1000 Hz ±10%. Here are links to some semiconductor companies that manufacture the LM555 timer: Note that there is also a CMOS version of the 555 (LMC555). For this assignment we are only interested in the regular (bipolar) version of the 555 timer. A description of the functionality and the operation of the 555 Timer can also be found below.
  3. Simulate your design (from the previous task) in LTspice. A sample schematic is shown below. Check that the 555 timer actually oscillates (using a transient simulation) and measure the frequency of the oscillation. Look at the waveforms at pin 3 of the 555 timer and at the timing capacitor. If the frequency of the waveform produced is not correct, reevaluate your component values.
  4. Build your design of the astable multivibrator (after testing in LTspice) on the breadboard and measure its performance using the NI myDAQ. In particular, look at the waveform of how the timing capacitor charges and discharges and determine the frequency of the multivibrator using the NI myDAQ and the (virtual) oscilloscope. You may want to use the breadboard wiring diagram and the color code for resistors to help with your implementation of the circuit.
  5. Select, simulate (in LTspice) and implement a mini-project using one or more 555 timer ICs. Some links to get ideas from are:
  6. Relate this myDAQ experiment to ABET criteria 3a...3k.
  7. Document the design, the implementation, and the measurements of your experiments, and the conclusions and lessons learned in the form of a technical report (check the Technical Report Evaluation Rubric to see the grading criteria that will be used). Your report needs to have a title page, a short introduction, one or two paragraphs for each of tasks stated above, a conclusion, and sources/references that were used. Labeled and properly scaled graphs should be used where appropriate.

The 555 Timer

The 555 timer integrated circuit (IC) consists of a mix of analog and digital subsystems that can be used, together with a few external components, to implement various timing functions. The IC was introduced in 1971 by Signetics and it is still widely used. Low power CMOS versions, e.g., LMC555 or TLC555, and a dual version, called 556, that combines two 555s on a single chip, are also available. The basic principle that is used by all variants of the 555 IC is to measure the charge and discharge times of a capacitor.

The main functional components of the 555 timer are two voltage comparators, a voltage divider network with three equal resistors (5 kohm in the original version, hence the name 555), one set/reset flip-flop, a discharge transistor, and an output buffer that is capable of sinking/sourcing up to 200 mA. The figure below shows a functional block diagram of the 555 timer IC.

Functional blockdiagram of 555 timer circuit

A voltage comparator is a device with a + and a - input and an output. The output voltage goes high (+VCC in this case) if the + input has a more positive voltage than the - input. Otherwise, if the voltage at the - input is more positive than at the + input, the ouptut voltage goes low (GND in this case). The flip-flop is a single memory cell with complementary outputs Q and \Q (or "overline{Q}"). If the voltage at the S input goes high the flip-flop is set and the Q output goes high while the \Q output goes low. If the voltage at the R input goes high then the flip-flop is reset and the Q output goes low at the same time as the \Q output goes high. If the voltage at the \RES (reset) input is low, then the flip-flop is reset, regardless of the states of the R and S inputs. The discharge transistor acts as a switch that is open when the flip-flop is set (\Q low) and closed when the flip-flop is reset (\Q high).

The three identical resistors are used to obtain the reference voltages VCC/3 and 2VCC/3 if the CTRL input is left open. The flip-flop is then set if the voltage at the \TRIG (trigger) input goes below VCC/3. To reset the flip-flop either the voltage at the THRESH (threshold) input has to go above 2VCC/3 or the \RES (reset) input has to go low. The two main modes in which the 555 timer typically operates are the monostable or one-shot mode and the astable or oscillatory mode.

Astable Operation. If the 555 timer circuit is connected as shown in the next figure (THRESH and \TRIG connected together at the upper terminal of the timing capacitor C), then it will run as a multivibrator. In this case three external components, RA, RB and C are required. If you build this circuit, it is recommended to use a 10 nF capacitor from pin 5 (CTRL) to GND and a 100 nF capacitor from +Vcc to GND. These two capacitors, whose function is to suppress noise and deliver energy during the transition time of the output, are not shown in the schematic below for clarity.

555 timer wired for astable operation

In the astable operation mode the 555 timer circuit triggers itself whenver the voltage across C goes below VCC/3, thereby setting the ouptut OUT high and turning off the discharge transistor. The capacitor C is then charged through RA+RB until the threshold level of 2VCC/3 is reached. At that point the upper comparator resets the flip-flop so that the OUT pin goes low and the discharge transistor turns on. Now C is discharged through resistor RB until the trigger level of VCC/3 is reached and the cycle starts anew.

The 555 Timer in LTspice

The figure below shows the schematic of an astable mutlivibrator in LTspice. The NE555 part is in the "Misc" component library.

555 timer schematic in LTspice

Note that the component is shown with the pins in the same location as the actual integrated circuit (seen from above). Thus, it's easy to use this schematic as a guideline for wiring up the actual circuit.

The Breadboard

The breadboard is organized as rows and columns of interconnected contact points. The top and bottom rows are typically connected horizontally across the whole length of the board and used to distribute power to the individual circuit elements. The remaining contact points are connected vertically, usually in groups of 5. The picture below shows a breadboard (click to enlarge) that has one horizontal row at the top and one at the bottom. In between are two sets of columns, each of which consists of 5 contacts.

View of Breadboard

The wiring pattern of this breadboard is shown below.

Wiring Pattern of Breadboard

Resistors and the Color Code

Examples of real resistors are shown in the picture below (click on the picture to enlarge). Resistors convert electrical energy to thermal energy or heat. Thus, in addition to the specification of the resistance in ohms, resistors are also characterized by the maximum power (i.e., the maximum rate at which electrical energy can be converted to thermal energy) they can handle without getting damaged. The six leftmost resistors are 1/4 W resistors. The next three are 1/2 W resistors, and the last two are 5 W resistors.

Different Examples of Resistors

To save space and increase readability, many resistors are marked using a color code rather than being labeled directly with a numerical value in ohms. Most resistors that you will use in the lab have 4 colored rings. The first two are the first two significant digits of the value, the third is a multiplier for that value, and the fourth gives the tolerance of the value. Occasionally you might see a fifth colored ring. In some cases this indicates the failure rate level of the resistor (i.e., an indication of the reliability of the resistor), in other cases 5 rings are used for high precision resistors (±1% tolerance or better) that need three significant digits to specify their value. The following table shows the meaning of the different colors in the different positions for resistors with 4 (or 5 for 1% resistors) colored rings.

Color First Digit Second Digit Third Digit
1% Resistors
Multiplier Tolerance
Black 0 0 0 1  
Brown 1 1 1 10 ±1%
Red 2 2 2 100 ±2%
Orange 3 3 3 1000 (=1k)  
Yellow 4 4 4 10k  
Green 5 5 5 100k  
Blue 6 6 6 1000k (=1M)  
Violet 7 7 7 10M  
Gray 8 8 8 100M  
White 9 9 9 1000M (=1G)  
Gold       0.1 ±5%
Silver       0.01 ±10%

Standard values in which resistors are manufactured are shown in the following table.

 Standard Resistor Values (Multiply by 0.1,1,10,100,1000,10000,100000) 

The most commonly used values are shown in bold in the first line and the least often used values are shown in the fields shaded in gray. By using the multipliers, several ranges of magnitude are covered, e.g., the value 22 in the table leads to standard resistor values of

2.2, 22, 220, 2.2k, 22k, 220k, 2.2M

The most common tolerances for 1/8, 1/4, and 1/2 W resistors are 5% and 1%.