Build an Electronic Night Light Electrical and Computer Engineering, Northeastern University What Do Electrical Engineers Do? • Design computers, cell phones, MP3 players, plasma TVs, electrical power systems, satellite communications, radar, GPS… About how many electrons are in a short wire? • We make electrons do useful things. >1020! Making electrons move - - + - - - 1.5 volts - - - - - - - - - - - - Voltage: determines how hard the electrons are pushed. 120 volts 170 volts peak! 9 volts Parts List • Resistors • 1,000 ohms: • 10,000 ohms: • 100,000 ohms: • Light emitting diode (LED) • Photoresistor • Transistor 2N 3904 The Power Supply and Protoboard set to 10 volts Horizontal rows of 5 are connected V- V+ All holes in each vertical column are connected switch the meter to measure current V Vresistor (ohms, W) 1,000 W 10,000 W 100,000 W LED brightness mA Circuit #1 V+ current (mA) The Results: resistor 1,000W LED brightness current (mA) bright 8 mA The Schematic - + 10 volts 10,000W dim 0.8 mA Resistor 100,000W almost out 0.08 mA Current: tells us how many electrons pass through the circuit every second. Resistor: resists the flow of current…like putting a kink in a garden hose. LED Our night light needs a device that can “see” light Photoresistor photons more light more electrons lower resistance Circuit #2 more light… lower resistance… brighter LED… backwards! Tricking the electrons Circuit #3 How does circuit #3 work? electrons take the path of least resistance - + - + - photoresistor: low R LED photoresistor: high R 10,000 ohms LED 10,000 ohms Actually, the current is in the direction of positive charges …thank you, Ben Franklin! - + - + + + photoresistor: low R LED photoresistor: high R 10,000 ohms LED 10,000 ohms We need amplification! …the LED is too weak… The Transistor V+ large current small current large current Circuit #4: the night light 1. the transistor has a flat face 2. the 1,000 ohm resistor is on top 3. the 100,000 ohm resistor is on the bottom 4. don’t forget the wire! 100k wire! How does this circuit work? remember, electrons take the path of least resistance - + LIGHTS ON photoresistor: low R + 100,000 ohms Transistor 1,000 ohms LED How does this circuit work? remember, electrons take the path of least resistance - + LIGHTS OFF photoresistor: high R + 100,000 ohms + Transistor 1,000 ohms LED QUESTIONS? Supplemental Materials Electronic Night Light Parts List 1000W RESISTOR (5%) (brown-black-red-gold) 10,000 W RESISTOR (5%) (brown-black-orange-gold) 100,000 W RESISTOR (5%) (brown-black-yellow-gold) CdS Photoresistor (note: the 100k resistor may need to be replaced with a larger or smaller value depending on the characteristics of the CdS photo resistor available.) Light emitting diodes (red, yellow, and/or green) Transistor (Part no. 2N3904) 1 – Protoboard; 1-Power supply or 9v battery; hook-up wire and wire cutters Written by J. Hopwood ECE Department, Northeastern University Title page photo credit: Prof. N. McGruer This lab is intended for college freshmen or high school students considering a career in engineering. It was developed for the “Building Bridges” program of Northeastern University, Boston, MA with support from the National Science Foundation.