ECET-402                 Week 3 Lab – Voltage-to-frequency converter and an alarm system

 
This lab actually consists of two separate labs.  They will be presented in Parts A and B.

 

Objective:

The objective of this lab is to (a) build a simple voltage-to-frequency converter and measure the output frequency as the input voltage changes, and (b) to build a simple security alarm system. 

 

Parts Needed

 


Voltage-to-Frequency Converter

1.      555 Timer

2.      3.3 k-ohm Resistor

3.      2 k-ohm Resistor

4.      10 k-ohm Resistor

5.      0.001 microfarad Capacitor (No. 102)

6.      12 VDC Power Supply

7.      Variable 0-12 VDC Power Supply

8.      Wires, wire cutter/stripper, breadboard

 

 

 

 

Alarm System

1.      Photoresistor (CdS cell), (1–10K (dark)

2.      5 V Relay                                          

3.      JK Flip-Flop 74112

4.      PN2222 Transistor

5.      Two,  1 kW Resistors

6.      10 kW Resistor

7.      NO Pushbutton

8.      5 V Buzzer (may be replaced by an LED)

9.      Laser Module (or a bright flashlight)

10.     Power source - 5 VDC


Part A – Voltage to Frequency Converter

 

Objective:

 

The objective of this part of the lab is to build a simple voltage-to-frequency converter and measure the output frequency as the input voltage changes. Several voltage-to-frequency or frequency-to-voltage converters are available commercially. Depending on the output signal of a sensor and input requirement of a controller, often it is required to convert a voltage level to frequency or vice versa.

 

 

Procedure:

 

1.      Build the circuit shown in Figure 1 on a breadboard:

 

Figure 1

 

2.      Connect output (pin 3) to an oscilloscope. 

 

3.      Adjust the variable voltage supply from 2 to 10 V and record measured frequency in Table 1 on the worksheet provided. Note that at 2 V the signal may not be stable or provide an accurate frequency reading, in which case you may start at 3 V.

 

4.      Use data from Table 1 to graph Voltage vs. Frequency in Figure 1 provided on the worksheet. Use an appropriate scale for frequency.

 

5.      Answer questions on the worksheet and turn in your work to week 3 lab dropbox.

                       

                       

Part B – A Simple Security Alarm System

 

Objective:

The objective of this lab is to build a simple security alarm system. If the path of a laser beam (or a bright light source) is crossed (say by an intruder), a buzzer alarm is activated. The buzzer will continue to sound even if the path of light is cleared.  The alarm can be silenced only by pressing a reset button.

 

Pre-lab

 

A photoresistor is a sensitive and inexpensive light sensor that its resistance decreases as light intensity increases.  It is also referred to as Light-Dependent Resistor (LDR), or photoconductor.  Photoresistors are made of material such as cadmium sulfide, in which case they may be called CdS cells.  A CdS cell can have a wide range of resistance from a few hundred ohms in bright light to several mega ohms in darkness.  The cell can react to a broad range of frequencies including infrared (IR), visible light, and ultraviolet (UV).  CdS cells are used in security alarm systems, street lights as an ON-OFF switch, camera light meters, etc.  A typical CdS cell is shown in Figure 2:

 



Figure 2 – A typical CdS cell

 

10 kW–100 kW rating of a photoresistor indicates an approximate resistance of 10 kW in bright light and 100 kW in dark.

 

Procedure:

 

1.      Construct the circuit shown in Figure 3 on a breadboard.  You may want to put the CdS cell inside of a tubular casing as shown in Figure 4, so that it would be in dark when an object blocks the path of light.  74112 is a dual J-K flip-flop IC. You will use only one of the flip-flops. Search the Internet and find the datasheet for 74112 IC.  Determine the pin out of the chip for wiring purposes.  There are three relays in your lab kit.  Use the one rated at 5V.  The other two are rated 12V and will be used in future labs.  For those of you who are not familiar with relays, additional information is provided at the end of this lab.

     

 

 


Figure 3 – The Security Alarm System

 

 

 


Figure 4

 

Note:  It is not really necessary to construct the tubular casing shown in Figure 4.  Normally, the laser or a bright flash light is shining at the light sensor.  To simulate the crossing of the light beam, cover the sensor by your hand to generate darkness.

 

The circuit is designed to sound the alarm whenever the light shining at the photoresistor changes from bright to dark.  Once the alarm is activated, it stays on even if light is reapplied to the sensor.  The alarm can be reset only by pressing the NO pushbutton.  Notice that the buzzer has polarities of + and -. The negative (the black wire) should be grounded

 

2.      Verify the functionality of your circuit.  Take a digital photo of your alarm system and attach to the worksheet.

 

3.      Write a brief report on the functionality of the alarm system, any issues you encountered, and any other comments you would like to add. 

 

4.      Answer the questions on the worksheet and submit week 3 dropbox.

 

Relays

 

Relays will be discussed in detail next week.  Here is a brief introduction for the purpose of this lab.  A relay has a coil and typically two or more sets of contacts.  Each set of contacts consists of a NO (normally open) contact and a NC (normally closed) contact.  If a voltage (usually 5-12 V depending on the rating of the relay) is applied to the coil, the NO contacts will close and the NC contacts will open.  That’s pretty much it!

 

Figure 5 on the next page shows the relay pin-out for your specific relay.  Please make sure to use the relay rated 5VDC from your lab kit.  Place the relay on the breadboard as shown in Figure 5 (in the middle track or valley).  With respect to the notch shown on the relay, the coil pins will be 1 and 16.  You may use pins 4 and 8 as the terminals for the NO contact needed for the lab.

 

 

Figure 5 – Relay pins

 

 

 

 

 

 ECET-402                                     Week 3 Lab Worksheet       Name:      

Voltage-to-frequency converter and an alarm system

Please complete this worksheet and submit to week 3 lab dropbox

This worksheet serves as coversheet

 

Part A – Voltage to Frequency Converter

 

1.      Change input voltage from 2–10 volts and record measured frequency in Table 1.

 

Input Voltage (V)

Output Frequency

2

 

3

 

4

 

5

 

6

 

7

 

8

 

9

 

10

 

 

Table 1

 

2.   Use the data obtained in Table 1 to graph Frequency vs. Voltage in Figure 1.

           

                         Frequency

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                         0    1    2    3    4    5    6    7    8    9   10    Voltage (V)

Figure 1

 

 

 

 

3.      Is the output frequency vs. voltage graph linear?

 

a)   Very Linear                 b)    Fairly Linear                    c)    Not Linear

 

4.      Research and describe some applications for Voltage to Frequency Converter circuits.

 

 

 

 

 

 

Part B – The Security Alarm System

 

1.      Verify the functionality of your circuit.  Take a digital photo of your alarm system and attach below.

 

 

 

 

 

 

2.      Write a brief report on the functionality of the alarm system, any issues you encountered, and any other comments you would like to add.

 

 

 

 

 

 

3.      What is the purpose of JK flip-flop in the circuit?






4.      What is the purpose of the transistor in the circuit?








 

 

 

5.      Describe how to modify the circuit so that the buzzer is activated after the light beam has been interrupted twice. The buzzer should then be turned off by pressing the Reset pushbutton. You may use the second JK flip-flop to build a counter circuit. You do not need to build the circuit. Just describe the process.

Note: If you build the circuit for this part, it may not work properly since the crossing of the beam would not represent clean (de-bounced) switching.  It may work sometimes but not every time!

 

 

 

 

 

 

 

 

 

6.      Submit the completed worksheet to week 3 lab dropbox.

 

 

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