ECET 402 Week 4 Lab Worksheet Relays and Relay Logic Controls Answer

ECET 402 Week 4 Lab Worksheet Relays and Relay Logic Controls Answer


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ECET 402 Week 4 Lab Worksheet Relays and Relay Logic Controls


The objective of this lab exercise is to identify the pin-out of a typical electromechanical relay, introduce the concept of relay logic diagrams, and wire a relay logic circuit for a given control process consisted of a phototransistor, a motor, switches, and pushbuttons.


Parts Needed

1. Two Relays - 12V DC Coil, DPDT

2. A 12 V DC Motor (from a previous course)

3. A Phototransistor (Photo-Darlington 08L14F1)

4. NO Pushbutton (START)

5. NC Pushbutton (STOP)

6. Two LEDs (green and red)

7. Two 560 Resistors

8. 12 V Power Supply

9. Breadboard, wires, wire cutter/stripper


Electromagnetic Relay:


An electromagnetic relay is a device that consists of a coil and one or more sets of electrical contacts.  Each set of contacts consists of a Normally Open (NO) and a Normally Closed (NC) contacts. If a voltage (typically 5-12 VDC) is applied across the coil of a relay, the NO contact will close and the NC contact will open.  The names of NO and NC are referred to the status of the contacts when the coil is not energized.  Figure 1 shows an electromagnetic relay.  Without voltage applied to the coil, the spring on the left keeps the hinged arm at upright position creating a NC contact at the top and a NO contact at the bottom.  Once a voltage is applied across the coil, the iron rod inside the coil will become a magnet and pulls the hinged arm downward, opening the NC contact and closing the NO contact.  Relay and contact symbols are shown on the right hand side of Figure 1.

Figure 2 – The ladder logic diagram for the given process


Here is a brief description of how the system works:  When the Start pushbutton is closed, Coil CR1 of relay-1 is energized.  Then the two NO contact CR1 (belonging to relay-1) are closed.  When we release the NO Start pushbutton, the coil CR1 remains energized.  This is called the latching of the process.  Since the second NO CR1 contact is closed, the motor and the green light turn on.  At normal operating temperatures of the motor, the thermostat switch is open.  However it closes if motor overheats.  When the thermostat switch is closed, it energizes the coil of a second relay CR2.   Once coil CR2 is energized, it opens the NC contact CR2 located just before the motor, turning both the motor and the green light off.  Note that when the thermostat switch is closed, the red light is turned on.




Not all relays have identifiable pins. However they can easily be identified using the simple instructions provided in lab procedure. Two of the pins are coil terminals.  Depending on the relay, a measurement of 200-1000 Ω resistance is expected across these two pins. Your relays have two sets of contacts. Each set consists of a NO and a NC contact. Each contact has two terminals; however, in each set of contacts, one pin is shared by the two contacts as shown in Figure 3. Again, C1 and C2 are common terminals between the two contacts on each side and it does not mean that they have to be connected to the common ground. They are simply the other terminal of the contacts.   Figure 3 is a generic diagram and does not necessarily represent the order of the pins on your relay.


The phototransistor supplied with this lab consists of a Darlington pair transistor that is often used as a current amplifier or driver. The transistor has a glass dome on the top. When light is penetrated, a small base current is produced that can turn the transistor on or off like a switch. In this case the base terminal is not connected to any external source.  Figure 4 shows a physical transistor and wiring diagram for a switching application.



1. Steps 1-6 below describe a simple way to identify the pins of a relay in case they are not identified or a datasheet is not available.  Please make sure you know which pins on your relays represent the coil, the NO contacts, and the NC contacts.  Place one of the 12V relays from your lab kit on the center of a breadboard as shown in Figure 5. Note the small notch on the left side of the relay. Pins are labeled similar to ICs (1–16 counter clockwise as shown).  Look at the pins at the bottom of the relay and notice that the pins are numbered accordingly (though some pins are missing).  Cut eight pieces of 2-inch long wires, strip both ends, and place them on the breadboard as shown in Figure 5, so that each wire connects to one of the pins of the relay.

Figure 5 – Relay pins

2. Set your DMM to measure resistance.  Note that when the two test leads of DMM touch each other, you get a reading of almost zero (meaning a “short” or a “NC contact” in our case). If the two test leads of DMM are not touching, you get a reading of OVL (meaning an “open” or a “NO contact” in our case).


3. Measure the resistance between any two of the eight pins and continue the process until you identify the two pins that would give you a reading of 700-750 Ω. These pins are the two terminals of the coil. Use the corresponding pin numbers of the coil from Figure 5 and enter them in Table 1 provided on the worksheet.


4. Connect the coil pins that you identified in step 3 to a 12VDC and ground. The polarity is not important here. Make sure the power supply is off.


5. Now, on each side of the relay, three pins need to be identified (4, 6, and 8 on one side, and 9, 11, and 13 on the other side).  Determine the two pins on one side that would give you a reading of “open” or OVL with the Power Supply off and a reading of almost zero with the power on. These must be the two terminals of the NO1 contact.  Similarly identify two pins on the same side that would give you a reading of almost zero Ohms with the power off and a reading of “open” or OVL with the power on.  These must be the terminals of the NC1 contact.  Note that the pin (C1) is shared by the NO1 and NC1 contacts.  Enter the corresponding pin numbers in Table 1 on the worksheet.


6. Repeat step 5 to identify the pins of the NO and NC contacts on the other side.  Enter the corresponding numbers in Table 1.


7. Design a ladder logic diagram for the process given below.  Make sure you read the process a few times to fully understand the process.  Attach your design to the worksheet.  You may use Visio or Paint to draw the ladder logic diagram.  Remember that LEDs require current limiting resistors.  Use a 560 Ω resistor is series with each LED.


Process: The process starts with a NO pushbutton and can be stopped at any time by pressing an emergency NC pushbutton.  As soon as the process starts, a dc motor runs provided that the light is dim in the room. Whenever the motor is running, a green light LED will be on. During the process a photo sensor detects light. If the light source becomes bright such as turning on a flashlight, the motor stops, the green LED is turned off, and a red LED will turn on. Without much light, the motor and the green LED are turned back on and the red LED is turned off.  


8. Construct the circuit you designed in step 7 and verify the functionality of your circuit. Make sure that the phototransistor is not located under a bright light.  It may be necessary to dim the light in the room.  Use a flashlight to activate the phototransistor.  Complete Table 2 on the worksheet and attach a digital photo of your circuit as well.


9. Modify your design as described on the worksheet.  Include the new design on the worksheet.


10. Write a brief report on your experience and state any issues you had during the lab (if any).


11. Complete the worksheet and submit to week 4 lab dropbox.

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