A schematic diagram is a drawing which shows how to connect electronic components together to
form a circuit.  In this article, we will examine a fairly simple schematic diagram and the actual circuit it describes.  The project shown is an automatic nightlight using LEDs as the light source.
When the room lights go out, the night light turns on.

Schematic Diagram - Automatic Nightlight

In this schematic diagram the components are drawn in blue, the connecting wires are green, the positive power is red and the negative power or ground is black.  Using red for positive and black for ground is a standard.

Schematic DiagramCredit: Walter Cushing (c) 2011

The above schematic is for an automatic nightlight.   This circuit is featured in the book Electronic Circuits for the Evil Genius.

A number of components are used in this circuit.  Here is a list of components and a brief
description of what they do.

  • Diode  - The diode is used to pass current (electricity) in only one  direction.

  • Transistor - The transistor works like a valve.  Power (+) is applied to the
    collector (C) and ground (-) to the emitter (E). Increasing power (+) to the base is like closing the valve.  As the current increases, the valve closes until current stops flowing between the
    collector and emitter.

  • Resistors - Resistors are used to restrict the flow of current in a circuit.  As an  analogy, imagine pinching a garden hose to reduce the flow of water.   A resistor 'pinches' or restricts the flow of current in a circuit.

  • LED (Light Emitting Diode) - This type of diode also passes current in only one direction.  It also produces light when in operation.  If an LED is installed backwards it may burst which, of course, destroys the LED.

  • Photoresistor - A photoresistor changes resistance when light falls on it.  In this
    case, the resistance increases as the light decreases.

  • Potentiometer - The potentiometer is a variable resistor.  By turning the shaft, you
    change the resistance from its minimum to its maximum.  These are  often used for volume controls on radios, stereos, televisions, etc.

  • Battery Connector - Makes it easier to connect power to your circuit.

  • 9V Battery - Power source.

  • Experimenter's Breadboard - This is a reusable base for building electronic projects.  They
    are inexpensive and greatly speed up circuit building, modification, and disassembly.

To help Identify the components they are laid out next to their schematic symbol in the following photo.


Schematic SymbolsCredit: (c) 2011 Walter Cushing

The next photo shows the circuit fully wired up.  I used green wire for interconnections.



Fully Wired CircuitCredit: (c) 2011 Walter Cushing


This is just an introduction to reading schematic diagrams. To learn more
about Electronics as a hobby, you may wish to read the following book
for beginners.