• If you are citizen of an European Union member nation, you may not use this service unless you are at least 16 years old.


Lab 1: Orientation and LED

Page history last edited by praveen2@... 6 years, 2 months ago


Today we will have a gentle introduction to the EE 47 Lab, including how to use the equipment at your stations and how to solder. You will also build some simple circuits to light up an LED. After the guided tour, please work your way through Parts B and C.


In The Report

Include your responses to all of the green questions in your lab report. Show your calculations. Also include a video of your LED being switched on and off (see part C). Please follow the Lab Report Guidelines. Please upload as 'wiki' pages instead of documents.


Part A. Tour and Orientation!

1. Where tools and parts are located.

2. Project storage cabinets and practices.

3. Overview of equipment:


          Power supply


4. How does a breadboard work?

5. Safety video.

6. How to solder: Show an instructor a solder connection you've made using one of the scrap boards.


Part B. Familiarize Yourself with Lab Equipment

1. Make sure you can log into a lab station computer.

2. Make sure you are added as a user to this PBWorks workspace!

3. Click on the 'Pages & Files' tab at the top of the page. Create a folder titled 'Lastname Firstname' (well, using your name) under 'Summer 2015' -> 'Summer 2015 Students' -> 'Lastname Firstname' (no quotes). We will look for lab reports here.

4. Learn to use your power supply and multimeter:

    Please follow this power supply and multimeter tutorial.

5. Learn to use your oscilloscope:

    Please follow this oscilloscope tutorial.



Part C. Make an LED Light Up!


1. Basic LED Circuit

Grab a breadboard from an instructor. You will use 'banana' plug connectors to hook up the power supply to your breadboard, then jumper wires to hook up power and ground to your supply rails on your breadboard. Power your breadboard by hooking up the power and ground cables from the bench power supply (set to +5V) to your breadboard. Use jumper wire to connect these to the red (power) and blue (ground) rails on your breadboard. Be sure to keep your supply powered off until you've checked the circuit and power-ground shorts (see below).


Build the following circuit on your breadboard. Use a 150 Ohm resistor, and a yellow, green or red LED to start out with.


Because LEDs are diodes, they have directionality (resistors and wires don't). The longer arm of the LED should be on the end of the circuit with higher potential. (Remember, the long arm reaches for power!)




Check your circuit before connecting it to power. While getting the circuit right is important to having your LED light up, the more crucial thing to check is to make sure that power is not shorted to ground. Let's repeat that: DON'T SHORT POWER TO GROUND! Shorting power to ground makes really large amounts of current to flow through your circuits, and is liable to damage your components and the power supply. You can check for a short before turning on the supply by hooking up your multimeter to power and ground on your circuit, and checking continuity. If it beeps, you've got a short. You can also check the resistance between power and ground. Obviously, if the resistance is 0 or very small, you have a short that you need to fix.


If your circuit is good, the LED should light up and stay on. If it doesn't, you probably need to switch the orientation of the LED.


2. Controlling the Brightness of LEDs

Now add a potentiometer (or a pot, which is a variable resistor) in series with the resistor to control the brightness of your LED. Do NOT remove the resistor entirely, or it is very possible you will destroy your LED. The max continuous DC forward current rating for our red LED is 30 mA. The max continuous DC forward current rating for our yellow and green LEDs is 25 mA. Here are the datasheets for your red, yellow and green LEDs. Under 'absolute maximum ratings' you will see values that should never be exceeded, and in fact, we want to design circuits conservatively, so we're not right at the edge of failure.




Questions a. and b. are theoretical questions that you should be able to answer using the datasheets and calculations.


a. What resistance do you need to limit current to 30 mA (if using red LED) or 25 mA (if using yellow or green)? Be sure to state which color LED you are using. This resistance refers to the total resistance in series with the LED.


Hint: Make sure that you account for the forward voltage drop (Vf) of the LED that you're using.


b. Is the resistance from question (a) a maximum or minimum resistance? That is, in which direction if you change the resistance (higher or lower) would the LED likely fail.


c. What is the resistance range of the potentiometer?


3. Basic LED Circuit with Switch

Next, we'll insert a switch into the circuit. The momentary switches in yourkit are "normal open," meaning that the circuit is interrupted in the idle state, when the switch is not pressed. Pressing the switch closes the circuit until you let go again.




a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?


4. Battery-Powered LED with Switch on Breadboard

Now, let's make things a little more portable. Disconnect the breadboard from the power supply. Use the battery clip and a 9V battery to power your LED circuit. You'll need to replace your 150 Ohm resistor for the higher voltage. Be sure to do the calculation below so you know what resistance to use. You can choose a conservative resistance so you are not at the edge of failure.


a. Using this battery, what is the minimum resistance required for use with your LED?


5. Make a Video of Your LED Being Switched On and Off.

Yes, we know it's funny to make a video of a LED, but this is good practice for you. Read How to take short range videos using the lab camera, and then record a video of your circuit in action. It's a chance for you to figure out how to arrange things so that you can shoot and poke your circuit at the same time, how to get things that are close to the camera in focus, and how to upload things and embed the video onto your lab page. The easiest way to embed is to upload your video to youtube (unlisted or not), then click 'Insert' -> 'Video' -> 'YouTube.' As an added benefit, you can show your friends and family what you're doing with your fancy Stanford education.


Congrats! Many of you made your first LED light up! After you're done, please return the parts and breadboards. You will get kits of your own starting next week!

Comments (0)

You don't have permission to comment on this page.