BE Lesson 7: Make an automatic nightlight

• What is a Light Dependent Resistor (LDR) photocell?

• Use a photocell to make an automatic nightlight.

• Solder the nightlight circuit onto a protoboard.

© 2012 C. Rightmyer, Licensed under The MIT OSI License, 20 July 2012

What is an Light Dependent Resistance (LDR) photocell?

physical appearance

electronic symbol

more input light = less resistance = more current through it

Photocell allows hot water circulation only during the daytime(unless tank temperature is too hot)

photocell

BE ckt 7-1. Automatic nightlight using Jameco #202403 photocell

+

v

LED

2N 3904B

C

E

Q1

photocell

330

100K pot

P1

Adapted from Electronic Kourseware Interactive, Discover Electronics, www.eki.com

BE ckt 7-1. Hookup diagram

LED

Photocell 2N 3904(face down)

100K pot

+

9.0

volts

330

Jameco #202403 photocell

BE ckt 7-2. Automatic nightlight soldered on protoboard

+

v

LED

2N 3904B

C

E

Q1

202403 Photocell

330

16K

P1

Adapted from Electronic Kourseware Interactive, Discover Electronics, www.eki.com

Protoboard version of ckt 7-2

Front of protoboard (ckt 7-2)

16k = Orange-Orange-Orange 330 ohm = Orange-Orange-Brown

PC = photocell 202403

16k

+

330

LED

+

-

2N3904

PC

Front of protoboard (ckt 7-2)

16k = Orange-Orange-Orange 330 ohm = Orange-Orange-Brown

PC = photocell 202403

16k

+

330

LED

+

-

2N3904

PC